BGP Route Reflector

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. Let me show you an example picture:

IBGP 6 routers full mesh

Above we have a network with 6 IBGP routers. Using the full mesh formula we can calculate the number of IBGP peerings:

N(N-1)/2

So that will be:

6(6-1=5) / 2 = 15 IBGP peerings.

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

IBGP Route Reflector Example

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:

IBGP Route Reflector Route Update

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 peerings. Of course there’s a solution to this, we can have multiple route reflectors in our network. I’ll give you some examples later.

The route reflector can have three type of peerings:

  • 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.

Rules

When a route reflector forwards a route, there are a couple of rules:

  1. A route learned from a non-RR client is advertised to RR clients but not to non-RR clients.
  2. A route learned from a RR client is advertised to both RR clients and non-RR clients. Even the RR client that advertised the route will receive a copy and discards it because it sees itself as the originator.
  3. A route learned from an EBGP neighbor is advertised to both RR clients and non-RR clients.

Here are three illustrations to see these rules in action.

Rule 1

Bgp Route Reflector Rule1

Rule 2

Bgp Route Reflector Rule2

Rule 3

Bgp Route Reflector Rule3

 

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

Configuration

We’ll use a simple example, 3 IBGP routers with a single route reflector:

AS 123 IBGP Route Reflector

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 my 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

That’s all we have to configure. Let’s use some show commands to verify our work.

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 2
Paths: (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 192.168.23.2
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path
*>i1.1.1.1/32       192.168.12.1             0    100      0 i

So what do we see here? Let me explain…

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Forum Replies

  1. Hi Rene, all of your BGP Tutorials are so great please make another advanced BGP topics too, such as :
    - BGP Peer-Group
    - BGP Community
    - BGP Local-AS
    - BGP Load-Sharing
    - BGP Summarization
    - etc (if any)

  2. Hi Black,

    Glad to hear that you like it. I’ll add some of these tutorials, don’t worry :slight_smile:

    Rene

  3. Thanks Rene, I’ll be waiting for that.

  4. Hi Rene,
    I also want to thank you for BGP tutorials. Its very nicely and simply written.

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