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NSDI '05 Paper    [NSDI '05 Technical Program]

Detecting BGP Configuration Faults with Static Analysis

Nick Feamster and Hari Balakrishnan
MIT Computer Science and Artificial Intelligence Laboratory
{feamster, hari}@csail.mit.edu
http://nms.csail.mit.edu/rcc/

Abstract

The Internet is composed of many independent autonomous systems (ASes) that exchange reachability information to destinations using the Border Gateway Protocol (BGP). Network operators in each AS configure BGP routers to control the routes that are learned, selected, and announced to other routers. Faults in BGP configuration can cause forwarding loops, packet loss, and unintended paths between hosts, each of which constitutes a failure of the Internet routing infrastructure.
This paper describes the design and implementation of rcc, the router configuration checker, a tool that finds faults in BGP configurations using static analysis. rcc detects faults by checking constraints that are based on a high-level correctness specification. rcc detects two broad classes of faults: route validity faults, where routers may learn routes that do not correspond to usable paths, and path visibility faults, where routers may fail to learn routes for paths that exist in the network. rcc enables network operators to test and debug configurations before deploying them in an operational network, improving on the status quo where most faults are detected only during operation. rcc has been downloaded by more than sixty-five network operators to date, some of whom have shared their configurations with us. We analyze network-wide configurations from 17 different ASes to detect a wide variety of faults and use these findings to motivate improvements to the Internet routing infrastructure.

1  Introduction

This paper describes the design, implementation, and evaluation of rcc, the router configuration checker, a tool that uses static analysis to detect faults in Border Gateway Protocol (BGP) configuration. By finding faults over a distributed set of router configurations, rcc enables network operators to test and debug configurations before deploying them in an operational network. This approach improves on the status quo of "stimulus-response" debugging where operators need to run configurations in an operational network before finding faults.
Network operators use router configurations to provide reachability, express routing policy (e.g., transit and peering relationships [28], inbound and outbound routes [3], etc.), configure primary and backup links [17], and perform traffic engineering across multiple links [14]. Configuring a network of BGP routers is like writing a distributed program where complex feature interactions occur both within one router and across multiple routers. This complex process is exacerbated by the number of lines of code (we find that a 500-router network typically has more than a million lines of configuration), by configuration being distributed across the routers in the network, by the absence of useful high-level primitives in today's configuration languages, by the diversity in vendor-specific configuration languages, and by the number of ways in which the same high-level functionality can be expressed in a configuration language. As a result, router configurations are complex and faulty [3,24].
Faults in BGP configuration can seriously affect end-to-end Internet connectivity, leading to lost packets, forwarding loops, and unintended paths. Configuration faults include invalid routes (including hijacked and leaked routes); contract violations [13]; unstable routes [23]; routing loops [8,10]; and persistently oscillating routes [1,19,35]. Section 2 discusses the problems observed in operational networks in detail. We find that rcc can detect many of these configuration faults.
Detecting BGP configuration faults poses several challenges. First, defining a correctness specification for BGP is difficult: its many modes of operation and myriad tunable parameters permit a great deal of flexibility in both the design of a network and in how that design is implemented in the configuration itself. Second, this high-level correctness specification must be used to derive a set of constraints that can be tested against the actual configuration. Finally, BGP configuration is distributed-analyzing how a network configuration behaves requires both synthesizing distributed configuration fragments and representing the configuration in a form that makes it easy to test constraints. This paper tackles these challenges and makes the following three contributions:
First, we define two high-level aspects of correctness-path visibility and route validity-and use this specification to derive constraints that can be tested against the BGP configuration. Path visibility says that BGP will correctly propagate routes for existing, usable IP-layer paths; essentially, it states that the control path is propagating BGP routes correctly. Route validity says that, if routers attempt to send data packets via these routes, then packets will ultimately reach their intended destinations.
Second, we present the design and implementation of rcc. rcc focuses on detecting faults that have the potential to cause persistent routing failures. rcc is not concerned with correctness during convergence (since any distributed protocol will have transient inconsistencies during convergence). rcc's goal is to detect problems that may exist in the steady state, even when the protocol converges to some stable outcome.
Third, we use rcc to explore the extent of real-world BGP configuration faults; this paper presents the first published analysis of BGP configuration faults in real-world ISPs. We have analyzed real-world, deployed configurations from 17 different ASes and detected more than 1,000 BGP configuration faults that had previously gone undetected by operators. These faults ranged from simple "single router" faults (e.g., undefined variables) to complex, network-wide faults involving interactions between multiple routers. To date, rcc has been downloaded by over 65 network operators.
Although rcc is actually intended to be used before configurations are deployed, rcc discovered many faults that could potentially cause failures in live, operational networks. These include: (1) faults that could have caused network partitions due to errors in how external BGP information was being propagated to routers inside an AS, (2) faults that cause invalid routes to propagate inside an AS, and (3) faults in policy expression that caused routers to advertise routes (and hence potentially forward packets) in a manner inconsistent with the AS's desired policies. Our findings indicate that configuration faults that can cause serious failures are often not immediately apparent (i.e., the failure that results from a configuration fault may only be triggered by a specific failure scenario or sequence of route advertisements). If rcc were used before BGP configuration was deployed, we expect that it would be able to detect many immediately active faults.
Our analysis of real-world configurations suggests that most configuration faults stem from three main causes. First, the mechanisms for propagating routes within a network are overly complex. The main techniques used to propagate routes scalably within a network (e.g., "route reflection with clusters") are easily misconfigured. Second, many configuration faults arise because configuration is distributed across routers: even simple policy specifications require configuration fragments on multiple routers in a network. Third, configuring policy often involves low-level mechanisms (e.g., "route maps", "community lists", etc.) that should be hidden from network operators.
The rest of this paper proceeds as follows. Section 2 provides background on BGP configuration. Section 3 describes the design of rcc. Sections 4 and 5 discuss rcc's path visibility and route validity tests. Section 6 describes implementation details. Section 7 presents configuration faults that rcc discovered in 17 operational networks. Section 8 addresses related work, and Section 9 concludes.

