Refer to the exhibit.
R2 and R3 are routers connected using Ethernet services from a service provider and can receive
pings from each other. OSPF is configured as the routing protocol but adjacency is not happening.
According to the output of the show commands in the exhibit, what could be the most likely cause
of the problem?
A.
Ethernet interfaces were configured as point-to-point.
B.
Process IDs are not matching.
C.
Configured bandwidths do not match on both interfaces.
D.
Broadcasts and multicast are not being propagated over the Ethernet services.
E.
OSPF cost does not match on both interfaces.
Explanation:
OSPF Adjacencies
Occurs through exchange of Hello packets
After adjacency established, link-state databases (LSDBs) are synched
Two OSPF neighbors on point-to-point link form full adjacency with each other
In LANs, all routers form adjacency with the DR and BDR; updates need to be sent only to DR,
which updates all other routers; all other routers on LAN are called DROTHERS and maintain a
partial neighbor relationship with each other
Once adjacencies are established, LSAs are exchanged through a reliable mechanism. LSAs are
flooded to ensure topological awareness. LSAs have a sequence number and a lifetime value.
LSAs convey the cost of links used for the SPF calculation. The cost metric is based on interface
bandwidth. The LSA aging timer is 30-minute default.
Here are the details of the exchange process between two routers on a LAN (Router 1 and Router
2) and the OSPF adjacency states involved:
Step 1 Router 1 begins in the down state because it is not exchanging OSPF information with any
other router.
It sends Hello packets via multicast address 224.0.0.5(all SPF).
Step 2 Router 2 receives the OSPF Hello and adds Router 1 in its list of neighbors. This is the
beginning of the init state.
Step 3 Router 2 sends a unicast Hello packet response to Router 1.
Step 4 Router 1 receives the Hello and notes that it is listed in the packet. It ads Router 2 to its list
of neighbors. Router 1 knows that it has bidirectional communication with Router 2. This is known
as the Two-Way State.
Step 5 In LAN environment, DR and BDR elected
Step 6 In LAN environment, Hello packets function as keepalive mechanism every 10 seconds.Cisco General Network Theory Quick Reference Sheets