InIBGP (Internal Border Gateway Protocol), all routers within the same AS (Autonomous System) must have a logical full-mesh topology. This means that every IBGP router must be able to communicate with every other IBGP router directly or indirectly to ensure proper route propagation.

Step-by-Step Breakdown:

Logical Full Mesh:

In an IBGP setup, routers do not re-advertise routes learned from one IBGP peer to another IBGP peer. This rule is in place to prevent routing loops within the AS.

To ensure full route propagation, alogical full meshis required, meaning every IBGP router must peer with every other IBGP router in the AS. This can be done either directly or via route reflection or confederation.

Physical Full Mesh Not Required:The physical topology does not need to be a full mesh, but the BGP peering relationships must form a logical full mesh. Techniques like route reflectors or BGP confederations can reduce the need for manual full-mesh peering.

Juniper Reference:

IBGP Configuration: IBGP logical full mesh requirements can be simplified usingroute reflectorsto avoid the complexity of manually configuring many IBGP peers.