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Network Working Group                                         Y. RekhterRequest for Comments: 1655        T.J. Watson Research Center, IBM Corp.Obsoletes:1268                                                 P. GrossCategory: Standards Track                                            MCI                                                                 Editors                                                               July 1994Application of the Border Gateway Protocol in the InternetStatus of this Memo   This document specifies an Internet standards track protocol for the   Internet community, and requests discussion and suggestions for   improvements.  Please refer to the current edition of the "Internet   Official Protocol Standards" (STD 1) for the standardization state   and status of this protocol.  Distribution of this memo is unlimited.Abstract   This document, together with its companion document, "A Border   Gateway Protocol 4 (BGP-4)", define an inter-autonomous system   routing protocol for the Internet.  "A Border Gateway Protocol 4   (BGP-4)" defines the BGP protocol specification, and this document   describes the usage of the BGP in the Internet.   Information about the progress of BGP can be monitored and/or   reported on the BGP mailing list (bgp@ans.net).Acknowledgements   This document was originally published asRFC 1164 in June 1990,   jointly authored by Jeffrey C. Honig (Cornell University), Dave Katz   (MERIT), Matt Mathis (PSC), Yakov Rekhter (IBM), and Jessica Yu   (MERIT).   The following also made key contributions toRFC 1164 -- Guy Almes   (ANS, then at Rice University), Kirk Lougheed (cisco Systems), Hans-   Werner Braun (SDSC, then at MERIT), and Sue Hares (MERIT).   We like to explicitly thank Bob Braden (ISI) for the review of the   previous version of this document.   This updated version of the document is the product of the IETF BGP   Working Group with Phill Gross (MCI) and Yakov Rekhter (IBM) as   editors.Rekhter & Gross                                                 [Page 1]

RFC 1655                   BGP-4 Application                   July 1994   John Moy (Proteon) contributedSection 7 "Required set of supported   routing policies".   Scott Brim (Cornell University) contributed the basis forSection 8   "Interaction with other exterior routing protocols".   Most of the text inSection 9 was contributed by Gerry Meyer   (Spider).   Parts of the Introduction were taken almost verbatim from [3].   We would like to acknowledge Dan Long (NEARNET) and Tony Li (cisco   Systems) for their review and comments on the current version of the   document.1. Introduction   This memo describes the use of the Border Gateway Protocol (BGP) [1]   in the Internet environment. BGP is an inter-Autonomous System   routing protocol. The network reachability information exchanged via   BGP provides sufficient information to detect routing loops and   enforce routing decisions based on performance preference and policy   constraints as outlined inRFC 1104 [2]. In particular, BGP exchanges   routing information containing full AS paths and enforces routing   policies based on configuration information.   As the Internet has evolved and grown over in recent years, it has   become painfully evident that it is soon to face several serious   scaling problems. These include:       - Exhaustion of the class-B network address space. One         fundamental cause of this problem is the lack of a network         class of a size which is appropriate for mid-sized         organization; class-C, with a maximum of 254 host addresses, is         too small while class-B, which allows up to 65534 addresses, is         too large to be densely populated.       - Growth of routing tables in Internet routers are beyond the         ability of current software (and people) to effectively manage.       - Eventual exhaustion of the 32-bit IP address space.   It has become clear that the first two of these problems are likely   to become critical within the next one to three years.  Classless   inter-domain routing (CIDR) attempts to deal with these problems by   proposing a mechanism to slow the growth of the routing table and the   need for allocating new IP network numbers. It does not attempt to   solve the third problem, which is of a more long-term nature, butRekhter & Gross                                                 [Page 2]

