Need for IBGP?

RouterOS Forwarding Protocols
1

Hi,

I have a small question regarding a redundant BGP (router + uplinks) setup.
This is the current situation:
R1R2BGP.png
R1:
/routing bgp peer
add name=IBGP remote-address=172.16.17.2 remote-as=11111
add in-filter=IPV4-IN name=R1ISP-1 out-filter=Own-subnets remote-address=xx.xx.xx.100 remote-as=11112
add in-filter=IPV4-IN name=R1ISP-2 out-filter=Own-subnets remote-address=xx.xx.xx.101 remote-as=11112


R2:
/routing bgp peer
add name=IBGP remote-address=172.16.17.1 remote-as=11111
add in-filter=IPV4-IN name=R2ISP-1 out-filter=Own-subnets remote-address=xx.xx.xx.102 remote-as=11112
add in-filter=IPV4-IN name=R2ISP-2 out-filter=Own-subnets remote-address=xx.xx.xx.102 remote-as=11112


Own AS in this example: 11111
Upstream ISP AS: 11112

Both routers are receiving two full internet tables from the Upstream ISP (11112) and one full table
from each other (the IBGP peer). I was wondering why i have connectivity issues once i disconnect
the IBGP? Both have an address in each subnet in one L2 domain.
Why do i need to have the IBGP?

2

an iBGP peering ensures that both border routers have a full view of the routes of the other router and also the interior of the network (if you’re iBGP based on the inside)

In most cases, an iBGP peering is desired between two routers in the same ASN that take in full tables. Otherwise, traffic could leave one border router and come back into the other border router and in the worst case, be blackholed or at the very least take a suboptimal path.