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
authenticationkey b0s0n router
ospf 1 routerid 1.1.1.1
network 10.10.10.0 0.0.0.255 area 1
network 192.168.51.48 0.0.0.3 area 1
area 0 authentication
You issue the following commands on Router2:
interface serial 0/0 ip ospf
authenticationkey b0s0n router
ospf 2routerid 2.2.2.2 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.)

A.
an OSPF area mismatch

B.
an OSPF authentication mismatch

C.
an OSPF process ID mismatch

D.
an OSPF router ID mismatch

Explanation:
Of the available choices, an Open Shortest Path First (OSPF) area mismatch is most likely the reason thatRouter1 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 1. The
Serial 0/0 interface on Router2 has been configured to operate in area 0, which is also known as the backbone
area.
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.
Although a mismatched authentication key or a mismatched authentication type could cause two OSPF routers
to not form an adjacency, the OSPF authentication type and key in this scenario are correctly configured. The
Serial 0/0 interface on Router1 is configured to use an authentication key of b0s0n. The Serial 0/0 interface on
Router1 is also configured to use an authentication key of b0s0n. In addition, each router’s OSPF process is
configured to use plaintext authentication in OSPF Area 0. If the correct area were configured between the
Serial 0/0 interfaces on the routers, OSPF authentication would succeed.
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.

Cisco: Sample Configuration for Authentication in OSPF: Configurations for Plain Text Authentication