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OSPF Practice Questions

15 original questions covering ENARSI 1.10.a to 1.10.d. Most are exhibit-based, like the exam. Answer every one before opening any explanation. Target: 12 of 15 (80%).

1. Blueprint 1.10.b

Refer to the exhibit. R1 and R2 are connected on GigabitEthernet0/1. What is the most likely cause of the problem?

R1# show ip ospf neighbor
Neighbor ID     Pri   State           Dead Time   Address         Interface
10.255.255.2      1   INIT/DROTHER    00:00:33    10.1.12.2       GigabitEthernet0/1

R2# show ip ospf neighbor
R2#
  • A. The hello timers do not match
  • B. The two routers are in different areas
  • C. The interface MTUs do not match
  • D. Something drops R1's hellos before R2 processes them, such as an inbound ACL on R2
Answer: D

R1 receives R2's hellos (so it lists R2), but those hellos do not contain R1's router ID, which means R2 never accepted a hello from R1. That is one-way communication, typically an inbound ACL or CoPP policy on R2. Timer or area mismatches (A, B) would make R1 discard R2's hellos too, so R1 would not list R2 at all. An MTU mismatch (C) shows up later, in EXSTART or EXCHANGE.

2. Blueprint 1.10.b, 1.10.c

Five routers share one Ethernet segment with default OSPF settings. On R4, show ip ospf neighbor shows two neighbors in FULL/DR and FULL/BDR and two neighbors in 2WAY/DROTHER. What should the engineer do?

  • A. Set the network type to point-to-point on all five routers
  • B. Clear the OSPF process on the DR
  • C. Nothing, because this is normal for a DROTHER
  • D. Raise R4's priority so that it forms full adjacencies
Answer: C

A DROTHER forms FULL adjacencies only with the DR and BDR and stays in 2WAY with other DROTHERs. Point-to-point (A) supports only one neighbor and breaks a five-router segment. Changing priority (D) or clearing the DR (B) is not needed because nothing is wrong.

3. Blueprint 1.10.b

Refer to the exhibit. R1 and R2 have lost their adjacency after a change window. What is the cause?

R1# debug ip ospf adj
OSPF-1 ADJ   Gi0/1: Rcv pkt from 10.1.12.2 : Mismatched Authentication type. Input packet specified type 0, we use type 2
  • A. R2 uses a different MD5 key string
  • B. R2 has no authentication enabled on the link, while R1 uses cryptographic authentication
  • C. R2 uses plain text authentication
  • D. R1 and R2 use different MD5 key IDs
Answer: B

Type 0 is null authentication and type 2 is cryptographic (MD5 or HMAC-SHA). R2 sends unauthenticated packets while R1 expects cryptographic ones. A wrong key string or key ID (A, D) produces a Mismatched Authentication Key message with both sides on type 2. Plain text (C) would be type 1.

4. Blueprint 1.10.b, 1.10.c

Refer to the exhibit. Area 0 uses MD5 authentication. After the change below was applied on all area 0 routers, every physical adjacency stayed up, but routes from area 3 disappeared from area 0. What fixes the problem?

R2(config)# router ospf 1
R2(config-router)# area 0 authentication message-digest

R2# show ip ospf virtual-links
Virtual Link OSPF_VL0 to router 10.255.255.4 is down
  Transit area 1, via interface GigabitEthernet0/2
  • A. Configure area 1 authentication message-digest on R2 and R4
  • B. Add message-digest-key 1 md5 <key> to the area 1 virtual-link 10.255.255.4 command on R2, and the matching key on R4
  • C. Change the transit area to area 3
  • D. Remove the virtual link and make area 3 a stub area
Answer: B

A virtual link is an area 0 interface, so it inherits area 0 authentication. The physical area 0 interfaces already had keys, but the virtual link did not. Area 1 authentication (A) is unrelated to the virtual link. The transit area is the shared area 1, not the disconnected area 3 (C). Making area 3 a stub (D) does not connect it to the backbone.

5. Blueprint 1.10.a

Refer to the exhibit. R2 must exchange IPv4 and IPv6 routes with R3 over Gi0/2 using OSPFv3. Which statement explains the output?

R2# show ospfv3 neighbor

          OSPFv3 1 address-family ipv4 (router-id 10.255.255.2)

Neighbor ID     Pri   State           Dead Time   Interface ID    Interface
10.255.255.1      0   FULL/  -        00:00:35    3               GigabitEthernet0/1

          OSPFv3 1 address-family ipv6 (router-id 10.255.255.2)

Neighbor ID     Pri   State           Dead Time   Interface ID    Interface
10.255.255.1      0   FULL/  -        00:00:31    3               GigabitEthernet0/1
10.255.255.3      0   FULL/  -        00:00:38    4               GigabitEthernet0/2
  • A. R3 has no IPv6 link-local address on its link to R2
  • B. OSPFv3 cannot carry IPv4 routes on a point-to-point link
  • C. The router IDs of R2 and R3 are the same
  • D. The Gi0/2 link is not in the OSPFv3 IPv4 address family on R2 or on R3
Answer: D

Each OSPFv3 address family is a separate instance with its own neighbor table. R3 is FULL in the IPv6 address family, so the link-local address and basic parameters are fine (A), and a duplicate router ID would stop both (C). The missing IPv4 adjacency means one end does not have ospfv3 1 ipv4 area <n> on that interface (or the IPv4 address family is passive there). Network type has nothing to do with it (B).

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