QuestionQ258

Routing Policy and Manipulation

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Refer to the exhibit. An engineer configured R1 to stop network 192.168.1.0/24 from being propagated outside area 5. After the change, users report that they cannot access any application servers that had worked previously. When checking the peer router’s routing table, the engineer finds that R1 has stopped propagating all routes outside area 5. What action must be taken to correct the issue?

  • A Change the prefix-list action to permit and add an explicit deny statement for network 192.168.1.0/24
  • B Add a permit statement for 0.0.0.0/0 le 32 to the end of the prefix-list to override the implicit deny
  • C Reconfigure the filter-list statement to apply in the inbound direction
  • D Configure an additional filter-list on R1 for inbound traffic to allow external routes into area 5
Explanation

A Cisco prefix list has an implicit deny for routes that do not match a permitted entry. The existing entry explicitly denies 192.168.1.0/24 and its more-specific prefixes (le 32), but it also implicitly denies every other route. Adding permit 0.0.0.0/0 le 32 after the deny permits all remaining IPv4 routes while the earlier, more-specific deny continues to block 192.168.1.0/24 from outbound OSPF advertisement.

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