RFC 1655                   BGP-4 Application                   July 1994   instead endeavors to ease enough of the short to mid-term   difficulties to allow the Internet to continue to function   efficiently while progress is made on a longer- term solution.   BGP-4 is an extension of BGP-3 that provides support for routing   information aggregation and reduction based on the Classless inter-   domain routing architecture (CIDR) [3].  This memo describes the   usage of BGP-4 in the Internet.   All of the discussions in this paper are based on the assumption that   the Internet is a collection of arbitrarily connected Autonomous   Systems. That is, the Internet will be modeled as a general graph   whose nodes are AS's and whose edges are connections between pairs of   AS's.   The classic definition of an Autonomous System is a set of routers   under a single technical administration, using an interior gateway   protocol and common metrics to route packets within the AS and using   an exterior gateway protocol to route packets to other AS's. Since   this classic definition was developed, it has become common for a   single AS to use several interior gateway protocols and sometimes   several sets of metrics within an AS. The use of the term Autonomous   System here stresses the fact that, even when multiple IGPs and   metrics are used, the administration of an AS appears to other AS's   to have a single coherent interior routing plan and presents a   consistent picture of which networks are reachable through it.   AS's are assumed to be administered by a single administrative   entity, at least for the purposes of representation of routing   information to systems outside of the AS.2. BGP Topological Model   When we say that a connection exists between two AS's, we mean two   things:      Physical connection:  There is a shared network between the two      AS's, and on this shared network each AS has at least one border      gateway belonging to that AS. Thus the border gateway of each AS      can forward packets to the border gateway of the other AS without      resorting to Inter-AS or Intra-AS routing.      BGP connection:  There is a BGP session between BGP speakers in      each of the AS's, and this session communicates those routes that      can be used for specific networks via the advertising AS.      Throughout this document we place an additional restriction on the      BGP speakers that form the BGP connection: they must themselves      share the same network that their border gateways share. Thus, aRekhter & Gross                                                 [Page 3]

RFC 1655                   BGP-4 Application                   July 1994      BGP session between adjacent AS's requires no support from either      Inter-AS or Intra-AS routing. Cases that do not conform to this      restriction fall outside the scope of this document.   Thus, at each connection, each AS has one or more BGP speakers and   one or more border gateways, and these BGP speakers and border   gateways are all located on a shared network. Note that BGP speakers   do not need to be a border gateway, and vice versa. Paths announced   by a BGP speaker of one AS on a given connection are taken to be   feasible for each of the border gateways of the other AS on the same   shared network, i.e. indirect neighbors are allowed.   Much of the traffic carried within an AS either originates or   terminates at that AS (i.e., either the source IP address or the   destination IP address of the IP packet identifies a host on a   network internal to that AS).  Traffic that fits this description is   called "local traffic". Traffic that does not fit this description is   called "transit traffic". A major goal of BGP usage is to control the   flow of transit traffic.   Based on how a particular AS deals with transit traffic, the AS may   now be placed into one of the following categories:      stub AS: an AS that has only a single connection to one other AS.      Naturally, a stub AS only carries local traffic.      multihomed AS: an AS that has connections to more than one other      AS, but refuses to carry transit traffic.      transit AS: an AS that has connections to more than one other AS,      and is designed (under certain policy restrictions) to carry both      transit and local traffic.   Since a full AS path provides an efficient and straightforward way of   suppressing routing loops and eliminates the "count-to-infinity"   problem associated with some distance vector algorithms, BGP imposes   no topological restrictions on the interconnection of AS's.3. BGP in the Internet3.1 Topology Considerations   The overall Internet topology may be viewed as an arbitrary   interconnection of transit, multihomed, and stub AS's.  In order to   minimize the impact on the current Internet infrastructure, stub and   multihomed AS's need not use BGP.  These AS's may run other protocols   (e.g., EGP) to exchange reachability information with transit AS's.   Transit AS's using BGP will tag this information as having beenRekhter & Gross                                                 [Page 4]

