iBGP Split Horizon Rule

Introduction

• Presented by Lazarus at telecom.Tech.
• Focus on iBGP split horizon rule.

Definition of iBGP Split Horizon Rule

• States that an iBGP router must not advertise any route learned from one iBGP peer to another iBGP peer.
• Different from distance vector protocols (EIGRP, RIP) where routes learned from neighbors can be advertised to other neighbors.

Purpose of the Rule

• Avoids routing loops and suboptimal routing.
• Ensures each router receives route information only from the iBGP peer that is the source of the route.

Full Mesh Peering Requirement

• Each iBGP router in an Autonomous System (AS) must peer with every other iBGP router (full mesh peering).
• Example topology with R1, R2, R3, and R4 illustrates full mesh.
• R1 can advertise the 20.20.20.0/24 network without violating the split horizon rule.

Consequences of Violating the Rule

• If R2 re-advertises the route to R3 or R4, it leads to suboptimal routing.
• All routers in the AS could erroneously believe multiple paths exist for the same network, causing confusion and routing loops.

Importance of Route Source

• R1, as the source of 20.20.20.0/24, is the only router that should advertise that route.
• If AS size increases (e.g., 10 routers), the chaos increases without enforcing the split horizon rule.

Automatic Enforcement

• The iBGP split horizon rule is automatically operational in routers from most vendors (including Cisco).

Exceptions to the Rule

1. Route Reflectors

   • Certain routers can peer only with a route reflector instead of all other routers.
   • Route reflectors can re-advertise routes learned from clients, violating the split horizon rule.

2. BGP Confederations

   • Introduces sub-ASes within an AS.
   • Routers can re-advertise iBGP-learned routes between sub-ASes without violating the split horizon within each sub-AS.

3. Selective Route Advertisements

   • Necessary for traffic engineering, load balancing, or policy controls.
   • May involve bypassing the split horizon rule to ensure availability of necessary routes.

4. Multipath Load Sharing

   • Involves advertising multiple paths to the same destination over iBGP.
   • Helps in redundancy and load balancing but must be done carefully to avoid routing loops.

Conclusion

• Summary of the iBGP split horizon rule and its importance.
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