Here are some guidelines for scaling service provider networks:
* BGP carries customer and provider routes
* IGPs carry only internal routes used to supply routers with an understanding of the next-hop-IP. This may include loopback IPs for IBGP neighborships.
* Do not redistribute BGP into your IGP
* IBGP does not scale well as a full mesh, and create too much update traffic
* Use route-summarization whenever possible
Route Reflectors overcome the full mesh requirement of IBGP neighborship.
Here is how a route reflector will behave.
* When a router receives an update from an external peer, it will propagate that advertisement to all peers (eBGP and iBGP).
* When a router receives an update from a non-client internal peer, if it is a router reflector, it will propagate that advertisement to all clients and eBGP peers.
* When a route reflector receives an update from a client, it will be reflected to all iBGP peers.
Route-reflectors may be single points of failure unless clusters are used. Clusters allow for redundancy without problems such as routing loops.
A hierarchy of route-reflectors may be used to overcome scaling very large autonomous systems.
Confederations allow a large autonomous system to be carved up into smaller AS numbers. To the outside world, the autonomous systems participating in the BGP confederation are seen as a single AS. This can help overcome scalability by reducing peering.
An iBGP full mesh is needed for member-autonomous systems. eBGP neighborships can be used in any manner to provide connectivity between all participating member-ASs.
bgp cluser-id cluster-id – Configured the route reflector cluster
neighbor ip-address route-reflector-client – Informs a route reflector of its clients
router bgp member-as-number – Configures the member-AS of a router within a confederation
bgp confederation identifier external-as-number – Configures the external AS
bgp confederation peers list-of-intra-confederation-as – Informs an intermember EBGP speaker in a confederation of the other member-autonomous systems participating in the confederation