eBGP Multihop Configuration

eBGP Multihop Configuration
Situations do exist where the remote BGP speaker you are going to communicate with is not
directly connected. When this occurs, it’s known as an eBGP multihop. An eBGP multihop will
occur when the connection to the remote BGP speaker is not a direct connection between the
local BGP speaker’s egress interface and the remote BGP speaker’s ingress interface. There are
a couple of different reasons why this could occur:
 There is another router in between the local BGP speaker and the remote BGP speaker that
cannot run BGP.
 You are sourcing the BGP connection from a loopback interface on at least one of the BGP
speakers involved.
There is one important item that you must take note of when implementing eBGP multihop.
The two BGP speakers participating must have a route between them. This route can be learned
utilizing one of the different IGPs. The route cannot be a default route. If this route does not
exist, the BGP speakers will never form a peer relationship because they don’t know how to find
each other. In order to configure eBGP multihop, you must enter the following command on
both BGP speakers involved in router configuration mode:
neighbor address ebgp-multihop [ttl]
address - IP address of the remote device.
ttl - an optional parameter that can be set to inform the device the maximum
number of hops away the neighbor is. The default value is 1.
With this in mind, take a look at Figure 8.14.
In this figure, R2 will not be participating in BGP. We will also be using the loopback interfaces
of R1 and R3 as the address of each BGP speaker. Remember from the previous example,
if loopback interfaces are used, the update-source keyword must also be used. The IP
addresses for R1 and R3 are as follows:
R1 Lo0-1.1.1.1
R3 Lo0-3.3.3.3
For this configuration, we will assume that R1 and R3 know how to reach each other. Let’s
go ahead and enable BGP and set the peers on this network:
R1#conf t
Enter configuration commands, one per line. End with CNTL/Z.
R1(config)#router bgp 100
R1(config-router)#neighbor 3.3.3.3 remote-as 200
R1(config-router)#^Z
R1#
Configuring BGP 267
R3#conf t
Enter configuration commands, one per line. End with CNTL/Z.
R3(config)#router bgp 200
R3(config-router)#neighbor 1.1.1.1 remote-as 100
R3(config-router)#^Z
R3#