Lab 9.1.3 Route Reflectors

Objectives:

  • Configure a Route Reflector to reduce the IBGP mesh.

Scenario:

The graphic above illustrates a simple IBGP configuration with three IBGP speakers (Routers A, B, and C). Without route reflectors, when Router C receives a route from an external neighbor, it must advertise it to both Routers A and B. Routers A and B do not readvertise the IBGP learned route to other IBGP speakers because the routers do not pass routes learned from internal neighbors on to other internal neighbors, thus preventing a routing information loop.

With route reflectors, all IBGP speakers need not be fully meshed because there is a method to pass learned routes to neighbors. In this model, an internal BGP peer is configured to be a route reflector responsible for passing IBGP learned routes to a set of IBGP neighbors. In the diagram above, Router A is configured as a route reflector. When the route reflector receives routes advertised from Router C, it advertises them to Router B, and vice versa. This scheme eliminates the need for the IBGP session between Routers B and C.

Your task in this lab is to configure Router A as a route reflector since there is not a full IBGP mesh in AS 100.

Tasks:

  1. Configure all the interfaces to have the appropriate IP addresses as shown in the diagram. Use the /24 subnet mask.
  2. Configure RIP as your IGP. Then enable BGP routing on all routers. Use the network command to advertise the LAN on Router A and the router in AS 200. Don't forget your neighbor statements!!! They are not shown. Advertise the following routes:

    Network 2.0.0.0 from Router B
    Network 170.10.0.0. from Router A
    Network 5.0.0.0 from Router D in AS 200

    Example: Router(config)#Router BGP 200
           Router(config-router)#network 5.0.0.0 mask 255.255.255.0

  1. Examine the routing table of each router in AS 100. Do you see network 5.0.0.0 in each routing table?



    Which router is missing the entry?



    Why is the network entry missing?

  1. From Router A, can you use an extended ping with source address 170.10.0.1 (interface E0) and successfully reach 5.0.0.1 (E0 of the router in AS 200)?

If the answer to the above question is yes, then good job in your configuration! If not, did you use the neighbor next-hop-self command? Why is this command necessary?

  1. Verify connectivity between the routers.
  1. Log on to Router A. We are going to configure it as a route reflector. The commands to do this are as follows:

    RouterA(config)#router bgp 100 
    RouterA(config-router)# neighbor 2.2.2.2 route-reflector-client 
    RouterA(config-router)#neighbor 3.3.3.2 route-reflector-client

  1. Save your configuration and log on to Router B. Issue the clear ip bgp * command. Now check the BGP routing table with the show ip bgp command. Do you see an entry for network 5.0.0.0?

In how large of a network do you think route reflectors would be useful in?



Which is better, a full IBGP mesh or route reflectors?



Why?

  1. Save you configurations as you will need them for the next lab on Route Filtering based on NLRI.