2  Background and Motivation

Today's Internet comprises over 17,000 independently operated ASes that exchange reachability information using BGP [31]. BGP distributes routes to destination prefixes via incremental updates. Each router selects one best route to a destination, announces that route to neighboring routers, and sends updates when the best route changes. Each BGP update contains several attributes. These include the destination prefix associated with the route; the AS path, the sequence of ASes that advertised the route; the next-hop, the IP address that the router should forward packets to in order to use the route; the multi-exit discriminator (MED), which a neighboring AS can use to specify that one route should be more (or less) preferred than routes advertised at other routers in that AS; and the community value, which is a way of labeling a route.
BGP's configuration affects which routes are originated and propagated, how routes are modified as they propagate, which route each router selects from multiple options, and how routes propagate between routers. A single AS can have anywhere from two or three routers to many hundreds of routers. A single router's configuration can range from a few hundred lines to more than 10,000 lines. In practice, a large backbone network may have more than a thousand different policies configured across hundreds of routers.
figures/nanog_table.png
Figure 1: Number of threads discussing routing faults on the NANOG mailing list.
To understand the extent to which this complex configuration is responsible for the types of failures that occur in practice, we studied the archives of the North American Network Operators Group (NANOG) mailing list, where network operators report operational problems, discuss operational issues, etc. [27]. Because the list has received about 75,000 emails over the course of ten years, we first clustered the emails by thread and pruned threads based on a list of about fifteen keywords (e.g., "BGP", "issue", "loop", "problem", "outage"). We then reviewed these threads and classified each of them into one or more of the categories shown in Figure 1.
This informal study shows some clear trends. First, many routing problems are caused by configuration faults. Second, the same types of problems continually appear. Third, BGP configuration problems continually perplex even experienced network operators. A tool that can detect configuration faults will clearly benefit network operators.

3  rcc Design

rcc analyzes both single-router and network-wide properties of BGP configuration and outputs a list of configuration faults. rcc checks that the BGP configuration satisfies a set of constraints, which are based on a correctness specification. Figure 2 illustrates rcc's high-level architecture.
We envision that rcc has three classes of users: those that wish to run rcc with no modifications, those that wish to add new constraints concerning the existing specification, and those that wish to augment the high-level specification. rcc's modular design allows users to specify other constraints without changing the system internals. Some users may wish to extend the high-level specification to include other aspects of correctness (e.g., safety [20]) and map those high-level specifications to constraints on the configuration.
Section 3.1 describes how we factor distributed configuration to reason about its behavior and how rcc generates a normalized representation of the configuration that facilitates constraint checking. To detect configuration faults, we must specify, at a high level, correct behavior for an Internet routing protocol; we outline this specification in Section 3.2. Using this high-level specification and our method for reasoning about configuration as a guide, we must then derive the actual correctness constraints that rcc can check against the normalized configuration. Section 3.3 explains this process.
figures/rcc_arch.png
Figure 2: Overview of rcc.

3.1  Factoring Routing Configuration

figures/factoring.png
Figure 3: Factoring BGP configuration.
In this section, we describe a systematic approach to analyze BGP configuration. We factor a network's configuration into the following three categories:
1. Dissemination. A router's configuration determines which other routers that router will exchange BGP routes with. A router has two types of BGP sessions: those to routers in its own AS (internal BGP, or "iBGP") and those to routers in other ASes (external BGP, or "eBGP"). A small AS with only two or three routers may have only 10 or 20 BGP sessions, but large backbone networks typically have more than 10,000 BGP sessions, more than half of which are iBGP sessions. Dissemination primarily concerns flexibility in iBGP configuration.
The session-level BGP topology determines how BGP routes propagate through the network. In small networks, iBGP is configured as a "full mesh" (every router connects to every other router). To improve scalability, larger networks typically use route reflectors. A route reflector selects a single best route and announces that route to all of its "clients". Route reflectors can easily be misconfigured (we discuss iBGP misconfiguration in more detail in Section 4). Incorrect iBGP topology configuration can create persistent forwarding loops and oscillations [20].
2. Filtering. A router's configuration can prevent a certain route from being accepted on inbound or readvertised on outbound. Configuring filtering is complicated because global behavior depends on the configuration of individual routers. A router may "tag" an incoming route to control whether some other router in the AS filters the route.
3. Ranking. Any given router may learn more than one route to a destination, but must select a single best route. Configuration allows a network operator to specify which route is the most preferred route to the destination among several candidates.
Configuration also manipulates route attributes for one of the following reasons: (1) controlling how a router ranks candidate routes, (2) controlling the "next hop" IP address for the advertised route, and (3) labeling a route to control whether another router filters it.
rcc implements the normalized representation as a set of relational database tables. This approach allows constraints to be expressed independently of router configuration languages. As configuration languages evolve and new ones emerge, only the parser must be modified. It also facilitates testing network-wide properties, since all of the information related to the network's BGP configuration can be summarized in a handful of tables. A relational structure is natural because many sessions share common attributes (e.g., all sessions to the same neighboring AS often have the same policies), and many policies have common clauses (e.g., all eBGP sessions may have a filter that is defined in exactly the same way). Table 1 summarizes these tables; Section 6.1 details how rcc populates them.
Table Description
global options router, various global options (e.g., router ID)
sessions router, neighbor IP address, eBGP/iBGP, pointers to policy, options (e.g.RR client)
prefixes router, prefix originated by this router
import/export filters normalized representation of filter: IP range, mask range, permit or deny
import/export policies normalized representation of policies
loopback address(es) router, loopback IP address(es)
interfaces router, interface IP address(es)
static routes static routes for prefixes
undefined references summary of policies and filters that a BGP configuration referenced but did not define
bogon prefixes prefixes that should always be filtered on eBGP sessions [7]
Table 1: Normalized configuration representation.

3.2  Defining a Correctness Specification

rcc's correctness specification uses our previous work on the routing logic [10] as a starting point. rcc checks two aspects of correctness: path visibility and route validity. In the context of BGP, a route is a BGP message that advertises reachability to some destination via an associated path. A path is a sequence of IP hops (i.e., routers) between two IP addresses. We say that a path is usable if it: (1) reaches the destination, and (2) conforms to the routing policies of ASes on the path.
Path visibility implies that every router learns at least one route for each destination it can reach via a usable path. Path visibility may be violated by problems with either dissemination or filtering. An example of a path visibility violation is an iBGP configuration that prevents the dissemination of BGP routes to external destinations, even though usable paths to those destinations exists.
Route validity implies that every route learned by a router describes a usable path, and that this path corresponds to the actual path taken by packets sent to the destination. Problems with dissemination or filtering can cause route validity violations. A forwarding loop is an example of a route validity violation: a router learns a route for a destination, but traffic sent on the corresponding path never reaches that destination.
rcc finds path visibility and route validity violations in BGP configuration only. To make general statements about path visibility and route validity, rcc assumes that the internal routing protocol (i.e., interior gateway protocol, or "IGP") used to establish routes between any two routers within a AS is operating correctly. BGP requires the IGP to operate correctly because iBGP sessions may traverse multiple IGP hops and because the "next hop" for iBGP-learned routes is typically several IGP hops away.
The correctness specification that we have presented addresses static properties of BGP, not dynamic behavior (i.e., its response to changing inputs, convergence time, etc.). BGP, like any distributed protocol, may experience periods of transient incorrectness in response to changing inputs. rcc detects faults that cause persistent failures. Previous work has studied sufficient conditions on the relationships between iBGP and IGP configuration that must be satisfied to guarantee that iBGP converges [20]; these constraints require parsing the IGP configuration, which rcc does not yet check. The correctness specifications and constraints in this paper assume that, given stable inputs, the routing protocol eventually converges to some steady state behavior.
Currently, rcc only detects faults in the BGP configuration of a single AS (a network operator typically does not have access to the BGP configuration from other ASes). Because an AS's BGP configuration explicitly controls both dissemination and filtering, many configuration faults, including partitions, route leaks, etc., are evident from the BGP configuration of a single AS.

