IGP routing protocols

[M.Sultan]

Hi everyone,

Why do we need IGP (OSPF/EIGRP) before we run iBGP ???

Regards

[MHM Cisco World]

Not only iBGP'

Also eBGP need igp if the neighbor is not direct connect.

And more iBGP need IGP to achieve full mesh.

[Joseph W. Doherty]

You don't need an IGP but in almost any topology, it would be difficult to meet iBGP's needs without an IGP.

Also understand for what iBGP is intended for, this design is not a flaw.

[David Ruess]

While its not Required it definitely helps. Think of the IGPs as the helper addresses and the BGP neighbors as the Source/Destination. An IGP can peer with routers on the same link. You cant peer OSPF/EIGRP neighbors multiple hops away. So you establish IGP adjacencies to provide connectivity in the middle while your edge devices can run BGP. Just like the iBGP neighbors can be multiple hops away it just needs a route to the neighbor IP address. The IGP provides that next hop resolution.

-David

[Richard Burts]

One way to look at it is that BGP chooses routes based on parameters like how many AS are in the AS path. So every advertisement of a prefix in your network would look the same. If one advertisement for for a prefix that was 8 hops away on very slow links and another advertisement for that prefix was one hop on a high speed link they would look the same to BGP. You could differentiate those advertisements when running an IGP.

[WarHawk]

It’s a common design pattern that it used in a lot of environments. The quick and dirty is:

1. For BGP to form adjacencies (peering sessions) it first needs to know how to reach its neighbour.

2. We can do this with static routing but best practise says we should use a dynamic routing protocol.

3. This leaves us with picking an IGP in order for each router to dynamically tell its neighbors what it knows. From here each router will have connectivity to each other in order to form those BGP adjacencies (whether iBGP or eBGP).

Running OSPF/EIGRP as the IGP usually means fast failover as convergence and is a tried and tested method.