Route ReflectorCreated By Ahmed Raafat

What is Route Reflector (RR)? Route reflectors (RR) are one method to get rid of the full-mesh of

IBGP peers in your network. The other method is BGP confederations.

The route reflector allows all IBGP speakers within your autonomous network to learn about the available routes without introducing loops.

Above we have a network with 6 IBGP routers. Using the full mesh formula we can calculate the number of IBGP peering: N(N-1)/2

So that will be: 6(6-1)/2= 15 IBGP peering.

When we use a route reflector our network could look like this:

We still have 6 routers but each router only has an IBGP peering with the route reflector on top. When one of those IBGP routes advertises a route to the route reflector, it will be “reflected” to all other IBGP routers.

This simplifies our IBGP configuration a lot but there’s also a downside. What if the route reflector crashes? It’s a single point of failure when it comes to IBGP peering. Of course there’s a solution to this, we can have multiple route reflectors in our network.

The route reflector can have three types of peering: EBGP neighbor IBGP client neighbor IBGP non-client neighbor

When you configure a route reflector you have to tell the router whether the other IBGP router is a client or non-client. A client is an IBGP router that the route reflector will “reflect” routes to, the non-client is just a regular IBGP neighbor.

When a route reflector forwards a route, there are a couple of rules: A route learned from an EBGP neighbor can be forwarded to another EBGP

neighbor, a client and non-client. A route learned from a client can be forwarded to another EBGP neighbor,

client and non-client. A route learned from a non-client can be forwarded to another EBGP

neighbor and client, but not to a non-client.

The third rule makes sense, this is our normal IBGP split horizon behavior.

Now you have an idea what the route reflector is about, let’s take a look at some configurations.

Configuration

In this example we have 3 IBGP routers. With normal IBGP rules, when R2 receives a route from R1 it will not be forwarded to R3 (IBGP split horizon). We will configure R2 as the route reflector to get around this. Let’s configure R1 and R3 first:

R1(config)#router bgp 123 R1(config-router)#neighbor 192.168.12.2 remote-as 123

R3(config)#router bgp 123 R3(config-router)#neighbor 192.168.23.2 remote-as 123

The configuration of R1 and R3 is exactly the same as a normal IBGP peering. Only the configuration on the route reflector is special:

R2(config)#router bgp 123 R2(config-router)#neighbor 192.168.12.1 remote-as 123 R2(config-router)#neighbor 192.168.12.1 route-reflector-client

R2(config-router)#neighbor 192.168.23.3 remote-as 123 R2(config-router)#neighbor 192.168.23.3 route-reflector-client

Here’s the magic.. When we configure the route reflector we have to specify its clients. In this case, R1 and R3. In the topology I have added a loopback interface on R1, let’s advertise that in BGP to see what it looks like on R2 and R3:

R1(config)#router bgp 123 R1(config-router)#network 1.1.1.1 mask 255.255.255.255

Verification

First we’ll look at R2, see if it learned anything: R2#show ip bgp 1.1.1.1

BGP routing table entry for 1.1.1.1/32, version 2Paths: (1 available, best #1, table Default-IP-Routing-Table)Flag: 0x820 Advertised to update-groups: 1 Local, (Received from a RR-client) 192.168.12.1 from 192.168.12.1 (192.168.12.1) Origin IGP, metric 0, localpref 100, valid, internal, best

R2 shows us that this route was received from a route reflector client. Did it advertise anything to R3? Let’s find out:

R2#show ip bgp neighbors 192.168.23.3 advertised-routes

BGP table version is 2, local router ID is 192.168.23.2Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, r RIB-failure, S StaleOrigin codes: i - IGP, e - EGP, ? - incomplete

Network Next Hop Metric LocPrf Weight Pathi1.1.1.1/32 192.168.12.1 0 100 0 i