3.3  Deriving Constraints and Detecting Faults

Problem Possible Active Fault
Path Visibility
Dissemination Problems
Signaling partition: Router may learn a suboptimal route
   - of route reflectors or none at all.
   - within a RR "cluster"
   - in a "full mesh"
Routers with duplicate: Routers may incorrectly drop routes.
   - loopback address
   - cluster ID
iBGP configured on one end Routers won't exchange routes.
iBGP not to loopback iBGP session fails when one interface fails.
Route Validity
Filtering Problems
transit between peers
Network carries traffic "for free".
inconsistent export to peer
Violation of contract.
inconsistent import
Possible unintentional "cold potato" routing.

eBGP session:
   - w/no filters
   - w/undef. filter
   - w/undef. policy
filter:
   - w/missing prefix
policy:
   - w/undef. AS path
   - w/undef. community
   - w/undef. filter

  • leaked internal routes
  • re-advertising bogus routes
  • accepting bogus routes from neighbors
  • unintentional transit between peers
Dissemination Problems
prepending with bogus AS
AS path is no longer valid.
originating unroutable dest.
Creates a blackhole.
incorrect next-hop Other routers may be unable to reach the routes for a next-hop that is not in the IGP.
Miscellaneous
Decision Process Problems

nondeterministic MED
age-based tiebreaking

Route selection depends on message order.
Table 2: BGP configuration problems that rcc detects and their potentially active faults.
Deriving constraints on the configuration itself that guarantee that the correctness specification is satisfied is challenging. We reason about how the aspects of configuration from Section 3.1 affect each correctness property and derive appropriate constraints for each of these aspects. Specifically, Table 2 summarizes the correctness constraints that rcc checks, which follow from determining which aspects of configuration (from Section 3.1) affect each aspect of the correctness specification (from Section 3.2). These constraints are an attempt to map the path visibility and route validity specifications to constraints on BGP configuration that can be checked against the actual configuration.
Ideally, operators would run rcc to detect configuration faults before they are deployed. Some of rcc's constraints detect faults that would most likely become active immediately upon deployment. For example, a router that is advertising routes with an incorrect next-hop attribute will immediately prevent other routers that use those routes from forwarding packets to those destinations. In this case, rcc can help the operator diagnose configuration faults and prevent them from introducing failures on the live network.
Many of the constraints in Table 2 concern faults that could remain undetected even after the configuration has been deployed because they remain masked until some sequence of messages triggers them. In these cases, rcc can help operators find faults that could result in a serious failure. Section 4 describes one such path visibility fault involving dissemination in iBGP in further detail. In other cases, checking constraints implies some knowledge of high-level policy (recall that a usable path conforms to some high-level policy). In the absence of a high-level policy specification language, rcc must make inferences about a network operator's intentions. Section 5 describes several route validity faults where rcc must make such inferences.

3.4  Completeness and Soundness

figures/faults2.png
Figure 4: Relationships between faults and failures.
rcc's constraints are neither complete nor sound; that is, they may not find all problematic configurations, and they may complain about harmless deviations from best common practice. However, practical static analysis techniques for program analysis are typically neither complete nor sound [25]. Figure 4 shows the relationships between classes of configuration faults and the class of faults that rcc detects. Latent faults are faults that are not actively causing any problems but nonetheless violate the correctness constraints. A subset of latent faults are potentially active faults, for which there is at least one input sequence that is certain to trigger the fault. For example, an import policy that references an undefined filter on a BGP session to a neighboring AS is a potentially active fault, which will be triggered when that neighboring AS advertises a route that ought to have been filtered. When deployed, a potentially active fault will become active if the corresponding input sequence occurs. An active fault constitutes a routing failure for that AS.
Some active faults may ultimately appear as end-to-end failures. For example, if an AS advertises an invalid route (e.g., a route for a prefix that it does not own) to a neighboring AS whose import policy references an undefined filter, then some end hosts may not be able to reach destinations within that prefix. Note that a potentially active fault may not always result in an end-to-end failure if no path between the sources and destinations traverses the routers in the faulty AS.
rcc detects a subset of latent (and hence, potentially active) faults. In addition, rcc  may also report some false positives: faults that violate the constraints but are benign (i.e., the violations would never cause a failure). Ideally, rcc would detect fewer benign faults by testing the BGP configuration against an abstract specification. Unfortunately, producing such a specification requires additional work from operators, and operators may well write incorrect specifications. One of rcc's advantages is that it provides useful information about configuration faults without requiring any additional work on the part of operators.
Our previous work [10] presented three properties in addition to path visibility and route validity: information flow control (this property checks if routes "leak" in violation of policy), determinism (whether a router's preference for routes depends on the presence or absence of other routes), and safety (whether the protocol converges) [21]. This work treats information flow control as a subset of validity. rcc does not check for faults related to determinism and safety. Determinism cannot be checked with static analysis alone. Safety is a property that typically requires access to configurations from multiple ASes; in recent work, we have explored how to guarantee safety with access to configurations of only a single AS [11].

