300-410 Question #75: Real Exam Question with Answer & Explanation

Question

Refer to the exhibit. A junior engineer updated a branch router configuration. Immediately after the change, the engineer receives calls from the help desk that branch personnel cannot reach any network destinations. Which configuration restores service and continues to block 10.1.1.100/32? BRANCH-RTR# router eigrp 100 network 10.4.31.0 0.0.0.7 network 10.100.100.1 0.0.0.0 distribute-list route-map FILTER-IN in FastEthernet0/0 eigrp router-id 10.100.100.1 ! ip prefix-list 102 seq 10 permit 10.1.1.100/32 ! route-map FILTER-IN deny 10 match ip address prefix-list 102 !

Options

• Aroute-map FILTER-IN deny 5
• Bip prefix-list 102 seq 15 permit 0.0.0.0/32 le 32
• Cip prefix-list 102 seq 5 permit 0.0.0.0/32 le 32
• Droute-map FILTER-IN permit 20

Explanation

The current route-map contains only a 'deny' statement, causing an implicit 'deny all' at the end that blocks all other routes, so adding a 'permit' statement ensures desired routes are accepted.

Common mistakes.

• A. Adding another deny statement at a lower sequence number would not resolve the issue of the implicit deny all blocking all valid routes.
• B. Modifying the ip prefix-list 102 with a permit 0.0.0.0/32 le 32 at seq 15 would add a permit-all entry after the specific permit, but since the route-map's deny 10 matches this prefix-list, it would then deny all traffic, worsening the problem.
• C. Modifying the ip prefix-list 102 with a permit 0.0.0.0/32 le 32 at seq 5 would cause everything to match prefix-list 102, which is then used by deny 10 in the route-map, resulting in all routes being denied.

Concept tested. Route-map implicit deny, distribute-list

Reference. https://www.cisco.com/c/en/us/td/docs/ios/iproute/command/ip_route_c_book/irc_pi1.html#wp1010372

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