Question

An ABR named R2 connects area 10 (containing subnets 172.16.0.0/24 through 172.16.3.0/24) to area 0. You configure 'area 10 range 172.16.0.0 255.255.252.0' on R2 to reduce the number of Type 3 LSAs advertised into the backbone. After the change, engineers notice R2 has installed a new route in its own routing table: '172.16.0.0/22 is a summary, 00:00:15, Null0'. A junior engineer asks why this route appeared and what purpose it serves. What is the correct explanation?

Accepted Answer

The Null0 discard route is automatically generated by the ABR summarization to prevent routing loops for traffic destined to summarized but nonexistent subnets within the range

Answer

The Null0 route indicates a misconfiguration; the area range command should never install a route on the ABR itself and points to an LSDB corruption

Answer

The Null0 route means all four component subnets have gone down, so R2 is blackholing the entire summary until they recover

Answer

The Null0 route is created because 'no discard-route internal' was configured, forcing R2 to drop inter-area traffic