Exchange info external BGP

[EAleg]

Can external bgp (AS100) exchange info with another external bgp (AS200) across is-is, ospf and so on. or only across directly connected?

[MHM Cisco World · Accepted solution]

Yes it can, 
config loopback 
and use multi hop under bgp. 
Note:- loopback for each peer must be seen from other peer.

[EAleg]

yeah man, thk, i used this "neighbor 192.168.4.1(example) ebgp-multihop 3 (example)", and it works!

[pman]

Yes you can,

For example by creating a tunnel interface in each of the eBGP peers.
But you can also do this using a CE-PE solution.
First it is important to understand your customer needs.
Why you are required to accomplish this and what goal you are trying to achieve?

[Giuseppe Larosa]

Hello @EAleg ,

without redistribution of BGP into IS-IS or OSPF routers on the path would not be able to route packets with destinations that are BGP learned prefixes.

The exceptions to the above scenario are the following:

a) there is MPLS forwarding enabled in addition to IS-IS or OSPF and LSPs to each eBGP endpoint are working for recursion traffic for eBGP learned prefixes is sent over these LSPs so routers on the path don't need to know how to route the packet they just do label swap.

b) a tunnel like a point to point GRE tunnel is configured between the two routers so that traffic can be sent over it

c) On each side you are redistributing BGP routes into the IGP so that each router knows how to route.

of course you should use ebgp multihop to be able to setup the eBGP session. Use a value that allows to setup the eBGP session.

Hope to help

Giuseppe