a reason for IBGP when OSPF is already running

axfalk · ‎03-23-2013

Could someone pls think of a reason to run IBGP between the 2 hosts if an IGP (OSPF) protocol is already running?

Thanks..

_ Greg

Edwin Summers · ‎03-23-2013

One example comes to mind...

The two routers are border routers, perhaps connected to different providers. Maybe you get full BGP tables or even partial BGP tables. Depending on your network topology and hardware, it's likely not feasible to dump these entire tables (redistribute) into OSPF. Run iBGP between the border routers and let them figure out between themselves which is the best exit point for a particular external network. The IGP is responsible for getting traffic destined for external networks to the border routers. This provides the best exit point for your traffic, while keeping all of the external routes out of your IGP tables.

Just one example.

Marcel Zehnder · ‎03-23-2013

Another reason: MP-BGP for MPLS services like MPLS-VPN.

Regards

Peter Paluch · ‎03-23-2013

Hi Greg,

Edwin has tackled the main point very nicely - you run both OSPF and iBGP because they both carry different sets of routes. OSPF carries the internal networks within your autonomous system (AS). iBGP carries networks from outside your AS. Especially with full internet routing table, you do not want to redistribute those hundreds of thousands of routes into your OSPF - it would not be able to carry them all, the burden would be overwhelming. So if you need routers within your AS to know both about your internal networks and also about networks outside your AS, you need to run both OSPF and BGP, and because your BGP speakers are in the same AS, they would be running iBGP to each other.

There are applications for iBGP if, for example, you run multicast with icongruent multicast/unicast topologies, MPLS L3VPNs, 6PE or 6VPE, VPLS, multipoint LSPs and some others - but in all these cases the information carried in BGP is different than the information carried in OSPF, and OSPF would be unsuitable or outright unable to carry it.

To be honest, I do not see a point in running both OSPF and iBGP between routers if both these protocols are intended to carry the same set of routes all the time.

Best regards,

Peter