How to configure EBGP (External BGP)

In this lesson I will show you how to configure EBGP (External BGP) and how to advertise networks. I will be using the following topology:

Let’s start with a simple topology. Just two routers and two autonomous systems. Each router has a network on a loopback interface which we are going to advertise in BGP.

R1(config)#router bgp 1
R1(config-router)#neighbor 192.168.12.2 remote-as 2

R2(config)#router bgp 2
R2(config-router)#neighbor 192.168.12.1 remote-as 1

Use the router bgp command with the AS number to start BGP. Neighbors are not configured automatically this is something you’ll have to do yourself with the neighbor x.x.x.x remote-as command. This is how we configure external BGP.

R1# %BGP-5-ADJCHANGE: neighbor 192.168.12.2 Up

R2# %BGP-5-ADJCHANGE: neighbor 192.168.12.1 Up

If everything goes ok you should see a message that we have a new BGP neighbor adjacency.

R1(config)#router bgp 1
R1(config-router)#neighbor 192.168.12.2 password MYPASS

R2(config)#router bgp 2
R2(config-router)#neighbor 192.168.12.1 password MYPASS

If you like you can enable MD5 authentication by using the neighbor password command. Your router will calculate a MD5 digest of every TCP segment that is being sent.

R1#show ip bgp summary 
BGP router identifier 1.1.1.1, local AS number 1
BGP table version is 1, main routing table version 1

Neighbor     V    AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd
192.168.12.2 4     2      10      10        1    0    0 00:07:12        0

R2#show ip bgp summary 
BGP router identifier 2.2.2.2, local AS number 2
BGP table version is 1, main routing table version 1

Neighbor     V    AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd
192.168.12.1 4     1      11      11        1    0    0 00:08:33        0

Show ip bgp summary is an excellent command to check if you have BGP neighbors. You also see how many prefixes you received from each neighbor.