Scenario
Refer to the topology. Your company has decided to connect the main office with three other remote branch offices using point-to-point serial links.
You are required to troubleshoot and resolve OSPF neighbor adjacency issues between the main office and the routers located in the remote branch offices.
Use appropriate show commands to troubleshoot the issues and answer all four questions.
Instructions
An OSPF neighbor adjacency is not formed between R3 in the main office and R5 in the Branch2 office. What is causing the problem?
A.
There is an area ID mismatch.
B.
There is a PPP authentication issue; a password mismatch.
C.
There is an OSPF hello and dead interval mismatch.
D.
There is a missing network command in the OSPF process on R5.
Explanation:
Here are the relevant parts of the router configs:
R1interface Loopback0description ***Loopback***ip address 192.168.1.1 255.255.255.255ip ospf 1 area 0!interface Ethernet0/0description **Connected to R1-
LAN**ip address 10.10.110.1 255.255.255.0ip ospf 1 area 0!interface Ethernet0/1description **Connected to L2SW**ip address 10.10.230.1 255.255.255.0ip ospf
hello-interval 25ip ospf 1 area 0!router ospf 1log-adjacency-changes
R2!interface Loopback0description **Loopback**ip address 192.168.2.2 255.255.255.255ip ospf 2 area 0!interface Ethernet0/0description **Connected to R2-
LAN**ip address 10.10.120.1 255.255.255.0ip ospf 2 area 0!interface Ethernet0/1description **Connected to L2SW**ip address 10.10.230.2 255.255.255.0ip ospf 2
area 0!router ospf 2log-adjacency-changes
R3username R6 password CISCO36!interface Loopback0description **Loopback**ip address 192.168.3.3 255.255.255.255ip ospf 3 area 0!interface
Ethernet0/0description **Connected to L2SW**ip address 10.10.230.3 255.255.255.0ip ospf 3 area 0!interface Serial1/0description **Connected to R4-Branch1
office**ip address 10.10.240.1 255.255.255.252encapsulation pppip ospf 3 area 0!interface Serial1/1description **Connected to R5-Branch2 office**ip address
10.10.240.5 255.255.255.252encapsulation pppip ospf hello-interval 50ip ospf 3 area 0!interface Serial1/2description **Connected to R6-Branch3 office**ip address
10.10.240.9 255.255.255.252encapsulation pppip ospf 3 area 0ppp authentication chap!router ospf 3router-id 192.168.3.3!
R4!interface Loopback0description **Loopback**ip address 192.168.4.4 255.255.255.255ip ospf 4 area 2!interface Ethernet0/0ip address 172.16.113.1
255.255.255.0ip ospf 4 area 2!interface Serial1/0description **Connected to R3-Main Branch office**ip address 10.10.240.2 255.255.255.252encapsulation pppip
ospf 4 area 2!router ospf 4log-adjacency-changes
R5!interface Loopback0description **Loopback**ip address 192.168.5.5 255.255.255.255ip ospf 5 area 0!interface Ethernet0/0ip address 172.16.114.1
255.255.255.0ip ospf 5 area 0!interface Serial1/0description **Connected to R3-Main Branch office**ip address 10.10.240.6 255.255.255.252encapsulation pppip
ospf 5 area 0!router ospf 5log-adjacency-changes
R6username R3 password CISCO36!interface Loopback0description **Loopback**ip address 192.168.6.6 255.255.255.255ip ospf 6 area 0!interface Ethernet0/0ip
address 172.16.115.1 255.255.255.0ip ospf 6 area 0!interface Serial1/0description **Connected to R3-Main Branch office**ip address 10.10.240.10
255.255.255.252encapsulation pppip ospf 6 area 0ppp authentication chap!router ospf 6router-id 192.168.3.3!
Continue checking their connected interfaces with the “show running-config” command:
R3#show running-config<<output omitted>>!interface Serial1/1description **Connected to R5-Branch2 office**ip address 10.10.240.5
255.255.255.252encapsulation pppip ospf hello-interval 50ip ospf 3 area 0!<<output omitted>>
R5#show running-config<<output omitted>>!interface Serial1/0description **Connected to R3-Main Branch office**ip address 10.10.240.6
255.255.255.252encapsulation pppip ospf 5 area 0!<<output omitted>>
The only difference we can see here is the line “ip ospf hello-interval 50″ on R3. This command sets the number of seconds R3 waits before sending the next hello
packet out this interface. In this case after configuring this command, R3 will send hello packets to R5 every 50 seconds. But the default value of hello-interval is 10
seconds and R5 is using it. Therefore we can think of a hello interval mismatch problem here. You can verify with the “show ip ospf interface <interface>” command
on each router.
R3#sh ip ospf int s1/1Serial1/1 is up, line protocol is upInternet Address 10.10.240.5/30, Area 0Process ID 3, Router ID 192.168.3.3, Network Type
POINT_TO_POINT, Cost: 64Enabled by interface config, including secondary ip addressesTransmit Delay is 1 sec, State POINT_TO_POINT,Timer intervals
configured, Hello 50, Dead 200, Wait 200, Retransmit 5oob-resync timeout 200Hello due in 00:00:28Supports Link-local Signaling (LLS)Index 2/2, flood queuelength 0Next 0×0(0)/0×0(0)Last flood scan length is 0, maximum is 0Last flood scan time is 0 msec, maximum is 0 msecNeighbor Count is 0, Adjacent neighbor
count is 0Suppress hello for 0 neighbor(s)
R5#sh ip ospf int s1/0Serial1/0 is up, line protocol is upInternet Address 10.10.240.6/30, Area 0Process ID 5, Router ID 10.10.240.6, Network Type
POINT_TO_POINT, Cost: 64Enabled by interface config, including secondary ip addressesTransmit Delay is 1 sec, State POINT_TO_POINT,Timer intervals
configured, Hello 10, Dead 40, Wait 40, Retransmit 5oob-resync timeout 40Hello due in 00:00:04Supports Link-local Signaling (LLS)Index 1/1, flood queue length
0Next 0×0(0)/0×0(0)Last flood scan length is 0, maximum is 0Last flood scan time is 0 msec, maximum is 0 msecNeighbor Count is 0, Adjacent neighbor count is
0Suppress hello for 0 neighbor(s)
So we can see both hello and dead interval are mismatched because the dead interval always four times the value of hello interval, unless you manually configure
the dead interval (with the ip ospf dead-interval <seconds> command).