Question Of The Day: 18 December, 2008

Given the following output:

R1#show backup
Primary Interface   Secondary Interface   Status
—————–   ——————-   ——
Serial1/0.1         Serial1/1             waiting to revert (292 more seconds)

Which of the following is configured on R1?

A)
interface Serial1/0.1
 backup delay 60 300
 backup interface Serial1/1

B)
interface Serial1/0.1
 backup delay 300 0
 backup interface Serial1/1

C)
interface Serial1/1
 backup delay 60 300
 backup interface Serial1/1

D)
interface Serial1/1
 backup delay 300 0
 backup interface Serial1/1

Last QoD:

Assume that OSPF was configured on the router after any changes were made to the interfaces on R1.

Given the following information:

R1#sh ip int br | e ass
Interface                  IP-Address      OK? Method Status                Protocol
FastEthernet0/0            155.1.146.1     YES NVRAM  up                    up
Serial0/1                  155.1.13.1      YES NVRAM  up                    up
Loopback0                  150.1.1.1       YES NVRAM  administratively down down

R1#sh run | sec router ospf
router ospf 100
log-adjacency-changes
network 155.1.13.1 0.0.0.0 area 0

Answer	%
155.1.146.1	41%
150.1.1.1	40%
155.1.13.1	18%
0.0.0.0	1%

The answer: 155.1.146.1

The algorithm for selecting the OSPF router-id is basically:

1) If there is an OSPF router-id configured under the routing process use that.
2) If not, then select the highest active loopback* address
3) If no active loopbacks, then choose the highest active interface* address.

*These do not need to running OSPF

In this example there is no router-id configured under the routing process so the router looks for an active loopback.  The loopback is administratively shut down so it goes to the third step and chooses the highest active interface, which in this case is fa0/0’s IP address of 155.1.146.1

R1#sh ip protocol | section ospf
Routing Protocol is “ospf 100″
  Outgoing update filter list for all interfaces is not set
  Incoming update filter list for all interfaces is not set
  Router ID 155.1.146.1   
  Number of areas in this router is 1. 1 normal 0 stub 0 nssa
  Maximum path: 4
  Routing for Networks:
    155.1.13.1 0.0.0.0 area 0
 Reference bandwidth unit is 100 mbps
  Routing Information Sources:
    Gateway         Distance      Last Update
  Distance: (default is 110)