4  Path Visibility Faults

Recall that path visibility specifies that every router that has a usable path to a destination learns at least one valid route to that destination. It is an important property because it ensures that, if the network remains connected at lower layers, the routing protocol does not create any network partitions. Table 2 shows many conditions that rcc checks related to path visibility; in this section, we focus on iBGP configuration faults that can violate path visibility and explain how rcc detects these faults.
Ensuring path visibility in a "full mesh" iBGP topology is reasonably straightforward; rcc checks that every router in the AS has an iBGP session with every other router. If this condition is satisfied, every router in the AS will learn all eBGP-learned routes.
Because a "full mesh" iBGP topology scales poorly, operators often employ route reflection [2]. A subset of the routers are configured as route reflectors, with the configuration specifying a set of other routers as route reflector clients. Each route reflector readvertises its best route according to the following rules: (1) if the best route was learned from an iBGP peer, the route is readvertised to all of its route reflector clients; (2) if it was learned from a client or via an eBGP session, the route is readvertised on all iBGP sessions. A router does not readvertise iBGP-learned routes over regular iBGP sessions. If a route reflector client has multiple route reflectors, those reflectors must share all of their clients and belong to a single "cluster".
A route reflector may itself be a client of another route reflector. Any router may also have iBGP sessions with other routers. We use the set of reflector-client relationships between routers in an AS to define a graph G, where each router is a node and each session is either a directed or undirected edge: a client-reflector session is a directed edge from client to reflector, and other iBGP sessions are undirected edges. An edge exists if and only if (1) the configuration of each router endpoint specifies the loopback address of the other endpoint1 and (2) both routers agree on session options (e.g., MD5 authentication parameters). G should also not have partitions at lower layers. We say that G is acyclic if G has no sequence of directed and undirected edges that form a cycle. To ensure the existence of a stable path assignment, G should be acyclic.
figures/rr_example_fig.png
Figure 5: In this iBGP configuration, route r2 will be distributed to all the routers in the AS, but r1 will not. Y and Z will not learn of r1, leading to a network partition that won't be resolved unless another route to the destination appears from elsewhere in the AS.
Even a connected directed acyclic graph of iBGP sessions can violate path visibility. For example, in Figure 5, routers Y and Z do not learn route r1 to destination d (learned via eBGP by router W), because X will not readvertise routes learned from its iBGP session with W to other iBGP sessions. We call this path visibility fault an iBGP signaling partition; a path exists, but neither Y nor Z has a route for it. Note that simply adding a regular iBGP session between routers W and Y would solve the problem.
In addition to causing network partitions, iBGP signaling partitions may result in suboptimal routing. For example, in Figure 5, even if Y or Z learned a route to d via eBGP, that route might be worse than the route learned at W. In this case, Y and Z would ultimately select a suboptimal route to the destination, an event that an operator would likely fail to notice.
rcc detects iBGP signaling partitions. It determines if there is any combination of eBGP-learned routes such that at least one router in the AS will not learn at least one route to the destination. The following result forms the basis for a simple and efficient check.
Theorem 1 Suppose that the graph defined by an AS's iBGP relationships, G, is acyclic. Then, G does not have a signaling partition if, and only if, the BGP routers that are not route reflector clients form a full mesh.
Proof. Call the set of routers that are not reflector clients the "top layer" of G. If the top layer is not a full mesh, then there are two routers X and Y with no iBGP session between them, such that no route learned using eBGP at X will ever be disseminated to Y, since no router readvertises an iBGP-learned route.
Conversely, if the top layer is a full mesh, observe that if a route reflector has a route to the destination, then all its clients have a route as well. Thus, if every router in the top layer has a route, all routers in the AS will have a route. If any router in the top layer learns a route through eBGP, then all the top layer routers will hear of the route (because the top layer is a full mesh). Alternatively, if no router at the top layer hears an eBGP-learned route, but some other router in the AS does, then that route propagates up a chain of route reflectors (each client sends it to its reflector, and the reflector sends it on all its iBGP sessions) to the top layer, from there to all the other top layer routers, and from there to the other routers in the AS. &loz
rcc checks this condition by constructing the iBGP signaling graph G from the sessions table (Table 1). It assumes that the IGP graph is connected, then determines whether G is connected and acyclic and whether the routers at the top layer of G form a full mesh.

5  Route Validity Faults

BGP should satisfy route validity. Its configuration affects which routes each router accepts, selects, and re-advertises. Table 2 summarizes the route validity faults that rcc  checks. In this section, we focus on rcc's approach to detecting potential policy-related problems.
The biggest challenge for checking policy-related problems is that rcc  operates without a specification of the intended policy. Requiring operators to provide a high-level policy specification would require designing a specification language and convincing operators to use it, and it provides no guarantees that the results would be more accurate, since errors may be introduced into the specification itself. Instead, rcc  forms beliefs about a network operator's intended policy in two ways: (1) assuming that intended policies conform to best common practice and (2) analyzing the configuration for common patterns and looking for deviations from those patterns. rcc then finds cases where the configuration appears to violate these beliefs. It is noteworthy that, even in the absence of a policy specification, this technique detects many meaningful configuration faults and generates few false positives.

5.1  Violations of Best Common Practice

Typically, a route that an AS learns from one of its "peers" should not be readvertised to another peer. Checking this condition requires determining how a route propagates through a network. Figure 6 illustrates how rcc performs this check. Suppose that rcc is analyzing the configuration from AS X and needs to determine that no routes learned from Worldcom are exported to Sprint. First, rcc determines all routes that X exports to Sprint, typically a set of routes that satisfy certain constraints on their attributes. For example, router A may export to Sprint only routes that are "tagged" with the label "1000". (ASes often designate such labels to signify how a route was learned.) rcc then checks the import policies for all sessions to Worldcom, ensuring that no import policy will set route attributes on any incoming route that would place it in the set of routes that would be exported to Sprint.
Additionally, an AS should advertise routes with equally good attributes to each peer at every peering point. An AS should not advertise routes with inconsistent attributes, since doing so may prevent its peer from implementing "hot potato" routing,2 which typically violates peering agreements. Recent work has observed that this type of inconsistent route advertisement sometimes occurs in practice [13].
This violation can arise for two reasons. First, an AS may apply different export policies at different routers to the same peer. Checking for consistent export involves comparing export policies on each router that has an eBGP session with a particular peer. Static analysis is useful because it can efficiently compare policies on many different routers. In practice, this comparison is not straightforward because differences in policy definitions are difficult to detect by direct inspection of the distributed router configurations. rcc makes comparing export policies easy by normalizing all of the export policies for an AS, as described in Section 3.1.
Second, an iBGP signaling partition can create inconsistent export policies because routes with equally good attributes may not propagate to all peering routes. For example, consider Figure 5 again. If routers W and Z both learn routes to some destination d, then route W may learn a "better" route to d, but routers Y and Z will continue to select the less attractive route. If routers X and Y re-advertise their routes to a peer, then the routes advertised by X and Y will not be equally good. Thus, rcc also checks whether routers that advertise routes to the same peer are in the same iBGP signaling partition (as described in Section 4, rcc checks for all iBGP signaling partitions, but ones that cause inconsistent advertisement are particularly serious).