RFC 1655                   BGP-4 Application                   July 1994   learned by some method other than BGP. The fact that BGP need not run   on stub or multihomed AS's has no negative impact on the overall   quality of inter-AS routing for traffic that either destined to or   originated from the stub or multihomed AS's in question.   However, it is recommended that BGP be used for stub and multihomed   AS's as well. In these situations, BGP will provide an advantage in   bandwidth and performance over some of the currently used protocols   (such as EGP).  In addition, this would reduce the need for the use   of default routes and in better choices of Inter-AS routes for   multihomed AS's.3.2 Global Nature of BGP   At a global level, BGP is used to distribute routing information   among multiple Autonomous Systems. The information flows can be   represented as follows:                    +-------+         +-------+              BGP   |  BGP  |   BGP   |  BGP  |   BGP           ---------+       +---------+       +---------                    |  IGP  |         |  IGP  |                    +-------+         +-------+                    <-AS A-->         <--AS B->   This diagram points out that, while BGP alone carries information   between AS's, both BGP and an IGP may carry information across an AS.   Ensuring consistency of routing information between BGP and an IGP   within an AS is a significant issue and is discussed at length later   inAppendix A.3.3 BGP Neighbor Relationships   The Internet is viewed as a set of arbitrarily connected AS's. BGP   speakers in each AS communicate with each other to exchange network   reachability information based on a set of policies established   within each AS. Routers that communicate directly with each other via   BGP are known as BGP neighbors. BGP neighbors can be located within   the same AS or in different AS's. For the sake of discussion, BGP   communications with neighbors in different AS's will be referred to   as External BGP, and with neighbors in the same AS as Internal BGP.   There can be as many BGP speakers as deemed necessary within an AS.   Usually, if an AS has multiple connections to other AS's, multiple   BGP speakers are needed. All BGP speakers representing the same AS   must give a consistent image of the AS to the outside. This requiresRekhter & Gross                                                 [Page 5]

RFC 1655                   BGP-4 Application                   July 1994   that the BGP speakers have consistent routing information among them.   These gateways can communicate with each other via BGP or by other   means. The policy constraints applied to all BGP speakers within an   AS must be consistent. Techniques such as using a tagged IGP (see   A.2.2) may be employed to detect possible inconsistencies.   In the case of External BGP, the BGP neighbors must belong to   different AS's, but share a common network. This common network   should be used to carry the BGP messages between them. The use of BGP   across an intervening AS invalidates the AS path information. An   Autonomous System number must be used with BGP to specify which   Autonomous System the BGP speaker belongs to.4. Requirements for Route Aggregation   A conformant BGP-4 implementation is required to have the ability to   specify when an aggregated route may be generated out of partial   routing information. For example, a BGP speaker at the border of an   autonomous system (or group of autonomous systems) must be able to   generate an aggregated route for a whole set of destination IP   addresses (in BGP-4 terminology such a set is called the Network   Layer Reachability Information or NLRI) over which it has   administrative control (including those addresses it has delegated),   even when not all of them are reachable at the same time.   A conformant implementation may provide the capability to specify   when an aggregated NLRI may be generated.   A conformant implementation is required to have the ability to   specify how NLRI may be de-aggregated.   A conformant implementation is required to support the following   options when dealing with overlapping routes:       - Install both the less and the more specific routes       - Install the more specific route only       - Install the less specific route only       - Install neither route   By default a BGP speaker should aggregate NLRI representing subnets   to the corresponding network.   Injecting NLRI representing arbitrary subnets into BGP without   aggregation to the corresponding network shall be controlled via   configuration parameters.Rekhter & Gross                                                 [Page 6]

RFC 1655                   BGP-4 Application                   July 1994   Certain routing policies may depend on the NLRI (e.g., "research"   versus "commercial"). Therefore, a BGP speaker that performs route   aggregation should be cognizant, if possible, of potential   implications on routing policies when aggregating NLRI.5. Policy Making with BGP   BGP provides the capability for enforcing policies based on various   routing preferences and constraints. Policies are not directly   encoded in the protocol. Rather, policies are provided to BGP in the   form of configuration information.   BGP enforces policies by affecting the selection of paths from   multiple alternatives and by controlling the redistribution of   routing information.  Policies are determined by the AS   administration.   Routing policies are related to political, security, or economic   considerations. For example, if an AS is unwilling to carry traffic   to another AS, it can enforce a policy prohibiting this. The   following are examples of routing policies that can be enforced with   the use of BGP:     1.  A multihomed AS can refuse to act as a transit AS for other         AS's.  (It does so by only advertising routes to networks         internal to the AS.)     2.  A multihomed AS can become a transit AS for a restricted set of         adjacent AS's, i.e., some, but not all, AS's can use the         multihomed AS as a transit AS. (It does so by advertising its         routing information to this set of AS's.)     3.  An AS can favor or disfavor the use of certain AS's for         carrying transit traffic from itself.   A number of performance-related criteria can be controlled with the   use of BGP:     1.  An AS can minimize the number of transit AS's. (Shorter AS         paths can be preferred over longer ones.)     2.  The quality of transit AS's. If an AS determines that two or         more AS paths can be used to reach a given destination, that AS         can use a variety of means to decide which of the candidate AS         paths it will use. The quality of an AS can be measured by such         things as diameter, link speed, capacity, tendency to become         congested, and quality of operation. Information about these         qualities might be determined by means other than BGP.Rekhter & Gross                                                 [Page 7]

