iBGP & Route Reflection

What you'll explore

1. 01

   Why iBGP Demands a Full Mesh

   eBGP prevents loops with the AS_PATH — a router drops any route already carrying its own AS. Inside an AS the AS number never changes, so that mechanism is blind. iBGP's answer is a hard rule: a route learned from an iBGP peer is never re-advertised to another iBGP peer. Correct for loops, brutal for scale — every router must therefore peer directly with every other, which is n*(n-1)/2 sessions. Ten routers is 45; a hundred is 4,950. The session count is computed server-side and shown in the cascade, not in the browser.

2. 02

   The Split-Horizon Trap

   The no-readvertise rule is iBGP split horizon, and it is exactly what breaks as the AS grows. Add one router and you don't add one session — you add a session to every existing router. The mesh grows quadratically while your team grows linearly, and a single missed session silently blackholes prefixes for the routers that never heard the UPDATE. This is the pain Route Reflection and Confederation exist to remove.

3. 03

   Route Reflection Lifts the Rule Safely

   A Route Reflector is allowed to do the forbidden thing — reflect an iBGP route to other iBGP peers — but only because it also adds two attributes so loops can still be caught. ORIGINATOR_ID records the router that first injected the route (a client ignores a reflection of its own origination); CLUSTER_LIST records every cluster the route passed through. Clients need no mesh among themselves: they peer only with the reflector, collapsing the session count dramatically.

4. 04

   Clusters & Multi-Cluster Loop Prevention

   Each reflector (or redundant reflector set) is a cluster with a CLUSTER_ID. When a reflector forwards a route it prepends its own CLUSTER_ID to the CLUSTER_LIST. If a reflector ever receives a route whose CLUSTER_LIST already contains its own CLUSTER_ID, the route looped and is dropped. This is what makes hierarchical, multi-cluster designs safe — and why a single reflector is a single point of failure that a redundant cluster removes.

5. 05

   Confederation — The Other Answer

   Confederation attacks the same full-mesh problem differently: it splits one AS into several sub-ASes that speak eBGP-like sessions to each other while each sub-AS runs its own small internal mesh. Loop prevention comes back via AS_PATH — the sub-AS numbers are carried in confederation segments, which the best-path comparison counts. It scales like Route Reflection but changes your AS_PATH semantics, so the two are rarely mixed. When to use which is a design judgment the cascade spells out.