Recursive look-ups in iBGP

DANMOH009

Thinking of doing the CCNP Route or CCNA SP next,

so started doing a bit of research on topics and reading up a bit. Took a major interest in BGP so ive been doing a bit of reading on that, then i came to recursive look-ups! OMG!!!!

What is going on? can someone please explain to me how this is done, where ever i read it and the way it works i just dont get it, can someone please help me out here and just tell me what is going on with it?

Cheers.

7of9

A recursive route lookup uses the BGP next-hop attribute to actually make a different route lookup. The IGP can use the destination network instead of the AS-path to determine where it gets sent. Even if the iBGP hasn’t converged, the routers will still know how to get to that network, since it will exist in the router it was advertised from, who will know the next-hop.

In iBGP, if you don't want to turn on synchronization between iBGP peers, recursive route lookups allow you to use the next-hop attribute to find a path to a network that the IGP is aware of, but has not converged with the iBGP peers yet, so essentially, it's like iBGP is shuffling the route lookup over to the IGP.

Links for more - Networking 101: Understanding iBGP
Better - Route My World! » Blog Archive » BSCI: BGP Attributes II

mattau

as above. the key is the next hop ip.

the next hop advertised could be 5 hops away. but from your perspective on the device you are sitting on, you arent directly connected to it.

its just like saying. how do i get to mcdonolds from my house? I obviously dont live right across the road to them (directly connected next hop)

instead you need to look up in your street directory how do I get to mcdonalds.
The street directory says, go down smith street

It works like that in a router. Except the street directory is built by your IGP to give you the routes to get to everyone in your own AS, instead of whoever makes street directorys. google maps or something