5.2  Configuration Anomalies

figures/policy_closure.png
Figure 6: How rcc computes route propagation.
When the configurations for sessions at different routers to a neighboring AS are the same except at one or two routers, the deviations are likely to be mistakes. This test relies on the belief that, if an AS exchanges routes with a neighboring AS on many sessions and most of those sessions have identical policies, then the sessions with slightly different policies may be misconfigurations. Of course, this test could result in many false positives because there are legitimate reasons for having slightly different import policies on sessions to the same neighboring AS (e.g., outbound traffic engineering), but it does provide a useful sanity check.

6  Implementation

figures/rcc_design.png
Figure 7: Overview of rcc implementation.
rcc is implemented in Perl and has been downloaded by over 65 network operators. The parser is roughly 60% of the code. Much of the parser's logic is dedicated to policy normalization. Figure 7 shows an overview of rcc, which takes as input the set of configuration files collected from routers in a single AS using a tool such as "rancid" [30]. rcc  converts the vendor-specific BGP configuration to a vendor-independent normalized representation. It then checks this normalized format for faults based on a set of correctness constraints. rcc's functionality is decomposed into three distinct modules: (1) a preprocessor, which converts configuration into a more parsable version; (2) a parser, which generates the normalized representation; and (3) a constraint checker, which checks the constraints.
figures/if.png
Figure 8: BGP configuration in normalized format.

6.1  Preprocessing and Parsing

The preprocessor adds scoping identifiers to configuration languages that do not have explicit scoping (e.g., Cisco IOS) and expands macros (e.g., Cisco's "peer group", "policy list", and "template" options). After the preprocessor performs some simple checks to determine whether the configuration is a Cisco-like configuration or a Juniper configuration, it launches the appropriate parser. Many configurations (e.g., Avici, Procket, Zebra, Quarry) resemble Cisco configuration; the preprocessor translates these configurations so that they more closely resemble Cisco syntax.
The parser generates the normalized representation from the preprocessed configuration. The parser processes each router's configuration independently. It makes a single pass over each router's configuration, looking for keywords that help determine where in the configuration it is operating (e.g., "route-map" in a Cisco configuration indicates that the parser is entering a policy declaration). The parser builds a table of normalized policies by dereferencing all filters and other references in the policy; if the reference is defined after it is referenced in the same file, the parser performs lazy evaluation. When it reaches the end of a file, the parser flags any policies references in the configuration that it was unable to resolve. The parser proceeds file-by-file (taking care to consider that definitions are scoped by each file), keeping track of normalized policies and whether they have already appeared in other configurations.
Figure 8 shows rcc's normalized representation for a fragment of Cisco IOS. In rcc, this normalized representation is implemented as a set of mySQL database tables corresponding to the schema shown in Table 1. This Cisco configuration specifies a BGP session to a neighboring router with IP address 10.1.2.3 in AS 3. This statement is represented by a row in the sessions table. The second line of configuration specifies that the import policy (i.e., "route map") for this session is defined as " IMPORT_CUST" elsewhere in the file; the normalized representation represents the import policy specification as a pointer into a separate table that contains the import policies themselves. A single policy, such as IMPORT_CUST is represented as multiple rows in the policies table. Each row represents a single clause of the policy. In this example, IMPORT_CUST has two clauses: the first rejects all routes whose AS path matches the regular expression number "99" (specified as "^65000" elsewhere in the configuration), and the second clause accepts all routes that match AS path number "88" and community number "10" and sets the "local preference" attribute on the route to a value of 80. Each of these clauses is represented as a row in the policies table; specifications for regular expressions for AS paths and communities are also stored in separate tables, as shown in Figure 8.
rcc's normalized representation does not store the names of the policies themselves (e.g., "IMPORT_CUST", AS regular expression number "88", etc.). Rather, the normalized format only stores a description of what the route policy does (e.g., "set the local preference value to 80 if the AS path matches regular expression ^3"). Two policies may be written using entirely different names, regular expression numbers, or even in different languages, but if the policies perform the same operations, rcc will recognize that they are in fact the same policy.

6.2  Constraint Checking

We implemented each correctness condition in Table 2 by executing SQL queries against the normalized format and analyzing the results of these queries in Perl.
rcc checks many constraints by executing simple queries against the normalized representation. Checking constraints against the normalized representation is simpler than analyzing distributed router configurations. Consider the test in Table 2 called "iBGP configured on one end"; this constraint requires that, if a router's configuration specifies an iBGP session to some IP address, then (1) that IP address should be the loopback address of some other router in the AS, and (2) that other router should be configured with an iBGP session back to the first router's loopback address. rcc tests this constraint as a single, simple "select" statement that "joins" the loopbacks and sessions tables. Other tests, such as checking properties of the iBGP signaling graph, require reconstructing the iBGP signaling graph using the sessions table.
As another example, to check that no routing policy in the AS prepends any AS number other than its own, rcc  executes a "select" query on a join of the sessions and policies tables, which returns the ASes that each policy prepends (if any) and the routers where each policy is used. rcc then checks the global table to ensure that that for each router, the AS number configured on the router matches the ASes that any policy on that router prepends.

7  Evaluating Operational Networks with rcc 

Our goal is to help operators move away from today's mode of stimulus-response reasoning by allowing them to check the correctness of their configurations before deploying them on a live network. rcc has helped network operators find faults in deployed configurations; we present these findings in this section. Because we used rcc  to test configurations that were already deployed in live networks, we did not expect rcc to find many of the types of transient misconfigurations that Mahajan et al. found [24] (i.e., those that quickly become apparent to operators when the configuration is deployed). If rcc were applied to BGP configurations before deployment, we expect that it could prevent more than 75% of the "origin misconfiguration" incidents and more than 90% of the "export misconfiguration" incidents described in that study.3

