Which of the following items are NOT required to match for two routers to form an OSPF adjacency?
A.
Area IDs
B.
Hello/Dead timers
C.
Passwords (if OSPF authentication has been configured)
D.
Process IDs
Explanation:
All of the listed items must match except for the process IDs. The process IDs are locally significant, which keeps multiple instances of OSPF separate on a router,
and do not need to match between neighboring routers for the adjacency to form. Process identifiers can be valued from 1 to 65535.
Adjacencies must be formed before routing updates can be exchanged. OSPF routers will form neighbor adjacencies on common subnets if the following three
items match:
Area IDs
Hello/Dead timers
Passwords (if OSPF authentication has been configured)
Once an adjacency has been formed it will be maintained by the exchange of Hello messages. On a broadcast medium like Ethernet, they will be sent every 10
seconds. On point-to-point links, they will be sent every 30 seconds.
The show ip ospf interface interface number command can be used to display the state of the DR/BDR election process.
Consider the following output:
The timer intervals’ configured output reveals that RouterA is showing a Hello timer of 10 seconds and a Dead timer of 40 seconds. RouterB has a Hello timer of 30
seconds and a Dead timer of 60 seconds. Hello/Dead timers have to match before OSPF routers will form an adjacency. If you executed the debug ip ospf events
command on one of the routers, the router at serial /01 will not form a neighbor relationship because of mismatched hello parameters:
RouterA# debug ip ospf events
OSPF events debugging is on
RouterA#
*Nov 9 05:41:21.456:OSPF:Rcv hello from 10.16.2.3 area 0 from Serial0/1
192.168.35.1
*Nov 9 05:41:21.698:OSPF:Mismatched hello parameters from
192.168.35.1
Hellos are used to establish neighbor adjacencies with other routers. On a point-to-point network, hello packets are sent to the multicast address 224.0.0.5, which is
also known as the ALLSPFRouters address.Area IDs have to match for OSPF routers to form an adjacency. Both of these routers have the interface correctly configured in matching Area 0.
The interface priorities do not have to match for OSPF routers to form an adjacency. Interface priorities can be configured to control which OSPF router becomes
the designated router (DR) or backup designated router (BDR) on a multi-access network segment.
Objective:
Routing Fundamentals
Sub-Objective:
Configure, verify, and troubleshoot single area and multi-area OSPFv2 for IPv4 (excluding authentication, filtering, manual summarization, redistribution, stub,
virtual-link, and LSAs)Cisco > Home > Support > Technology Support > IP > IP Routing > Design > Design TechNotes > OSPF Neighbor Problems Explained
