BGP - [Part 20] - ROUTE REFLECTOR

BGP Route Reflector - 

• As per BGP loop avoidance mechanism-Split Horizon Rule(Within IBGP), when a router learn routes from an IBGP peer, that router does not advertise the same routes to another IBGP peer.
• Check the below diagram, there are three Routers within the same Autonomous system, and all of these are IBGP peer of each other, Router R1 will send the routes to R2, and Router R2 will learn these routes, but Router R2 cannot advertise the same routes to Router R3.

• So according to Split Horizon rule, all iBGP peers within an AS must be fully meshed. But we can break this rule via Route Reflector.
• So we used the Route Reflector to break the split horizon rule and to reduced number of IBGP peering.
• Below is the full mesh topology, you can calculate the number of BGP peering within an AS with the formula N*(N-1)/2, N is the number of routers, As per below topology N=5,

5*(5-1)/2 = 10, so 10 peering is required for 5 routers

• We can reduce the number of BGP peering within an AS by Route Reflector. So now no need to connect each BGP peer to every BGP peer. Each BGP speaker will peer with a router reflector only. 
• All BGP speakers will send Routing advertisements to the route reflector and then Route reflector will sent out to all of the other BGP speakers. So in RR BGP updates will flow from the server to the clients, without the clients having to interact with each other.

Now you can check, there is only 5 BGP Peering in below topology.

There are two types of internal peers to a route reflector

• Client - Check the below topology, a route from a client peer is reflected to all the non-client peers and client peers

• Non-Client - Check the below topology, a route from a non-client iBGP peer reflect to all the client peers only, it will not reflect to non client peers

• So only the route reflector is know, who is a client and who is a non-client. A client is an IBGP router for which route reflector will reflect routes
• And a non-client is just another regular iBGP peer. Because of this, the route reflector must follow the BGP split horizon rule, hence routes from a non-client are only reflected to clients.
• We can configure the Route Reflector via this command "neighbor [neighbor ip address] route-reflector-client"

Check the below configuration sample - 

Router1(config)# router bgp 200
Router1(config-router)# neighbor 10.10.10.1 remote-as 200
Router1(config-router)# neighbor 10.10.10.1 route-reflector-client
Router1(config-router)# neighbor 10.10.10.2 remote-as 200
Router1(config-router)# neighbor 10.10.10.2 route-reflector-client

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Anubhav Upadhyay

Hello and welcome to networktopic Blog. My name is Anubhav. I am a Senior Network Egineer. I have created this blog specially to serve interview questions and answer on Network Routing and Switching, I will try my best to serve correct and updated networking knowledge for you as per my corporate experience.

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