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

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—6-2 Outline Overview Network Design with Route Reflectors Potential Network Issues Hierarchical Route Reflectors Summary

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—6-3 Network Design with Route Reflectors Route reflector rules Route reflector rules divide a transit AS into smaller areas (called clusters). Each cluster contains route reflectors and route reflector clients. Routers that do not support route reflector functionality act as a one-router cluster or as a route reflector client.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—6-4 Network Design with Route Reflectors (Cont.) IBGP session rules All clients in a cluster must establish IBGP sessions with and only with all route reflectors in the cluster. An IBGP full mesh between all route reflectors within the AS is required. Routers that are not route reflectors can participate in the IBGP full mesh or be route reflector clients.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—6-5 Network Design with Route Reflectors ― Example

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—6-6 Potential Network Issues Issue Clients do not have sessions with all reflectors in a cluster. Clients have sessions with reflectors in several clusters. Clients have IBGP sessions with other clients. Result Clients will not receive all IBGP routes. Clients will receive duplicate copies of the same route. Potential problems that can occur when you deviate from the route reflector network design rules:

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—6-7 Hierarchical Route Reflectors Problem: In very large networks, a single layer of route reflectors might not be enough. Solution: A hierarchy of route reflectors can be established. –A route reflector can be a client of another route reflector. –The hierarchy can be as deep as needed.

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

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—6-9 Summary All route reflectors in a cluster should have IBGP sessions to all clients in the cluster. The route reflectors also participate in the IBGP full mesh, and they should have no other IBGP sessions. When the route reflector clients do not have IBGP sessions with all route reflectors in the cluster, they might not receive all IBGP routes. When the clients have additional IBGP sessions with routers that are not their route reflectors, they receive unnecessary IBGP routes and potentially encounter a routing loop. Route reflector clusters can be built in hierarchies. A router that is a route reflector in one cluster can act as client in another cluster.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—6-10