The Serial 0/0 interfaces on Router1 and Router2 are directly connected on the 192.168.51.48/30 network. You issue the following commands on Router1:
interface serial 0/0 ip ospf messagedigestkey 1 md5 b0s0n router ospf 1routerid 220.127.116.11 network 10.10.10.0 0.0.0.255 area 1 network 192.168.51.48 0.0.0.3 area 0 area 0
authenticationYou issue the following commands on Router2:
interface serial 0/0 ip ospf authenticationkey b0s0n router ospf 2routerid 18.104.22.168 network 10.10.20.0 0.0.0.255 area 2 network 192.168.51.48 0.0.0.3 area 0 area 0 authentication
Router1 and Router2 do not form an OSPF adjacency.
Which of the following is most likely the problem? (Select the best answer.)
- an OSPF area mismatch
- an OSPF authentication mismatch
- an OSPF process ID mismatch
- an OSPF router ID mismatch
Of the available choices, a mismatched authentication type is most likely to be the cause of the problem in this scenario. A mismatched authentication key or a mismatched authentication type could cause two Open
Shortest Path First (OSPF) routers to not form an adjacency. In this scenario, the Serial 0/0 interface on Router1 is configured to use a Message Digest 5 (MD5) authentication key of b0s0n. The Serial 0/0 interface on Router2, on the other hand, is configured to use a plaintext authentication key of b0s0n. If the correct authentication type were configured between the Serial 0/0 interfaces on the routers, OSPF authentication would succeed and an adjacency would be formed.
A mismatched process ID will not prevent an OSPF router from establishing an adjacency with a neighbor. An OSPF process ID is used to identify the OSPF process only to the local router. In this scenario, the router ospf 1 command has been issued on Router1, which configures Router1 with an OSPF process ID of 1. The router ospf 2 command has been issued on Router2, which configures Router2 with an OSPF process ID of 2. An OSPF area mismatch is not the reason that Router1 and Router2 do not form an adjacency in this scenario. In order to establish an adjacency, OSPF routers must be configured with the same area ID, Hello timer value, Dead timer value, and authentication password. In this scenario, the Serial 0/0 interface on Router1 has been configured to operate in area 0, which is also known as the backbone area. Similarly, the Serial 0/0 interface on Router2 has been configured to operate in area 0.
OSPF router IDs should never match between routers. A router ID is a unique 32bit identifier that resembles an IP address. A router ID conflict could cause routers to not form an adjacency. If you do not manually configure a router ID on an OSPF router, then the router ID is the highest IP address configured among loopback interfaces on the router, even if a physical interface is configured with a higher IP address. Cisco recommends using a loopback interface instead of a physical interface for the router ID? a loopback interface is never in the down state, thus OSPF is considered to be more stable when the router ID is configured from the IP address of a loopback interface. In this scenario, the router IDs on Router1 and Router2 have been manually configured by using the routerid ipaddresscommand.