Refer to the following.
Router # sh ip route eigrp
13.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
D 13.0.0.0/8 is a summary, 00:00:32, Null0
What happens to packets that are forwarded from the 13.0.0.0/8 network to the Null0
interface?
A.
Flagged
B.
Accepted
C.
Summarized
D.
Dropped
Explanation:
When an EIGRP router summarizes, it automatically builds a route to null0 for the
summarized route. The router to null0 prevents packets that do not match a specific entry in
the routing table from following a default route. (The route to null0 causes the packet to be
dropped).
Explanation forgets to MENTION that the MORE Specific route is what is used.
The logical effect here matters as you may have discontiguous networks.
10.10.1.0 and 10.10.3.0 with a summary of 10.10.0.0/22
hence things destined for 10.10.0.x/24 and 10.10.2.0/24 would arrive at the router…but there IS NO MORE SPECIFIC route for those two blocks…so they’d match ONLY the summary and be NULL0 routed….on purpose: dumping the traffic so it does not clutter up the queue or waste processor time.
Iit makes a lot of functional sense when you chew on the thought a bit.
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