BGP

[gauravpundir231]

Hi All,

Need your advise on a BGP query.

In many of my company's routers we have a BGP peer example peer ip: x.x.x.x however i am not able to find this subnet on any of the interface or loopback.

Can anyone pls advise what kind of peer is that. 

[Giuseppe Larosa]

Hello @gauravpundir231 ,

you can use show ip bgp summary to see the BGP neighbors of your local routers.

BGP peer = BGP neighbor so you cannot find a BGP peer address in one loopback of your device it is in another router, in the case of iBGP the BGP peer is typically the loopback of another device in the case of eBGP the BGP peer is usually the IP address of the neighbor on the border link.

You can use

show ip bgp summary

and then

show ip route x.x.x.x

for each BGP peer/neighbor listed in the output of first show command

Hope to help

Giuseppe

[gauravpundir231]

Hello Giuseppe,
Thanks for the response.

Yes, the nei is other router. but my point was both the peers should have same subnet IPs (Local & Nei).
Example

Neighbor V AS MsgRcvd MsgSent Up/Down State/PfxRcd

4.4.4.4 4 100 11 15 0:09:01 Estab/ 108

5.5.5.5 4 200 27 28 0:24:44 Estab/ 1004

if my nei is 4.4.4.4 then on my wan int there must be IP like 4.4.4.3 or 4.4.4.5
for second one, neighborship is there but no matching subnet on interface or loopback.

Is it possible to have iBGP peers but not in common subnet ?

[Giuseppe Larosa]

Hello @gauravpundir231 ,

>> Is it possible to have iBGP peers but not in common subnet ? 

Yes indeed the BGP TTL for iBGP messages is 255 and it is only 1 for eBGP messages

iBGP is configured between loopbacks that are logical interfaces always on and reachable if the router has at least one alive interface.

Of course the loopback addresses need to be advertised in the iGP routing protocol in use either OSPF, EiGRP or IS-IS.

This is why you don't need to share a common IP subnet with an iBGP peer

Hope to help

Giuseppe

[gauravpundir231]

Ok. Thanks Giuseppe for explaining
I will go thru the config again.

[paul driver]

Hello

Depending of what software is your running on your rtrs there is a hidden command for bgp peering that shows you if your bgp neighbors shares a directly connected common subnet, if it show a connected interface then you know the peering is local if it returns none then you know it isn't

show ip cef <neighbor> samecable