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Routing & Switching · Intermediate

Split Horizon

A loop-prevention rule that stops a router from advertising a route out the interface on which it was learned.

Definition

Split horizon prevents simple two-node routing loops in distance-vector protocols such as RIP and EIGRP. It can cause reachability problems on hub-and-spoke NBMA or DMVPN networks, where the hub must re-advertise spoke routes out the same tunnel interface, so it is often disabled there. iBGP has its own analogous rule that forbids re-advertising iBGP-learned routes to other iBGP peers.

Example

no ip split-horizon eigrp 100 on a DMVPN hub tunnel interface lets spokes learn each other's routes.

Exam tip

On DMVPN hubs running EIGRP, split horizon must be disabled for spoke-to-spoke route visibility.

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