I am trying to run intra and inter-domain routing using Quagga. I like to announce a prefix from AS7 and I like AS6 receive it and send it to AS2(as shown in the attached topology).

I know for intra topology I need to run OSPF. My question and my problem is about involving iBGP and eBGP routers in OSPF protocol. I mean, my guess is that I should use a public ip address for my eBGP routers. So do I need to run OSPF just on routers r3,r4,r10? Do I need to have a session between R6 and R5 or between R6 and R9? If I involve my eBGP routers in building my full mesh topology, then I need to connect R6 and for example R5 too. But as I am using their public ip addresses, then they can communicate without using the internal path. I am using their public ip addresses because I link AS7 could be able to talk to them.

Can you please help me to understand how can handle this?

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1 Answer 1


Your question is very confusing, and I think it is because you do not really understand some things. To get on the same page:

  • Every router interface, including loopback interfaces, has its own address, each in a different network.
  • eBGP is used between ASes.
  • With eBGP, you will almost certainly use the interface addresses of the two directly-connected interfaces for the neighbor eBGP routers (it is possible, with complications, to use non-directly-connected interface addresses, but it is rare).
  • iBGP is used inside a single AS.
  • iBGP requires a full mesh between iBGP speakers (you can use a mitigation like route reflectors or confederations).
  • An AS with an IGP makes it easy to connect iBGP to non-directly-connected interfaces, allowing the full mesh to be logical.
  • An AS with an IGP and multiple physical paths between iBGP speakers should use loopbacks for the iBGP neighbor router addresses.

With that in mind, based on your drawing:

  • Router 2 and Router 5 should peer via eBGP using their directly-connected interface addresses.
  • Router 2 and Router 9 should peer via eBGP using their directly-connected interface addresses.
  • Router 6 and Router 7 should peer via eBGP using their directly-connected interface addresses.
  • AS6 should run an IGP among all the routers.
  • Routers 5, 6, and 9 should have loopback addresses that are advertised by the IGP.
  • Routers 5, 6, and 9 should peer via iBGP to each other using the loopback addresses.
  • Routers 5 and 9 should have a direct link between them to prevent the ibgp peering between them from have to travel the entire AS.
  • Unless AS8 wants to be a transit AS between AS2 and AS7, then you will need some type of filtering applied to prevent advertising AS2 prefixes to AS9, and vice versa.

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