BGP vs. OSPF

[pamcmahon]

I am new to the LAN world, and I am learning all about configuring Cisco routers. My question is why would you want to use BGP on your external connections, and OSPF on your internal connections when BGP interior seems to take care of all internal connections.

[marikakis]

You need eBGP for the rest of the world to know you.

(And for you to know the rest of the world if a default route does not suffice.)

You either run eBGP for yourself or some provider does it for you.

Sometimes you need to run iBGP inside the core.

For example, when you have multiple exit points

and you need to choose the best exit point for outgoing traffic.

In environments that run iBGP,

the IGP is used to carry the BGP next-hop and local routes.

The BGP next-hops must be known for the BGP routes to be usable,

and an IGP makes this happen in a neat way.

An IGP such as OSPF is designed to cover the needs of interior routing.

For example, it converges quickly after topology changes.

BGP is designed to cover the needs of exterior routing.

For example, to control routing policies.

BGP is not optimized for fast convergence.

Even if you tune it, it will still be slow, and other problems will pop up.

It was just not designed with the same goals as OSPF was.

So, you definitely need an IGP for your interior routing.

M.

[ruwhite]

The basic issue is that BGP was designed to provide loop free routing between autonomous systems; iBGP, which is used within an AS, doesn't gaurentee loop free routing.

If you used iBGP as your IGP, you'd have to be certain to provide loop free routing within your network in other ways (route reflectors, full mesh iBGP, etc), and ways to find the next hop (iBGP won't let you recurse the next hop onto another iBGP route), and etc.

:-)

Russ.W