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:

bgp as1 as2

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 remote-as 2
R2(config)#router bgp 2
R2(config-router)#neighbor 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 Up
R2# %BGP-5-ADJCHANGE: neighbor 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 password MYPASS
R2(config)#router bgp 2
R2(config-router)#neighbor 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, 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 4     2      10      10        1    0    0 00:07:12        0
R2#show ip bgp summary 
BGP router identifier, 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 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.

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Forum Replies

  1. Hi,

    I would like to know how I can be able to see the ebgp prefixes from my OSPF lan segment without using redistribution of BGP to OSPF?

    I would send you the configs and topology I have made if you are interested?


  2. Hi Rane ,
    I am a big fan of your blog. I find your lessons very helpful . Can you please publish an lesson " how BGP works " Explanation of the process.

  3. Hi,

    If you need the prefixes that you learned through EBGP on your internal routers then you have two options:

    - Redistribute them into your IGP (not a good idea if you have a LOT of prefixes).
    - Run IBGP on your internal routers.

    You also need to consider if you really need all EBGP prefixes on your internal routers, maybe a default route in OSPF also does the job…


  4. Hi Rene,
    1st of all thank you for all of your precious writings.

    I started reading your BGP blog. But im struggling to find an easy way to start with BGP. Because topics are scattered all over the place. Can’t figure out which topics I should read first. Can you please help and show me the correct order that i should follow to become a BGP guru.

    Thank you

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