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I understand that routes learnt via iBGP can't be redistributed to other iBGP peers to avoid loops, but then why is a iBGP connection between two routers that don't have eBGP connections needed? They won't distribute anything, right?

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...why is a iBGP connection between two routers that don't have eBGP connections needed? They won't distribute anything, right?

Not quite right. It's not that two routers connected via iBGP cannot learn any prefixes from each other.

Suppose you have two WAN routers in your AS, A and B, both connected to separate WAN ASes via eBGP, and connected together via iBGP. Router A cannot learn prefixes from Router B about which Router B learned from other iBGP speakers in your AS, but it can learn about prefixes which Router B learned about from eBGP or an IGP.

Any prefixes which Router A learns about through iBGP from Router B cannot be passed on to any other iBGP speakers because those prefixes could be sent back to Router B, directly or indirectly. That's the reason for needing a full mesh or a mitigation such as route reflectors or confederations; other iBGP speakers will need to learn the prefixes which Router B will share with other iBGP speaker directly from Router B since Router A is unable to pass those prefixes on.

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  • But I'm asking specifically about two BGP routers that do NOT have any eBGP connection. Two internal routers in the AS. Why do they need a iBGP connection? – multigraph Jan 11 '16 at 19:13
  • Right, it's the same thing. The routers can't learn prefixes from each other that were leaned via iBGP, but they can still learn prefixes, via iBGP that were learned in other ways. These outer prefixes must be distributed to each an every other iBGP speak directly since the other iBGP speakers can't tell each other about these prefixes. – Ron Maupin Jan 11 '16 at 19:19
  • @multigraph, look at this question, especially the comments under Ron Trunk's answer. – Ron Maupin Jan 11 '16 at 19:21
  • By "other ways" what do you mean? – multigraph Jan 11 '16 at 19:23
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    Routes end up in a router's routing table via a variety of methods: directly connected, statically configured or from a routing protocol. A router can run multiple routing protocol, and it can redistribute routes learned via any of the routing protocols or other method into other routing protocols. Router B could have a directly connected route which it shares with Router A via iBGP, but router A can't share it with Router C via iBGP, only Router B can do that, hence a full mesh is required. – Ron Maupin Jan 11 '16 at 19:27
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Yes if the iBGP neighbors do not need to exchange prefixes with non directly connected router then sure you don't have to do a full mesh or a Route Reflector. So yes they won't distribute anything to non-connected peers.

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In a large network, the full-mesh requirement for iBGP can be a big challenge. Route Reflectors is an alternative to full-mesh iBGP configuration. Check out RFC4456

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