7.1  Analyzing Real-World Configurations

We used rcc to evaluate the configurations from 17 real-world networks, including BGP configurations from every router in 12 ASes. We made rcc  available to operators, hoping that they would run it on their configurations and report their results.
Network operators are reluctant to share router configuration because it often encodes proprietary information. Also, many ISPs do not like researchers reporting on mistakes in their networks. (Previous efforts have enjoyed only limited success in gaining access to real-world configurations [34].) We learned that providing operators with a useful tool or service increases the likelihood of cooperation. When presented with rcc, many operators opted to provide us with configurations, while others ran rcc on their configurations and sent us the output.
rcc detected over 1,000 configuration faults. The size of these networks ranged from two routers to more than 500 routers. Many operators insisted that the details of their configurations be kept private, so we cannot report separate statistics for each network that we tested. Every network we tested had BGP configuration faults. Operators were usually unaware of the faults in their networks.
Problem Latent Benign
Path Visibility
Dissemination Problems
Signaling partition:
   - of route reflectors 4 1
   - within a RR "cluster" 2 0
   - in a "full mesh" 2 0
Routers with duplicate:
   - loopback address 13 120
iBGP configured on one end 420 0
or not to loopback
Route Validity
Filtering Problems
transit between peers 3 3
inconsistent export to peer 231 2
inconsistent import 105 12
eBGP session:
   - w/no filters 21 -
   - w/undef. filter 27 -
   - w/undef. policy 2 -
filter:
   - w/missing prefix 196 -
policy:
   - w/undef. AS path 31 -
   - w/undef. community 12 -
   - w/undef. filter 18 -
Dissemination Problems
prepending with bogus AS 0 1
originating unroutable dest. 22 2
incorrect next-hop 0 2
Miscellaneous
Decision Process Problems
nondeterministic MED 43 0
age-based tiebreaking 259 0
Table 3: BGP configuration faults in 17 ASes.

7.2  Fault Classification and Summary

Table 3 summarizes the faults that rcc detected. rcc discovered potentially serious configuration faults as well as benign ones. The fact that rcc discovers benign faults underscores the difficulty in specifying correct behavior. Faults have various dimensions and levels of seriousness. For example, one iBGP partition indicates that rcc found one case where a network was partitioned, but one instance of unintentional transit means that rcc found two sessions that, together, caused the AS to carry traffic in violation of high-level policy. The absolute number of faults is less important than noting that many of the faults occurred at least once.
Figure 9 shows that many faults appeared in many different ASes. We did not observe any significant correlation between network complexity and prevalence of faults, but configurations from more ASes are needed to draw any strong conclusions. The rest of this section describes the extent of the configuration faults that we found with rcc.

7.3  Path Visibility Faults

The path visibility faults that rcc detected involve iBGP signaling and fall into three categories: problems with "full mesh" and route reflector configuration, problems configuring route reflector clusters, and incomplete iBGP session configuration. Detecting these faults required access to the BGP configuration for every router in the AS.
iBGP signaling partitions. iBGP signaling partitions appeared in one of two ways: (1) the top layer of iBGP routers was not a full mesh; or (2) a route reflector cluster had two or more route reflectors, but at least one client in the cluster did not have an iBGP session with every route reflector in the cluster. Together, these accounted for 9 iBGP signaling partitions in 5 distinct ASes, one of which was benign. While most partitions involved route reflection, we were surprised to find that even small networks had iBGP signaling partitions. In one network of only three routers, the operator had failed to configure a full mesh; he told us that he had "inadvertently removed an iBGP session". rcc also found two cases where routers in a cluster with multiple route reflectors did not have iBGP sessions to all route reflectors in that cluster.
rcc discovered one benign iBGP signaling partition. The network had a group of routers that did not exchange routes with the rest of the iBGP-speaking routers, but the routers that were partitioned introduced all of the routes that they learned from neighboring ASes into the IGP, rather than readvertising them via iBGP. The operator of this network told us that these routers were for voice-over-IP traffic; presumably, these routers injected all routes for this application into the IGP to achieve fast convergence after a failure or routing change. In cases such as these, BGP configuration cannot be checked in isolation from other routing protocols.
figures/errors_by_as_nsdi.png
Figure 9: Number of ASes in which each type of fault occurred at least once.
Route reflector cluster problems. In an iBGP configuration with route reflection, multiple route reflectors may serve the same set of clients. This group of route reflectors and its clients is called a "cluster"; each cluster should have a unique ID, and all routers in the cluster should be assigned the same cluster ID. If a router's BGP configuration does not specify a cluster ID, then typically a router's loopback address is used as the cluster ID. If two routers have the same loopback address, then one router may discard a route learned from the other, thinking that the route is one that it had announced itself. rcc found 13 instances of routers in distinct clusters with duplicate loopback addresses and no assigned cluster ID.
Different physical routers in the same AS may legitimately have identical loopback addresses. For example, routers in distinct IP-layer virtual private networks may route the same IPv4 address space.
Incomplete iBGP sessions. rcc discovered 420 incomplete iBGP sessions (i.e., a configuration statement on one router indicated the presence of an iBGP session to another router, but the other router did not have an iBGP session in the reverse direction). Many of these faults are likely benign. The most likely explanation for the large number of these is that network operators may disable sessions by removing the configuration from one end of the session without ever "cleaning up" the other end of the session.

7.4  Route Validity Faults

In this section, we discuss route validity faults. We first discuss filtering-related faults; we classify faults as latent unless a network operator explicitly told us that the fault was benign. We also describe faults concerning undefined references to policies and filters. Some of these faults, while simple to check, could have serious consequences (e.g., leaked routes), if rcc had not caught them and they had been activated. Finally, we present some interesting faults related to route dissemination, all of which were benign.

