Question

A network engineer reports that R3 is missing several specific subnets of 172.16.0.0/16 learned via EIGRP AS 100. The topology uses discontiguous subnets of 172.16.0.0/16 separated by a 10.0.0.0/8 backbone. On R1 and R2, 'show ip protocols' shows 'auto-summary' is in effect. Both R1 and R2 advertise the classful summary 172.16.0.0/16 toward each other across the 10.x backbone. What is the most likely cause of R3's missing routes, and what corrects it?

Accepted Answer

Auto-summarization on R1 and R2 causes both routers to advertise 172.16.0.0/16, creating an ambiguous classful summary; disable it with 'no auto-summary' under the EIGRP process on all routers.

Answer

Split horizon is dropping the specific subnets on the backbone interfaces; disable split horizon with 'no ip split-horizon eigrp 100' on the WAN interfaces.

Answer

The EIGRP administrative distance for internal routes is too high; lower it with 'distance eigrp 90 170' under the process.

Answer

A route map is filtering the specific subnets inbound; remove the distribute-list referencing the route map on R3.