RFC 1655                   BGP-4 Application                   July 1994     3.  Preference of internal routes over external routes.   For consistency within an AS, equal cost paths, resulting from   combinations of policies and/or normal route selection procedures,   must be resolved in a consistent fashion.   Fundamental to BGP is the rule that an AS advertises to its   neighboring AS's only those routes that it uses. This rule reflects   the "hop-by-hop" routing paradigm generally used by the current   Internet.6. Path Selection with BGP   One of the major tasks of a BGP speaker is to evaluate different   paths to a destination network from its border gateways at that   network, select the best one, apply appropriate policy constraints,   and then advertise it to all of its BGP neighbors. The key issue is   how different paths are evaluated and compared.  In traditional   distance vector protocols (e.g., RIP) there is only one metric (e.g.,   hop count) associated with a path. As such, comparison of different   paths is reduced to simply comparing two numbers. A complication in   Inter-AS routing arises from the lack of a universally agreed-upon   metric among AS's that can be used to evaluate external paths.   Rather, each AS may have its own set of criteria for path evaluation.   A BGP speaker builds a routing database consisting of the set of all   feasible paths and the list of networks reachable through each path.   For purposes of precise discussion, it's useful to consider the set   of feasible paths for a given destination network. In most cases, we   would expect to find only one feasible path. However, when this is   not the case, all feasible paths should be maintained, and their   maintenance speeds adaptation to the loss of the primary path. Only   the primary path at any given time will ever be advertised.   The path selection process can be formalized by defining a complete   order over the set of all feasible paths to a given destination   network. One way to define this complete order is to define a   function that maps each full AS path to a non-negative integer that   denotes the path's degree of preference. Path selection is then   reduced to applying this function to all feasible paths and choosing   the one with the highest degree of preference.   In actual BGP implementations, the criteria for assigning degree of   preferences to a path are specified as configuration information.   The process of assigning a degree of preference to a path can be   based on several sources of information:Rekhter & Gross                                                 [Page 8]

RFC 1655                   BGP-4 Application                   July 1994     1.  Information explicitly present in the full AS path.     2.  A combination of information that can be derived from the full         AS path and information outside the scope of BGP (e.g., policy         routing constraints provided as configuration information).   Possible criteria for assigning a degree of preference to a path are:       - AS count. Paths with a smaller AS count are generally better.       - Policy considerations. BGP supports policy-based routing based         on the controlled distribution of routing information.  A BGP         speaker may be aware of some policy constraints (both within         and outside of its own AS) and do appropriate path selection.         Paths that do not comply with policy requirements are not         considered further.       - Presence or absence of a certain AS or AS's in the path. By         means of information outside the scope of BGP, an AS may know         some performance characteristics (e.g., bandwidth, MTU, intra-         AS diameter) of certain AS's and may try to avoid or prefer         them.       - Path origin. A path learned entirely from BGP (i.e., whose         endpoint is internal to the last AS on the path) is generally         better than one for which part of the path was learned via EGP         or some other means.       - AS path subsets. An AS path that is a subset of a longer AS         path to the same destination should be preferred over the         longer path.  Any problem in the shorter path (such as an         outage) will also be a problem in the longer path.       - Link dynamics. Stable paths should be preferred over unstable         ones. Note that this criterion must be used in a very careful         way to avoid causing unnecessary route fluctuation. Generally,         any criteria that depend on dynamic information might cause         routing instability and should be treated very carefully.7. Required set of supported routing policies   Policies are provided to BGP in the form of configuration   information.  This information is not directly encoded in the   protocol. Therefore, BGP can provide support for very complex routing   policies. However, it is not required that all BGP implementations   support such policies.Rekhter & Gross                                                 [Page 9]

