Due to the conflicting opinions here I tested it in GNS3. I tried altering the MTU, hello timers and the area. The only one that produced this problem is the area change.
R1 (10.0.0.1) has changed its area ID from 0 to 1, and it can no longer form a neighbor relationship with R2 (10.0.0.2), which is located in the backbone area. All OSPF areas must be connected (physically or virtually) to the backbone area (area 0).
cisco_R3(config-router)#network 192.168.255.3 0.0.0.0 area 1
cisco_R3(config-router)#
*Dec 18 19:46:09.590: %OSPF-5-ADJCHG: Process 1, Nbr 192.168.255.2 on Ethernet0/0.10 from FULL to DOWN, Neighbor Down: Interface down or detached
cisco_R3(config-router)#
*Dec 18 19:46:09.590: %OSPF-6-AREACHG: 192.168.255.3/32 changed from area 0 to area 1
cisco_R3(config-router)#
*Dec 18 19:46:14.061: %OSPF-4-ERRRCV: Received invalid packet: mismatched area ID from backbone area from 192.168.255.2, Ethernet0/0.10
cisco_R3(config-router)#
*Dec 18 19:46:23.896: %OSPF-4-ERRRCV: Received invalid packet: mismatched area ID from backbone area from 192.168.255.2, Ethernet0/0.10
cisco_R3(config-router)#
cisco_R3(config-router)#do s runn | s ospf
router ospf 1
passive-interface default
no passive-interface Ethernet0/0.10
network 192.168.255.3 0.0.0.0 area 1
network 0.0.0.0 255.255.255.255 area 0
cisco_R3(config-router)#
cisco_R3(config-router)#no network 192.168.255.3 0.0.0.0 area 1
cisco_R3(config-router)#
*Dec 18 19:48:21.676: %OSPF-6-AREACHG: 192.168.255.3/32 changed from area 1 to area 0
cisco_R3(config-router)#
*Dec 18 19:48:21.752: %OSPF-5-ADJCHG: Process 1, Nbr 192.168.255.2 on Ethernet0/0.10 from LOADING to FULL, Loading Done
cisco_R3(config-router)#
Wow,
Yes, the root cause is area change. And this log message is not generated because of area change configuration. It is because of the reception of the Hello Packet.
I was tricked too, so I believe the answer is D.
But this message is
Looking more closely again:
---- there are three facilities that generated, particullarly:
1 --- AREACHG: for configuration change of the area
2 --- ERRRCV: for the hello packet
It is confusing, take your chance
I think A is correct and not D, because is asking for the "cause" of the log, and the cause of the log was triggered by an area change, the consecuence of the event cause a drop of adjacency.
Into the hello packet is the area ID, the area ID must be matched between peers to establish a relationship. In this way, a mismatch of the hello packet is the correct answer for me.
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