7.4.1  Filtering Problems

Decomposing policies across configurations on different routers can cause faults, even for simple policies such as controlling route export between peers. rcc discovered the following problems:
Transit between peers. rcc discovered three instances where routes learned from one peer or provider could be readvertised to another; typically, these faults occurred because an export policy for a session was intended to filter routes that had a certain community value, but the export policy instead referenced an undefined community.
Obsolete contractual arrangements can remain in configuration long after those arrangements expire. rcc discovered one AS that appeared to readvertise certain prefixes from one peer to another. Upon further investigation, we learned that the AS was actually a previous owner of one of the peers. When we notified the operator that his AS was providing transit between these two peers, he told us, "Historically, we had a relationship between them. I don't know what the status of that relationship is these days. Perhaps it is still active-at least in the configs!"
Inconsistent export to peer. We found 231 cases where an AS advertised routes that were not "equally good" at every peering point. It is hard to say whether these inconsistencies are benign without knowing the operator's intent, but roughly twenty of these inconsistencies were certainly accidental. For example, one inconsistency existed because of an undefined AS path regular expression referenced in the export policy; these types of inconsistencies have also been observed in previous measurement studies [13].
Inconsistent import policies. A recent measurement study observed that ASes often implement policies that result in late exit (or "cold potato") routing, where a router does not select the BGP route that provides the closest exit point from its own network [33].4 rcc  found 117 instances where an AS's import policies explicitly implemented cold potato routing, which supports this previous observation. In one network, rcc detected a different import policy for every session to each neighboring AS. In this case, the import policy was labeling routes according to the router at which the route was learned.
Inconsistent import and export policies were not always immediately apparent to us even after rcc detected them: the two sessions applied policies with the same name, and both policies were defined with verbatim configuration fragments. The difference resulted from the fact that the difference in policies was three levels of indirection deep. For example, one inconsistency occurred because of a difference in the definition for an AS path regular expression that the export policy referenced (which, in turn, was referenced by the session parameters).
rcc also detected filtering problems on single-router configurations:
Undefined references in policy definitions. Several large networks had router configurations that referenced undefined variables and BGP sessions that referenced undefined filters. These faults can sometimes result in unintentional transit or inconsistent export to peers or even potential invalid route advertisements. In one network, rcc found four routers with undefined filters that would have allowed a large ISP to accept and readvertise any route to the rest of the Internet (such a failure actually occurred in 1997 [32]); this potentially active fault could have been catastrophic if a customer had (unintentionally or intentionally) announced invalid routes, since ASes typically do not filter routes coming from large ISPs. This misconfiguration occurred even though the router configurations were being written with scripts; an operator had apparently made a mistake specifying inputs to the scripts. Operators can detect such faults using rcc.
Non-existent or inadequate filtering. Filtering can go wrong in several ways: (1) no filters are used whatsoever, (2) a filter is specified but not defined, or (3) filters are defined but are missing prefixes or otherwise out-of-date (i.e., they are not current with respect to the list of private and unallocated IP address space [7]).
Every network that rcc analyzed had faults in filter configuration. Some of these faults would have caused an AS to readvertise any route learned from a neighboring AS. In one case, policy misconfiguration caused an AS to transit traffic between two of its peers. Table 3 and Figure 9 show that these faults were extremely common: rcc found 21 eBGP sessions in 5 distinct ASes with no filters whatsoever and 27 eBGP sessions in 2 ASes that referenced undefined filters. Every AS had partially incorrect filter configuration, and most of the smaller ASes we analyzed either had minimal or no filtering. Only a handful of the ASes we analyzed appeared to maintain rigorous, up-to-date filters for private and unallocated IP address space. These findings agree with those of our recent measurement study, which also suggests that many ASes do not perform adequate filtering [12].
The reason for inadequate filtering seems to be the lack of a process for installing and updating filters. One operator told us that he would be willing to apply more rigorous filters if he knew a good way of doing so. Another operator runs sanity checks on filters and was surprised to find that many sessions were referring to undefined filters. Even a well-defined process can go horribly wrong: one operator intended to use a feed of unallocated prefixes to automatically install filters, but instead ended up readvertising them. Because there is a set of prefixes that every AS should always filter, some prefixes should be filtered by default.

7.4.2  Dissemination Problems

We describe configuration faults involving dissemination. rcc found only benign faults in this case.
Unorthodox AS path prepending practices. An AS will often prepend its own AS number to the AS path on certain outbound advertisements to affect inbound traffic. However, we found one AS that prepended a neighbor's AS on inbound advertisements in an apparent attempt to influence outbound traffic.5
iBGP sessions with "next-hop self". We found two cases of iBGP sessions that violated common rules for setting the next-hop attribute, both of which were benign. First, rcc detected route reflectors that appeared to be setting the "next hop" attribute. Although this practice is not likely to create active faults, it seemed unusual, since the AS's exit routers typically set the next hop attribute, and route reflectors typically do not modify route attributes. Upon further investigation, we learned that some router vendors do not allow a route reflector to reset the next-hop attribute. Even though the configuration specified that the session would reset the next-hop attribute, the configuration statement had no effect because the software was designed to ignore it. The operator who wrote the configuration specified that the next-hop attribute be reset on these sessions to make the configuration appear more uniform. Second, routers sometimes reset the next-hop on iBGP sessions to themselves on sessions to a route monitoring server to allow the operator to distinguish which router sent each route to the monitor.

7.5  Miscellaneous Tests

Non-deterministic route selection. rcc discovered more than two hundred routers that were configured such that the arrival order of routes affected the outcome of the route selection process (i.e., these routers had either one or both of the two configuration settings that cause nondeterminism). Although there are occasionally reasonably good reasons for introducing ordering dependencies (e.g., preferring the "most stable" route; that is, the one that was advertised first), operators did not offer good reasons for why these options were disabled. In response to our pointing out this fault, one operator told us,"That's a good point, but my network isn't big enough that I've had to worry about that yet." Non-deterministic features should be disabled by default.

7.6  Higher-level Lessons

Our evaluation of real-world BGP configuration from operational networks suggests five higher-level lessons about the nature of today's configuration process. First, operational networks-even large, well-known, and well-managed ones-have faults. Even the most competent of operators find it difficult to manage BGP configuration. Moreover, iBGP is misconfigured often; in fact, in the absence of a guideline such as Theorem 4.1, it is hard for a network operator to know what properties the iBGP signaling graph should have. Second, the majority of the configuration faults that rcc detected resulted from the fact that an AS's configuration is distributed across its routers. A routing architecture or configuration management system that enabled an operator to configure the network from a centralized location with a high-level language would likely prevent many serious faults. Third, although operators use tools that automate some aspects of configuration, these tools are not a panacea. In fact, we found cases where the incorrect use of these tools caused configuration faults. Fourth, maintaining network-wide policy consistency appears to be hard; invariably, in most ASes there are routers whose configuration appears to contradict the AS's desired policy. Finally, we found that route filters are poorly maintained. Routes that should never be seen on the global Internet (e.g., routes for private addresses) are rarely filtered, and the filters that are used are often misconfigured and outdated.

