Question

R1 is an EIGRP border router advertising a manual summary route of 10.10.0.0/16 out its interface toward R2. The summary covers subnets 10.10.1.0/24 through 10.10.8.0/24, all of which are reachable via R1. A network engineer notices that when R2 sends traffic destined to 10.10.20.5 (an address inside the summary range but with no matching specific subnet on R1), the packets are silently dropped at R1 instead of being forwarded elsewhere. Why does R1 drop this traffic?

Accepted Answer

When a manual summary is configured, EIGRP installs a route to Null0 for the summary prefix, so traffic matching the summary but no more-specific route is discarded.

Answer

R1's administrative distance for the summary (5) is lower than the connected routes, causing a routing loop that drops the packet.

Answer

The summary route must be advertised with a leak-map; without one, EIGRP blocks all traffic inside the summarized range.

Answer

Split horizon on R1 prevents the summary from being installed in the local routing table, so R1 has no route to 10.10.20.5.