eBGP for Internet Edge iBGP for?

[Bilal Nawaz]

Hello experts,

I understand why eBGP is required for multi-homing scenario, is an iBGP instance required when looking to advertise down a default route into an ospf enabled core? If someone could explain why iBGP is used in the scenario below, that would be great!

Kind Regards,

Bilal

[Edison Ortiz]

Bilal,

Your edge routers will more than likely be running the same AS # and ideally a BGP peering will be configured.

Once you configure the BGP peering, an iBGP design comes into place.

You don't use iBGP for redistributing BGP learned routes into OSPF.

iBGP is when BGP learned routes are exchanged between routers running the same AS #.

Regards,

[Peter Paluch]

Hello Bilal,

The iBGP session between the edge routers allows any edge router to converge through a different edge router/ISP if its own directly connected ISP fails. Imagine a default route being injected by both ISPs to your edge routers. Now if the connection of the left edge router to SP1 fails, it will lose its default route from SP1 and instead, it will learn the default route from the right edge router, thereby converging to SP2 for internet connectivity.

It is not always required to run iBGP between the edge routers, however. Especially if the SPs simply advertise a default route to you and nothing more, the iBGP does not carry anything what a common IGP could not carry as well. In simpler scenarios, therefore, the iBGP session is not required.

Best regards,

Peter