BGP configuration problem - iBGP works, eBGP doesn't

[miguelopes]

Hi guys, first post here!

I have some problemas with my BGP connections... I started by configuring OSPF in all routers. After all pinging in same AS, I started by trying to configure iBGP in R2 and R3 with loopbacks. And it worked, iBGP connection all good. But then, I just can't make R2 (remote-as 10) communicate with R7 (remote-as 205). When I do the command "sh ip bgp sum" in R2, it appears it has two neighbors, with R3, there is communication but with R7 there isn't and it appears as IDLE. Dont know what to do... Attached is complete work. Thank you!

[davidsudjiman]

Make sure you have L3 connectivity between R2 and R7.

What is the IP for the PtP link between R2 and R7? is it 203.0.113.0/30 or 203.0.113.6/30?

Are you using PtP link for the BGP peering or using loopback?

[Giuseppe Larosa]

Hello Miguel,

as noted by David you have mismatching IP addressing on the direct link R2-R7

when you use a /30 subnet you have only two possible hosts.

Change R7 ip address to 203.0.113.2/30 as 203.0.113.7 is in another IPv4 subnet!

To be correct 203.0.113.7 is the broadcast address of 203.0.113.4/30 and cannot be assigned to a router interface, so you probably just have a wrong network diagram.

If peering on loopbacks make use of the correct suggestions made by Paul Driver as eBGP packets are sent out with TTL=1 by default

Hope to help

Giuseppe

[sknighting]

As probably pointed out.

R2 Fa0/2 is on a different network to R7 Fa 0/2....  You have applied a /30 on R2 and a /24 on R7.

Change the subnet to /28 (255.255.255.240) if you want to keep the subnet size down but use the 4th octet to match your router number.

Or to keep it really simple and tie up with what you have already done on your other interfaces use a /24.

[paul driver]

Hello

at present I don't have access to your zip file so I cannot see your configuration 

when you say you have ospf enabled is this igp advertising the network for the external ask of R7

Do you have NLRI ( connectivity) to R7 and how are you peering to it? - Is it via its directed subnet or a loopback address of the rtrs?

If your sourcing the peering other then directly connect interfaces then you need to tell bgp this because its more than 1 hop away (TTL +1)

With these commands

update source xxxx(interface)

ebgp multi-hop xx 

I also assume this is an ebgp peering to R7 and not an ibgp you have setup?

As R2 is the external next hop for R7 - Your R3 ibgp peer will also see that subnet as the next hop but won't have its subset in its routing table so won't be able to reach the external routes-

So you need to tell R3 use R2 as the next hop for external networks using the " next-hop self " command on R2 towards R3

Res

paul