1. © 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—6-1 Scaling Service Provider Networks Introducing Route Reflectors

2. © 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—6-2 Outline Overview IBGP Scalability Issues in a Transit AS Route Reflector Split-Horizon Rules Redundant Route Reflectors Route Reflector Clusters Additional Route Reflector Loop-Prevention Mechanisms Summary

3. © 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—6-3 IBGP Scalability Issues in a Transit AS IBGP requires a full mesh between all BGP-speaking routers. Large number of TCP sessions Unnecessary duplicate routing traffic Solutions Route reflectors modify IBGP split-horizon rules. BGP confederations modify IBGP AS-path processing.

4. © 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—6-4 Route Reflector Split-Horizon Rules Classic IBGP: IBGP routes are not propagated to other IBGP peers. Full mesh of IBGP peers is therefore required. Route reflector can propagate IBGP routes to other IBGP peers. Full mesh of IBGP peers is no longer required.

5. © 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—6-5 Route Reflector Split-Horizon Rules (Cont.)

6. © 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—6-6 Route Reflector Split-Horizon Rules (Cont.)

7. © 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—6-7 Redundant Route Reflectors

8. © 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—6-8 Redundant Route Reflectors (Cont.)

9. © 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—6-9 Route Reflector Clusters A group of redundant route reflectors and their clients form a cluster. Each cluster must have a unique cluster-ID. Each time a route is reflected, the cluster-ID is added to the cluster-list BGP attribute. The route that already contains the local cluster-ID in the cluster-list is not reflected.

10. © 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—6-10 Route Reflector Clusters (Cont.)

11. © 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—6-11 Additional Route Reflector Loop-Prevention Mechanisms Every time a route is reflected, the router-ID of the originating IBGP router is stored in the originator-ID BGP attribute. A router receiving an IBGP route with originator-ID set to its own router-ID ignores that route. The BGP path selection procedure is modified to take into account cluster-list and originator-ID.

12. © 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—6-12 Summary BGP route reflectors were introduced to free the network designers from IBGP full-mesh requirements that prevent large networks from scaling. BGP route reflectors modify IBGP split-horizon rules in that all routes that are received from a route reflector client are sent to all other IBGP neighbors, and all routes that are received from a nonclient IBGP neighbor are sent to all route reflector clients. A route reflector is a single point of failure, and therefore redundancy should be implemented in a network containing route reflectors.

13. © 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—6-13 Summary (Cont.) Route reflector clusters were introduced in the BGP route reflector architecture to support redundancy, preventing IBGP routing loops in redundant route reflector designs. The originator-ID and cluster-list BGP attributes were introduced to prevent routing loops in route reflector environments.

14. © 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—6-14