RFC 1655                   BGP-4 Application                   July 1994   We are not attempting to standardize the routing policies that must   be supported in every BGP implementation; we strongly encourage all   implementors to support the following set of routing policies:     1.  BGP implementations should allow an AS to control announcements         of BGP-learned routes to adjacent AS's.  Implementations should         also support such control with at least the granularity of a         single network.  Implementations should also support such         control with the granularity of an autonomous system, where the         autonomous system may be either the autonomous system that         originated the route, or the autonomous system that advertised         the route to the local system (adjacent autonomous system).         Care must be taken when a BGP speaker selects a new route that         can't be announced to a particular external peer, while the         previously selected route was announced to that peer.         Specifically, the local system must explicitly indicate to the         peer that the previous route is now infeasible.     2.  BGP implementations should allow an AS to prefer a particular         path to a destination (when more than one path is available).         At the minimum an implementation shall support this         functionality by allowing to administratively assign a degree         of preference to a route based solely on the IP address of the         neighbor the route is received from. The allowed range of the         assigned degree of preference shall be between 0 and 2^(31) -         1.     3.  BGP implementations should allow an AS to ignore routes with         certain AS's in the AS_PATH path attribute.  Such function can         be implemented by using the technique outlined in [2], and by         assigning "infinity" as "weights" for such AS's. The route         selection process must ignore routes that have "weight" equal         to "infinity".8. Interaction with other exterior routing protocols   The guidelines suggested in this section are consistent with the   guidelines presented in [3].   An AS should advertise a minimal aggregate for its internal networks   with respect to the amount of address space that it is actually   using.  This can be used by administrators of non-BGP 4 AS's to   determine how many routes to explode from a single aggregate.   A route that carries the ATOMIC_AGGREGATE path attribute shall not be   exported into either BGP-3 or EGP2, unless such an exportation can be   accomplished without exploding the NLRI of the route.Rekhter & Gross                                                [Page 10]

RFC 1655                   BGP-4 Application                   July 19948.1 Exchanging information with EGP2   This document suggests the following guidelines for exchanging   routing information between BGP-4 and EGP2.   To provide for graceful migration, a BGP speaker may participate in   EGP2, as well as in BGP-4. Thus, a BGP speaker may receive IP   reachability information by means of EGP2 as well as by means of   BGP-4.  The information received by EGP2 can be injected into BGP-4   with the ORIGIN path attribute set to 1.  Likewise,  the information   received via BGP-4 can be injected into EGP2 as well. In the latter   case, however, one needs to be aware of the potential information   explosion when a given IP prefix received from BGP-4 denotes a set of   consecutive A/B/C class networks.  Injection of BGP-4 received NLRI   that denotes IP subnets requires the BGP speaker to inject the   corresponding network into EGP2.  The local system shall provide   mechanisms to control the exchange of reachability information   between EGP2 and BGP-4.  Specifically, a conformant implementation is   required to support all of the following options when injecting BGP-4   received reachability information into EGP2:       - inject default only (0.0.0.0); no export of any other NLRI       - allow controlled deaggregation, but only of specific routes;         allow export of non-aggregated NLRI       - allow export of only non-aggregated NLRI   The exchange of routing information via EGP2 between a BGP speaker   participating in BGP-4 and a pure EGP2 speaker may occur  only at the   domain (autonomous system) boundaries.8.2 Exchanging information with BGP-3   This document suggests the following guidelines for exchanging   routing information between BGP-4 and BGP-3.   To provide for graceful migration, a BGP speaker may participate in   BGP-3, as well as in BGP-4. Thus, a BGP speaker may receive IP   reachability information by means of BGP-3, as well as by means of   BGP-4.   A BGP speaker may inject the information received by BGP-4 into BGP-3   as follows.   If an AS_PATH attribute of a BGP-4 route carries AS_SET path   segments, then the AS_PATH attribute of the BGP-3 route shall be   constructed by treating the AS_SET segments as AS_SEQUENCE segments,Rekhter & Gross                                                [Page 11]

