3.7.bi EBGP, IBGP

1) a tcp connection request must have a matching source address in a bgp neighbor statement

R2#sh tcp brie
TCB       Local Address               Foreign Address             (state)
67F71058  2.2.2.2.179                 1.1.1.1.19300                ESTAB
67F709D4  2.2.2.2.179                 3.3.3.3.25648                ESTAB

R2#sh run | b bgp 123
router bgp 123
bgp log-neighbor-changes
neighbor 1.1.1.1 remote-as 123
neighbor 1.1.1.1 update-source Loopback1
neighbor 3.3.3.3 remote-as 123
neighbor 3.3.3.3 update-source Loopback1

2) the router bgp asn statement must match the neighbor router’s reference to that asn in its remote-as statement

R4#sh run | b router bgp
router bgp 45
bgp log-neighbor-changes
neighbor 3.3.3.3 remote-as 123
neighbor 3.3.3.3 ebgp-multihop 2
neighbor 3.3.3.3 update-source Loopback1

R3#sh run | b bgp 123
router bgp 123
bgp log-neighbor-changes
neighbor 1.1.1.1 remote-as 123
neighbor 1.1.1.1 password cisco
neighbor 1.1.1.1 update-source Loopback1
neighbor 2.2.2.2 remote-as 123
neighbor 2.2.2.2 update-source Loopback1
neighbor 4.4.4.4 remote-as 45
neighbor 4.4.4.4 ebgp-multihop 2
neighbor 4.4.4.4 update-source Loopback1

3) the bgp rid’s have to be unique…

R4#sh ip bgp neigh
BGP neighbor is 3.3.3.3,  remote AS 123, external link
BGP version 4, remote router ID 3.3.3.3

R3#sh ip bgp neigh 4.4.4.4
BGP neighbor is 4.4.4.4,  remote AS 45, external link
BGP version 4, remote router ID 4.4.4.4

4) if used, md5 authentication must be successful

*Feb 24 16:16:46.532: %TCP-6-BADAUTH: Invalid MD5 digest from 3.3.3.3(39907) to 1.1.1.1(179) tableid – 0
R1(config-router)#neigh 3.3.3.3 pass cisco
R1(config-router)#
*Feb 24 16:16:53.648: %BGP-5-NBR_RESET: Neighbor 3.3.3.3 reset (Peer closed the session)
*Feb 24 16:16:53.660: %BGP-5-ADJCHANGE: neighbor 3.3.3.3 Down Peer closed the session
*Feb 24 16:16:53.660: %BGP_SESSION-5-ADJCHANGE: neighbor 3.3.3.3 IPv4 Unicast topology base removed from session  Peer closed the session
R1(config-router)#
*Feb 24 16:17:06.304: %BGP-5-ADJCHANGE: neighbor 3.3.3.3 Up
R1(config-router)#