BGP Confederation

IBGP requires a full mesh of peerings to prevent routing loops.
As your network grows, maintaining all those sessions becomes complex.

Figure 1 – AS 65000 before the split: four routers, six iBGP sessions

The Full Mesh Problem

You have two solutions to break the full mesh requirement:

• Route Reflectors: One router reflects updates between its clients.

• Confederations: Divide one large AS into smaller, independent sub-ASes.

Figure 2 – Principle of BGP Confederation

Sub-AS Division

Confederations solve this by dividing AS 65000 into sub-AS 65001 and sub-AS 65002.

The full mesh does not disappear. It still applies inside each sub-AS, on a much smaller group of routers.

Route Reflectors and Confederations differ mainly in how heavily you must redesign your network topology.

RR vs Confederation

• Route Reflectors: (RFC 4456) Easy to deploy. AS numbers never change. They work well in simple topologies but can cause sub-optimal routing in complex networks.

• Confederations: (RFC 5065) Require a full BGP redesign, since every router changes AS number. They use eBGP-like rules between sub-ASes for better path control.

Figure 3 – One AS divided into two sub-ASes with session types

iBGP vs Confed eBGP

Each sub-AS keeps its own iBGP mesh:

• R1 and R2 establish an iBGP session in sub-AS 65001.

• R3 and R4 establish an iBGP session in sub-AS 65002.

The link between R2 and R3 uses confed eBGP.
This session acts like eBGP to bypass split horizon, but it keeps internal attributes intact.

Session Count

Two iBGP sessions, plus one confed eBGP.
The same four routers needed six in a single full mesh, so you get three sessions instead of six.

Confederations hide all internal sub-AS details from external peers.
Your entire network still looks like a single Autonomous System to the outside world.

Topology Hiding

External router R5 belongs to AS 65100 and connects to R4 via standard eBGP.

Figure 4 – AS 65000 split into sub-ASes facing external AS 65100

R5 sees only main AS 65000 and has zero visibility into sub-AS 65001 or sub-AS 65002.

Sub-AS Attribute Rules

As the update for LAN 192.168.1.0/24 travels from R1 to R5, specific rules modify its parameters:

• Rule 1 (Attribute Preservation): Across confed eBGP between sub-AS 65001 and sub-AS 65002, Next-Hop and Local Preference stay unchanged.

• Rule 2 (Sub-AS Stripping): Internal updates carry sub-AS numbers in parentheses, like AS_PATH: (65001). R4 strips them before sending the update to R5. They never count in the AS_PATH length.

Figure 5 – BGP update path through sub-ASes