RFC 1655                   BGP-4 Application                   July 1994   with the resulting AS_PATH being a single AS_SEQUENCE. While this   procedure loses set/sequence information, it doesn't affect   protection for routing loops suppression, but may affect policies, if   the policies are based on the content or ordering of the AS_PATH   attribute.   While injecting BGP-4 derived NLRI into BGP-3, one needs to be aware   of the potential information explosion when a given IP prefix denotes   a set of consecutive A/B/C class networks. Injection of BGP-4 derived   NLRI that denotes IP subnets requires the BGP speaker to inject the   corresponding network into BGP-3. The local system shall provide   mechanisms to control the exchange of routing information between   BGP-3 and BGP-4.  Specifically, a conformant implementation is   required to support all of the following options when injecting BGP-4   received routing information into BGP-3:       - inject default only (0.0.0.0), no export of any other NLRI       - allow controlled deaggregation, but only of specific routes;         allow export of non-aggregated NLRI       - allow export of only non-aggregated NLRI   The exchange of routing information via BGP-3 between a BGP speaker   participating in BGP-4 and a pure BGP-3 speaker may occur  only at   the autonomous system boundaries. Within a single autonomous system   BGP conversations between all the BGP speakers of that autonomous   system have to be either BGP-3 or BGP-4, but not a mixture.9. Operations over Switched Virtual Circuits   When using BGP over Switched Virtual Circuit (SVC) subnetworks it may   be desirable to minimize traffic generated by BGP. Specifically, it   may be desirable to eliminate traffic associated with periodic   KEEPALIVE messages.  BGP includes a mechanism for operation over   switched virtual circuit (SVC) services which avoids keeping SVCs   permanently open and allows it to eliminates periodic sending of   KEEPALIVE messages.   This section describes how to operate without periodic KEEPALIVE   messages to minimise SVC usage when using an intelligent SVC circuit   manager.  The proposed scheme may also be used on "permanent"   circuits, which support a feature like link quality monitoring or   echo request to determine the status of link connectivity.   The mechanism described in this section is suitable only between the   BGP speakers that are directly connected over a common virtual   circuit.Rekhter & Gross                                                [Page 12]

RFC 1655                   BGP-4 Application                   July 19949.1 Establishing a BGP Connection   The feature is selected by specifying zero Hold Time in the OPEN   message.9.2 Circuit Manager Properties   The circuit manager must have sufficient functionality to be able to   compensate for the lack of periodic KEEPALIVE messages:       - It must be able to determine link layer unreachability in a         predictable finite period of a failure occurring.       - On determining unreachability it should:                - start a configurable dead timer (comparable to a                  typical Hold timer value).                - attempt to re-establish the Link Layer connection.       - If the dead timer expires it should:                - send an internal circuit DEAD indication to TCP.       - If the connection is re-established it should:                - cancel the dead timer.                - send an internal circuit UP indication to TCP.9.3 TCP Properties   A small modification must be made to TCP to process internal   notifications from the circuit manager:       - DEAD: Flush transmit queue and abort TCP connection.       - UP: Transmit any queued data or allow an outgoing TCP call to         proceed.9.4 Combined Properties   Some implementations may not be able to guarantee that the BGP   process and the circuit manager will operate as a single entity; i.e.   they can have a separate existence when the other has been stopped or   has crashed.Rekhter & Gross                                                [Page 13]

RFC 1655                   BGP-4 Application                   July 1994   If this is the case, a periodic two-way poll between the BGP process   and the circuit manager should be implemented.  If the BGP process   discovers the circuit manager has gone away it should close all   relevant TCP connections.  If the circuit manager discovers the BGP   process has gone away it should close all its connections associated   with the BGP process and reject any further incoming connections.10. Conclusion   The BGP protocol provides a high degree of control and flexibility   for doing interdomain routing while enforcing policy and performance   constraints and avoiding routing loops. The guidelines presented here   will provide a starting point for using BGP to provide more   sophisticated and manageable routing in the Internet as it grows.Appendix A. The Interaction of BGP and an IGP   This section outlines methods by which BGP can exchange routing   information with an IGP. The methods outlined here are not proposed   as part of the standard BGP usage at this time.  These methods are   outlined for information purposes only.  Implementors may want to   consider these methods when importing IGP information.   This is general information that applies to any generic IGP.   Interaction between BGP and any specific IGP is outside the scope of   this section.  Methods for specific IGP's should be proposed in   separate documents.  Methods for specific IGP's could be proposed for   standard usage in the future.Overview   By definition, all transit AS's must be able to carry traffic which   originates from and/or is destined to locations outside of that AS.   This requires a certain degree of interaction and coordination   between BGP and the Interior Gateway Protocol (IGP) used by that   particular AS. In general, traffic originating outside of a given AS   is going to pass through both interior gateways (gateways that   support the IGP only) and border gateways (gateways that support both   the IGP and BGP). All interior gateways receive information about   external routes from one or more of the border gateways of the AS via   the IGP.   Depending on the mechanism used to propagate BGP information within a   given AS, special care must be taken to ensure consistency between   BGP and the IGP, since changes in state are likely to propagate at   different rates across the AS. There may be a time window between the   moment when some border gateway (A) receives new BGP routingRekhter & Gross                                                [Page 14]

