--- 1/draft-ietf-idr-route-reflect-00.txt 2006-02-04 23:31:49.000000000 +0100 +++ 2/draft-ietf-idr-route-reflect-01.txt 2006-02-04 23:31:49.000000000 +0100 @@ -1,20 +1,20 @@ INTERNET-DRAFT Tony Bates - MCI + MCI Ravi Chandra cisco Systems March 1996 BGP Route Reflection An alternative to full mesh IBGP - + Status of this Memo This document is an Internet Draft. Internet Drafts are working documents of the Internet Engineering Task Force (IETF), its Areas, and its Working Groups. Note that other groups may also distribute working documents as Internet Drafts. Internet Drafts are draft documents valid for a maximum of six months. Internet Drafts may be updated, replaced, or obsoleted by @@ -41,23 +41,23 @@ 1. Introduction Currently in the Internet today, BGP deployments are configured such that that all BGP speakers within a single AS must be fully meshed and any external routing information must be re-distributed to all other routers within that AS. This "full mesh" requirement clearly does not scale when there are a large number of IBGP speakers as is common in many of todays internet networks. - For n BGP speakers within an AS you must maintain n*n-1/2 unique IBGP - sessions. With finite resources in both bandwidth and router CPU this - clearly does not scale. + For n BGP speakers within an AS you must maintain n*(n-1)/2 unique + IBGP sessions. With finite resources in both bandwidth and router CPU + this clearly does not scale. This scaling problem has been well documented and a number of proposals have been made to alleviate this [2,3]. This document represents another alternative in alleviating the need for a "full mesh" and is known as "Route Reflection". It represents a change in the commonly understood concept of IBGP and the addition of two new optional transitive BGP attributes. 2. Design Criteria @@ -70,21 +70,21 @@ o Easy Migration It must be possible to migrate from a full mesh configuration without the need to change either topology or AS. This is an unfortunate management overhead of the technique proposed in [3]. o Compatibility - It must be possible for non compliment IBGP peers + It must be possible for non compliant IBGP peers to continue be part of the original AS or domain without any loss of BGP routing information. These criteria were motivated by operational experiences of a very large and topology rich network with many external connections. 3. Route Reflection The basic idea of Route Reflection is very simple. Let us consider the simple example depicted in Figure 1 below. @@ -98,28 +98,29 @@ IBGP \ ASX / IBGP \ / +-------+ | | | RTR-C | | | +-------+ Figure 1: Full Mesh IBGP - In ASX there are three IBGP speakers (routers RTR-A, RTR-B and RTR-C) - and each B, C). With the existing BGP model, if RTR-A receives an - external route, it must advertise it to both RTR-B and RTR-C. RTR-B - and RTR-C (as IBGP speakers) will not re-advertise these IBGP learned - routes to other IBGP speakers. + In ASX there are three IBGP speakers (routers RTR-A, RTR-B and RTR- + C). With the existing BGP model, if RTR-A receives an external route + and it is selected as the best path it must advertise the external + route to both RTR-B and RTR-C. RTR-B and RTR-C (as IBGP speakers) + will not re-advertise these IBGP learned routes to other IBGP + speakers. - If this rule is broken and RTR-C is allowed to reflect IBGP learned - routes, then it could re-distribute (or reflect) the IBGP routes + If this rule is relaxed and RTR-C is allowed to reflect IBGP learned + routes, then it could re-advertise (or reflect) the IBGP routes learned from RTR-A to RTR-B and vice versa. This would eliminate the need for the IBGP session between RTR-A and RTR-C as shown in Figure 2 below. +------ + +-------+ | | | | | RTR-A | | RTR-B | | | | | +-------+ +-------+ \ / @@ -123,29 +124,28 @@ | | | | +-------+ +-------+ \ / IBGP \ ASX / IBGP \ / +-------+ | | | RTR-C | | | +-------+ - Figure 2: Route Reflection IBGP - The Route Reflection scheme is based upon this principle. + + The Route Reflection scheme is based upon this basic principle. 