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350-401 · Question #623

A network engineer is configuring OSPF on a router. The engineer wants to prevent having a route to 177.16.0.0/16 learned via OSPF. In the routing table and configures a prefix list using the command

The correct answer is A. distribute-list prefix OFFICE in under the OSPF process B. ip prefix-list OFFICE seq 10 permit 0.0.0.0/0 Ie 32. To prevent a specific route from being learned via OSPF while allowing others, a deny entry in a prefix list must be applied as an inbound distribute-list, and a permit all entry must be added to the prefix list to avoid implicitly denying all other routes.

Submitted by amina.ke· Mar 6, 2026Infrastructure

Question

A network engineer is configuring OSPF on a router. The engineer wants to prevent having a route to 177.16.0.0/16 learned via OSPF. In the routing table and configures a prefix list using the command ip prefix-list OFFICE seq 5 deny 172.16.0.0/16. Which two identical configuration commands must be applied to accomplish the goal? (Choose two)

Options

  • Adistribute-list prefix OFFICE in under the OSPF process
  • Bip prefix-list OFFICE seq 10 permit 0.0.0.0/0 Ie 32
  • Cip prefix-list OFFICE seq 10 permit 0.0.0.0/0 ge 32
  • Ddistribute-list OFFICE out under the OSPF process
  • Edistribute-list OFFICE in under the OSPF process

How the community answered

(15 responses)
  • A
    47% (7)
  • C
    33% (5)
  • D
    7% (1)
  • E
    13% (2)

Why each option

To prevent a specific route from being learned via OSPF while allowing others, a `deny` entry in a prefix list must be applied as an inbound distribute-list, and a `permit all` entry must be added to the prefix list to avoid implicitly denying all other routes.

Adistribute-list prefix OFFICE in under the OSPF processCorrect

The `distribute-list prefix OFFICE in` command, applied under the OSPF router configuration, instructs the OSPF process to filter incoming routing updates using the `OFFICE` prefix list, thereby preventing the `172.16.0.0/16` route from being installed into the routing table.

Bip prefix-list OFFICE seq 10 permit 0.0.0.0/0 Ie 32Correct

Prefix lists have an implicit `deny all` at the end; therefore, to ensure that routes other than `172.16.0.0/16` are still permitted and learned, an explicit `permit 0.0.0.0/0 le 32` entry must be added to the `OFFICE` prefix list after the deny statement.

Cip prefix-list OFFICE seq 10 permit 0.0.0.0/0 ge 32

`ge 32` (greater than or equal to 32) would only permit host routes, which would deny most other prefixes; `le 32` (less than or equal to 32) is needed to permit all prefix lengths.

Ddistribute-list OFFICE out under the OSPF process

Applying the distribute-list `out` would filter routes *advertised* by this router, not routes *learned* by it, which is the goal.

Edistribute-list OFFICE in under the OSPF process

This option is identical to option A, providing the same correct configuration command but not a second distinct command.

Concept tested: OSPF route filtering with prefix lists

Source: https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_pi/configuration/15-mt/iri-15-mt-book/iri-cfg-prefix-list.html

Topics

#OSPF#Route Filtering#Prefix Lists#Distribute Lists

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