What is the split-horizon rule?

• A. Routes learned through iBGP are never propagated to other iBGP peers. This avoids routing loops.
• B. Routes learned via EBGP are never propagated to EBGP peers.
• C. All routes learned by iBGP are advertised to all iBGP peers.
• D. Routes learned through iBGP are always flooded to EBGP peers.

Answer: (A)

Split-horizon in BGP says that routes learned via an iBGP session are not advertised to other iBGP peers. This prevents routing loops in an iBGP mesh, since without a full mesh (or a route reflector), an iBGP-learned route could bounce between peers endlessly. Routes learned through iBGP can still be advertised to EBGP peers, and EBGP-learned routes can be shared with iBGP peers as needed. The rule is what makes a scalable iBGP design possible, often supplemented by route reflectors or confederations to avoid requiring a full mesh. The other statements are too absolute or inaccurate: EBGP routes are normally propagated to EBGP peers; iBGP routes are not forwarded to other iBGP peers; and iBGP routes are not inherently flooded to EBGP peers in all configurations.