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 will 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 an MD5 digest of every TCP segment 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.