A route reflector is BGP router that is allowed to break the iBGP loop avoidance rule. Route reflectors can advertise updates received from an iBGP peer to another iBGP peer under specific conditions.

By breaking the rules, route reflectors are used to eliminate the full mesh requirement and allow for building iBGP networks that scale easily and cleanly.

There are two primary timers in BGP. The first is the Hold Down timer, the other is the Keepalive Interval.

The Hold Down Timer indicates how long a router will wait between hearing messages from it's neighbor. The Hold Down Timer defaults to 180 seconds on a Cisco router, but can be reconfigured.

cisco default setting: 60 seconds
To be certain that a BGP session stays up and functional, Keepalive messages are exchanged. The Keepalive Interval counts down to zero and then sends out another Keepalive. There is no timer for route updates, as updates happen dynamically on an incremental basis.

external 20 and internal 200

The router looks up the BGP route and the BGP next hop to reach a destination in the remote AS. Then the router looks up the route to reach the BGP next hop using the IGP.

Additionally, there are four well-knowncommunities that can be referenced by name:
• No-export– prevents the route from being advertised outsidethe local AS to eBGP peers.
• No-advertise– prevents the route from being advertised to either internal or external peers.
• Internet – allows the route to be advertised outside the local AS.
• Local-AS – prevents the route from being advertised outside the local AS to either eBGP orconfederate peers.