IBGP full mesh requirement

BGP uses the AS_PATH as a loop detection and prevention mechanism because the ASN is prepended when advertising to an EBGP neighbor. IBGP peers do not prepend their ASN to the AS_PATH because the NLRIs would fail the validity check and would not install the prefix in the IP routing table.

No other method exists to detect loops with IBGP sessions. RFC 4271 prohibits the advertisement of an NLRI received from one IBGP peer to another. RFC 4271 states that all BGP routers within a single AS must be fully meshed to provide a complete loop-free routing table and prevent traffic black holing.

In the figure above, R1, R2, and R3 are all within AS65100.
  • R1 has an IBGP session with R2 and R2 has an IBGP session with R3.

  • R1 advertises the prefix to R2, which is processed and inserted into the R2 BGP table.

  • R2 does not advertise the NLRI to R3 because it received the prefix from an IBGP peer.

  • To resolve this issue, R1 must form a multi-hop IBGP session so that R3 can receive the prefix directly from R1.