4. Terminology and Concepts We use the term "Route Reflector" (RR) to represent an IBGP speaker - that - participates in the reflection. The internal peers of a RR are + that participates in the reflection. The internal peers of a RR are divided into two groups: 1) Client Peers 2) Non-Client Peers A RR reflects routes between these groups. A RR along with its client peers form a Cluster. The Non-Client peer must be fully meshed but the Client peers need not be fully meshed. The Client peers should not peer with internal speakers outside of their cluster. @@ -173,22 +173,24 @@ +-------+ +-------+ | RTR-D | IBGP | RTR-E | | Non- |---------| Non- | |Client | |Client | +-------+ +-------+ Figure 3: RR Components 5. Operation - When a route is received by a RR, it must do the following depending - on the type of the peer it is receiving a route from: + When a route is received by a RR, it selects the best path based on + its path selection rule. After the best path is selected, it must do + the following depending on the type of the peer it is receiving the + best path from: 1) A Route from a Non-Client peer Reflect to all other Clients. 2) A Route from a Client peer Reflect to all the Non-Client peers and also to the Client peers (Hence the Client peers are not required to be fully meshed). @@ -224,38 +226,45 @@ Usually a cluster of clients will have a single RR. In that case, the cluster will be identified by the ROUTER_ID of the RR. However, this represents a single point of failure so to make it possible to have multiple RRs in the same cluster, all RRs in the same cluster must be configured with a 4-byte CLUSTER_ID so that an RR can discern routes from other RRs in the same cluster. 7. Avoiding Routing Information Loops As IBGP learned routes are reflected, it is possible through mis- - configuration to form route redistribution loops. The Route + configuration to form route re-distribution loops. The Route Reflection method defines the following attributes to detect and avoid routing information loops. ORIGINATOR_ID ORIGINATOR_ID is a new optional, non-transitive BGP attribute of Type code 9. This attribute is 4 bytes long and it will be created by a RR. This attribute will carry the ROUTER_ID of the originator of the route in the local AS. A BGP speaker should not create an ORIGINATOR_ID attribute if one already exists If routing information comes back to the originator, it must be ignored. CLUSTER_LIST Cluster-list is a new optional, non-transitive BGP attribute of Type code 10. It is a sequence of CLUSTER_ID values representing the - reflection path that the route has passed. + reflection path that the route has passed. It is encoded as follows: + + 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | Attr. Flags |Attr. Type Code| Length | value ... + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + + Where Length is the number of octets. When a RR reflects a route from its Clients to a Non-Client peer, it must append the local CLUSTER_ID to the CLUSTER_LIST. If the CLUSTER_LIST is empty, it must create a new one. Using this attribute an RR can identify if the routing information is looped back to the same cluster due to mis-configuration. If the local CLUSTER_ID is found in the cluster-list, the advertisement will be ignored. 8. Implementation and Configuration Considerations @@ -265,37 +274,37 @@ Non-Clients. This could result is looping of routes. In some implementations, modification of the BGP path attribute, NEXT_HOP is possible. For example, there could be a need for a RR to modify NEXT_HOP for EBGP learned routes sent to its internal peers. However, this must not be possible for an RR to set on reflected IBGP routes as this breaks the basic principle of Route Reflection and will result in potential black holes. An RR should not modify any AS-PATH attributes (i.e. LOCAL_PREF, MED, - DPA)that could change consistent route selection. THis could + DPA)that could change consistent route selection. This could resulting in potential loops. The BGP protocol provides no way for a Client to identify itself dynamically as a Client to an RR configured BGP speaker and the simplest way to achieve this is by manual configuration. 9. Security Security considerations are not discussed in this memo. 10. Acknowledgments - The authors would like to thank Dennis Ferguson, Enke Chen, Paul - Traina and Tony Li for the many discussions resulting in this work. - This idea was developed from an earlier discussion between Tony Li - and Dimitri Haskin. + The authors would like to thank Dennis Ferguson, Enke Chen, John + Scudder, Paul Traina and Tony Li for the many discussions resulting + in this work. This idea was developed from an earlier discussion + between Tony Li and Dimitri Haskin. 11. References [1] Rekhter, Y., and Li, T., "A Border Gateway Protocol 4 (BGP-4)", RFC1771, March 1995. [2] Haskin, D., "A BGP/IDRP Route Server alternative to a full mesh routing", RFC1863, October 1995. [3] Traina, P. "Limited Autonomous System Confederations for BGP",