RFC 1655                   BGP-4 Application                   July 1994   information which was originated from another border gateway (B)   within the same AS, and the moment the IGP within this AS is capable   of routing transit traffic to that border gateway (B). During that   time window, either incorrect routing or "black holes" can occur.   In order to minimize such routing problems, border gateway (A) should   not advertise a route to some exterior network X via border gateway   (B) to all of its BGP neighbors in other AS's until all the interior   gateways within the AS are ready to route traffic destined to X via   the correct exit border gateway (B). In other words, interior routing   should converge on the proper exit gateway before/advertising routes   via that exit gateway to other AS's.A.2 Methods for Achieving Stable Interactions   The following discussion outlines several techniques capable of   achieving stable interactions between BGP and the IGP within an   Autonomous System.A.2.1 Propagation of BGP Information via the IGP   While BGP can provide its own mechanism for carrying BGP information   within an AS, one can also use an IGP to transport this information,   as long as the IGP supports complete flooding of routing information   (providing the mechanism to distribute the BGP information) and one   pass convergence (making the mechanism effectively atomic). If an IGP   is used to carry BGP information, then the period of   desynchronization described earlier does not occur at all, since BGP   information propagates within the AS synchronously with the IGP, and   the IGP converges more or less simultaneously with the arrival of the   new routing information. Note that the IGP only carries BGP   information and should not interpret or process this information.A.2.2  Tagged Interior Gateway Protocol   Certain IGPs can tag routes exterior to an AS with the identity of   their exit points while propagating them within the AS. Each border   gateway should use identical tags for announcing exterior routing   information (received via BGP) both into the IGP and into Internal   BGP when propagating this information to other border gateways within   the same AS. Tags generated by a border gateway must uniquely   identify that particular border gateway--different border gateways   must use different tags.   All Border Gateways within a single AS must observe the following two   rules:Rekhter & Gross                                                [Page 15]

RFC 1655                   BGP-4 Application                   July 1994     1.  Information received via Internal BGP by a border gateway A         declaring a network to be unreachable must immediately be         propagated to all of the External BGP neighbors of A.     2.  Information received via Internal BGP by a border gateway A         about a reachable network X cannot be propagated to any of the         External BGP neighbors of A unless/until A has an IGP route to         X and both the IGP and the BGP routing information have         identical tags.   These rules guarantee that no routing information is announced   externally unless the IGP is capable of correctly supporting it. It   also avoids some causes of "black holes".   One possible method for tagging BGP and IGP routes within an AS is to   use the IP address of the exit border gateway announcing the exterior   route into the AS. In this case the "gateway" field in the BGP UPDATE   message is used as the tag.   An alternate method for tagging BGP and IGP routes is to have BGP and   the IGP agree on a router ID.  In this case, the router ID is   available to all BGP (version 3 or higher) speakers.  Since this ID   is already unique it can be used directly as the tag in the IGP.A.2.3 Encapsulation   Encapsulation provides the simplest (in terms of the interaction   between the IGP and BGP) mechanism for carrying transit traffic   across the AS. In this approach, transit traffic is encapsulated   within an IP datagram addressed to the exit gateway. The only   requirement imposed on the IGP by this approach is that it should be   capable of supporting routing between border gateways within the same   AS.   The address of the exit gateway A for some exterior network X is   specified in the BGP identifier field of the BGP OPEN message   received from gateway A via Internal BGP by all other border gateways   within the same AS. In order to route traffic to network X, each   border gateway within the AS encapsulates it in datagrams addressed   to gateway A. Gateway A then performs decapsulation and forwards the   original packet to the proper gateway in another AS.   Since encapsulation does not rely on the IGP to carry exterior   routing information, no synchronization between BGP and the IGP is   required.Rekhter & Gross                                                [Page 16]