8  Related Work

We discuss related work in three areas: router configuration, model checking, and BGP convergence.
Router configuration. Mahajan et al. studied short-lived BGP misconfiguration by analyzing transient, globally visible BGP announcements from an edge network [24]. They defined a "misconfiguration" as a transient BGP announcement that was followed by a withdrawal within a small amount of time (suggesting that the operator observed and fixed the problem). They found that many misconfigurations are caused by faulty route origination and incorrect filtering. rcc can help operators find these faults; it can also detect faults that are difficult to quickly locate and correct. rcc also helps operators detect the types of misconfigurations found by Mahajan et al. [24] before deployment.
Some commercial tools analyze network configuration and highlight rudimentary errors [29]. Previous work has proposed tools that analyze intradomain routing configuration [15] and automate enterprise network configuration [6]. These tools detect router and session-level syntax errors only (e.g., undefined filters), a subset of the faults that rcc detects. rcc is the first tool to check network-wide properties using a vendor-independent configuration representation and the first tool that applies a high-level specification of routing protocol correctness.
Many network operators use configuration management tools such as "rancid" [30], which periodically archive router configuration and provide version tracking. When a network problem coincides with the configuration change that caused it, these tools can help operators revert to an older configuration. Unfortunately, a configuration change may induce a latent or potentially active fault, and these tools do not detect whether the configuration has these types of faults in the first place.
Model checking. Model checking has been successful in verifying the correctness of programs [18] and other network protocols [4,22,26]. Unfortunately, model checking is not appropriate for verifying BGP configuration because it depends heavily on exhausting the state-space within an appropriately-defined environment [25]. The behavior of an AS's BGP configuration depends on routes that arrive from other ASes, some of which, such as backup paths, cannot be known in advance [9].
Analysis of BGP safety and stability. Previous work has noted that BGP may not converge to a stable path assignment and stated sufficient conditions to guarantee that BGP will arrive at such an assignment [19,21,35]. This property is called safety. Gao and Rexford state sufficient conditions for safety in eBGP and observe that typical policy configurations satisfy these conditions [16]. (Griffin et al. note that analogous sufficient conditions apply to iBGP with route reflection [20].) In both cases, the sufficient conditions also require global knowledge of either rankings or the AS-level topology. rcc tests constraints that must hold on the configuration of a single AS. Our recent work derives necessary conditions that the configuration of each AS must satisfy to guarantee safety [11].

9  Discussion and Conclusion

In recent years, much work has been done to understand BGP's behavior, and much has been written about the wide range of problems it has. Some argue that BGP has outlived its purpose and should be replaced; others argue that faults arise because today's configuration languages are not well-designed. We believe that our evaluation of faults in today's BGP configuration provides a better understanding of the types of errors that appear in today's BGP configuration and the problems in today's configuration languages. Our findings should help inform the design of wide-area routing systems in the future.
Despite the fact that BGP is almost 10 years old, operators continually make the same mistakes as they did during BGP's infancy, and, regrettably, our understanding of what it means for BGP to behave "correctly" is still rudimentary. This paper takes a step towards improving this state of affairs by making the following contributions:
  • We define a high-level correctness specification for BGP and map that specification to conditions that can be tested with static analysis.
  • We use this specification to design and implement rcc, a static analysis tool that detects faults by analyzing the BGP configuration across a single AS. With rcc, network operators can find many faults before deploying configurations in an operational network. rcc has been downloaded by over 65 network operators.
  • We use rcc to explore the extent of real-world BGP misconfigurations. We have analyzed real-world, deployed configurations from 17 different ASes and detected more than 1,000 BGP configuration faults that had previously gone undetected by operators.
In light of our findings, we suggest two ways to make interdomain routing less prone to configuration faults. First, protocol improvements, particularly in intra-AS route dissemination, could avert many BGP configuration faults. The current approach to scaling iBGP should be replaced. Route reflection serves a single, relatively simple purpose, but it is the source of many faults, many of which cannot be checked with static analysis of BGP configuration alone [20]. The protocol that disseminates BGP routes within an AS should enforce path visibility and route validity; the Routing Control Platform [5] offers one possible solution.
Second, BGP should be configured with a centralized, higher-level specification language. Today's BGP configuration languages enable an operator to specify router-level mechanisms that implement high-level policy, but the distributed, low-level nature of the configuration languages introduces complexity, obscurity, and opportunities for misconfiguration rather than design flexibility or expressiveness. For example, rcc detects many faults in implementation of some high-level policies in low-level configuration; these faults arise because there are many ways to implement the same high-level policy, and the low-level configuration is unintuitive. Ideally, a network operator would never touch low-level mechanisms (e.g., the community attribute) in the common case. Rather than configuring routers with a low-level language, an operator should configure the network using a language that directly reflects high-level policies.

Acknowledgements

This work would not have been possible without the support and patience of many in the network operations community. We are grateful to Randy Bush, Jennifer Rexford, and Guy Tal for inspiration, suggestions, and feedback. We thank David Andersen, Tom Barron, Rob Beverly, Jay Borkenhagen, Grover Browning, Andres Gasson, Michael Hallgren, John Heasley, Jaroslaw Kowalczyk, Arnaud Le Taillanter, Simon Leinen, Ratul Mahajan, Hank Nussbacher, Scott Poretsky, Jeff Schiller, Nicolas Strina, and Matt Zekauskas for their help. We also thank Eddie Kohler (this paper's "shepherd"), Mike Walfish, and the anonymous NSDI reviewers for thoughtful feedback that improved this paper. This work was supported by a Cisco URP grant and by the NSF under Cooperative Agreement ANI-0225660. Nick Feamster is partially supported by an NSF Graduate Research Fellowship.

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Footnotes:

1 If a router establishes an iBGP session with a router's loopback address, then the iBGP session will remain active as long as that router is reachable via any IGP path between the two routers. If a router establishes an iBGP session with an interface address of another router, however, the iBGP session will go down if that interface fails, even if an IGP path exists between those routers.
2If ASes 1 and 2 are peers, then the export policies of the routers in AS 1 should export routes to AS 2 that have equal AS path length and MED values. If not, router X could be forced to send traffic to AS 1 via router Y ("cold potato" routing)
3rcc detects the following classes of misconfiguration described by Mahajan et al.: reliance on upstream filtering, old configuration, community, forgotten filter, prefix-based config, bad ACL or route map, and typo.
4Inconsistent import policy technically concerns how configuration affects ranking, and it is more often intentional than not. Nevertheless, this test occasionally highlights anomalies that operators are interested in correcting, and it serves as a useful sanity check when looking for other types of anomalies (such as dynamically detecting inconsistent route advertisements from a neighboring AS [13]).
5One network operator also mentioned that ASes sometimes prepend the AS number of a network that they want to prevent from seeing a certain route (i.e., by making that AS discard the route due to loop detection), effectively "poisoning" the route. We did not witness this poisoning in any of the configurations we analyzed.


This paper was originally published in the Proceedings of the 2nd Symposium on Networked Systems Design and Implementation,
May 2–4, 2005, Boston, MA, USA

Last changed: 2 May 2005 aw
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