Question Of The Day: 20 May, 2008

Topic: OSPF

You’ve rolled out your new OSPF auto-cost command to your network.  It looks like one of your colleagues has adjusted the OSPF cost of interface FastEthernet0/0.  That interface should have an OSPF cost of 1000.  What will it’s OSPF cost be based on the following configuration:

interface FastEthernet0/0
 description ->r2 fa0/0
 ip address 100.1.12.1 255.255.255.0
 ip ospf cost 10
 !
router ospf 100
 router-id 1.1.1.1
 auto-cost reference-bandwidth 100000
 network 100.1.12.1 0.0.0.0 area 0
 network 100.1.12.1 0.0.0.0 area 0

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Yesterday’s Question

Question Of The Day: 19 May, 2008 

You’ve rolled out your new OSPF auto-cost command to your network.  It looks like you’ve missed a router.  r1 is peered with r2 over an Ethernet connection.  r1 has been updated with the new reference bandwidth, but r2 has not.

r1
router ospf 100
 auto-cost reference-bandwidth 100000
 network 10.1.12.1 0.0.0.0 area 0
!
interface FastEthenet 0/0
 ip address 10.1.12.1 255.255.255.0

r2
router ospf 100
 network 10.1.12.2 0.0.0.0 area 0
!
interface FastEthenet 0/0
 ip address 10.1.12.2 255.255.255.0

Will these routers form an OSPF neighbor adjacency?

Answer: Yes, the routers will form an OSPF adjacency.

r1 is peered with r2:

r1(config)#do show ip ospf neighbor

Neighbor ID     Pri   State           Dead Time   Address         Interface
2.2.2.2           0   FULL/  -        00:00:37    10.1.12.2       FastEthernet0/0

r1(config-router)#do show ip ospf neighbor detail | i interface|State|is up
 Neighbor 2.2.2.2, interface address 10.1.12.2
    In the area 0 via interface FastEthernet0/0
    Neighbor priority is 0, State is FULL, 6 state changes
    Neighbor is up for 00:11:42

Now let’s change the bandwidth-reference on r1:

r1(config)#router ospf 100
r1(config-router)#auto-cost reference-bandwidth 100000
% OSPF: Reference bandwidth is changed.
        Please ensure reference bandwidth is consistent across all routers.

The peering does not drop:

r1(config)#do show ip ospf neighbor

Neighbor ID     Pri   State           Dead Time   Address         Interface
2.2.2.2           0   FULL/  -        00:00:35    10.1.12.2       FastEthernet0/0

r1(config)#do show ip ospf neighbor detail | i interface|State|is up
 Neighbor 2.2.2.2, interface address 10.1.12.2
    In the area 0 via interface FastEthernet0/0
    Neighbor priority is 0, State is FULL, 6 state changes
    Neighbor is up for 00:14:38

I guess that this should not come as a surprise as reference-bandwidth is not one of the required attributes to establish an OSPF neighbor adjacency.

r1#show ip ospf interface brief
Interface    PID   Area            IP Address/Mask    Cost  State Nbrs F/C
Se1/0        100   0               10.1.12.1/24       1000 P2P   1/1

r2#show ip ospf interface brief
Interface    PID   Area            IP Address/Mask    Cost  State Nbrs F/C
Fa0/0        100   0               10.1.12.2/24       1    P2P   1/1