RFC 1655                   BGP-4 Application                   July 1994   Some means of identifying datagrams containing encapsulated IP, such   as an IP protocol type code, must be defined if this method is to be   used.   Note that, if a packet to be encapsulated has length that is very   close to the MTU, that packet would be fragmented at the gateway that   performs encapsulation.A.2.4  Pervasive BGP   If all routers in an AS are BGP speakers, then there is no need to   have any interaction between BGP and an IGP.  In such cases, all   routers in the AS already have full information of all BGP routes.   The IGP is then only used for routing within the AS, and no BGP   routes are imported into the IGP.   For routers to operate in this fashion, they must be able to perform   a recursive lookup in their routing table.  The first lookup will use   a BGP route to establish the exit router, while the second lookup   will determine the IGP path to the exit router.   Since the IGP carries no external information in this scenario, all   routers in the AS will have converged as soon as all BGP speakers   have new information about this route.  Since there is no need to   delay for the IGP to converge, an implementation may advertise these   routes without further delay due to the IGP.A.2.5  Other Cases   There may be AS's with IGPs which can neither carry BGP information   nor tag exterior routes (e.g., RIP). In addition, encapsulation may   be either infeasible or undesirable. In such situations, the   following two rules must be observed:     1.  Information received via Internal BGP by a border gateway A         declaring a network to be unreachable must immediately be         propagated to all of the External BGP neighbors of A.     2.  Information received via Internal BGP by a border gateway A         about a reachable network X cannot be propagated to any of the         External BGP neighbors of A unless A has an IGP route to X and         sufficient time has passed for the IGP routes to have         converged.   The above rules present necessary (but not sufficient) conditions for   propagating BGP routing information to other AS's. In contrast to   tagged IGPs, these rules cannot ensure that interior routes to the   proper exit gateways are in place before propagating the routes toRekhter & Gross                                                [Page 17]

RFC 1655                   BGP-4 Application                   July 1994   other AS's.   If the convergence time of an IGP is less than some small value X,   then the time window during which the IGP and BGP are unsynchronized   is less than X as well, and the whole issue can be ignored at the   cost of transient periods (of less than length X) of routing   instability. A reasonable value for X is a matter for further study,   but X should probably be less than one second.   If the convergence time of an IGP cannot be ignored, a different   approach is needed. Mechanisms and techniques which might be   appropriate in this situation are subjects for further study.References   [1] Rekhter, Y., and T. Li, "A Border Gateway Protocol 4 (BGP-4),RFC1654, cisco Systems, T.J. Watson Research Center, IBM Corp., July       1994.   [2] Braun, H-W., "Models of Policy Based Routing",RFC 1104,       Merit/NSFNET, July 1989.   [3] Fuller, V., Li, T., Yu, J., and K. Varadhan, "Supernetting:  an       Address Assignment and Aggregation Strategy",RFC 1519, BARRNet,       cisco, MERIT, OARnet, September 1993.Rekhter & Gross                                                [Page 18]

RFC 1655                   BGP-4 Application                   July 1994Security Considerations   Security issues are not discussed in this memo.Authors' Addresses   Yakov Rekhter   T.J. Watson Research Center IBM Corporation   P.O. Box 218   Yorktown Heights, NY 10598   Phone:  (914) 945-3896   EMail: yakov@watson.ibm.com   Phill Gross   Director of Broadband Engineering   MCI Data Services Division   2100 Reston Parkway, Room 6001   Reston, VA 22091   Phone: +1 703 715 7432   Fax: +1 703 715 7436   EMail: 0006423401@mcimail.com   IETF BGP WG mailing list: bgp@ans.net   To be added: bgp-request@ans.netRekhter & Gross                                                [Page 19]