Refer to the exhibit. ROUTE Enterprises has many stub networks in their enterprise network, such as router B and its associated network. EIGRP is to be implemented on router A so that neither the prefix for the S/0/0/0 interface nor the prefixes from router B appear in the routing tables for the router in the enterprise network.
Which action will accomplish this goal?
Select the best response.
A.
Declare router B a stub router using the eigrp stub command.
B.
Use the passive-interface command for interface Serial0/0/0.
C.
Use a mask with the network command to exclude interface Serial0/0/0.
D.
Implement a distribute list to exclude the link prefix from the routing updates.
Why not D
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Answer: C
Explanation
If we declare router B a stub router then the routers in Enterprise Network still learn about the network for S0/0/0 interface and the network behind router B -> A is not correct.
If we use the passive-interface command on s0/0/0 interface then router A & B can not become neighbor because they don’t exchange hello messages -> A can not send traffic to the network behind B -> B is not correct.
Theoretically, we can use a distribute list to exclude both the link prefix and the prefix from router B but it is not efficient because:
+ We have many stub networks so we will need a “long” distribute list.
+ We declare networks in stub routers (like router B) while filter them out at router A -> it is a waste.
I am not totally sure about answer C because if we “use a mask with the network command to exclude interface Serial0/0/0″ then router A and B can not become neighbors and the situation is same as answer B. But from many discussions about this question, maybe C is the best answer.
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Just Lab It up and you will see the only answer is D; distribution-lists are used to filter routes using ACLs or Prefix-lists, you can filter the local route connected to int s0/0/0 and also the learned routes from RB, even when there are a lot of spoke routers, if ip addressing was properly designed you can filter all those routes with just one line in a prefix-list.
If you use wilcard masks to exclude int s0/0/0, Router B and router A will not become neighbors and we DO need them to be neighbors.
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