Categories R&S

OSPF – Implement and troubleshoot path preference – Bandwidth

Another way to influence OSPF path selection is to change the bandwidth configured under one interface.

 

Requirements :

Ensure that traffic from R4 to R3 loopback goes via R2.

 

Diagram:

OSPF - Implement and troubleshoot path preference – Bandwidth

 

Configuration and verification:

 

Same as in the previous post, let’s check what’s the current route to R3.

R4#sh ip route 33.33.33.33
Routing entry for 33.33.33.0/24
  Known via "ospf 10", distance 110, metric 3, type inter area
  Last update from 10.10.14.1 on Ethernet0/0, 00:47:44 ago
  Routing Descriptor Blocks:
  * 10.10.14.1, from 11.11.11.11, 00:47:44 ago, via Ethernet0/0
      Route metric is 3, traffic share count is 1

R4#sh ip ospf int Ethernet0/0
Ethernet0/0 is up, line protocol is up
  Internet Address 10.10.14.2/30, Area 2, Attached via Interface Enable
  Process ID 10, Router ID 44.44.44.44, Network Type POINT_TO_POINT, Cost: 1

R4#sh ip ospf int Ethernet0/1
Ethernet0/1 is up, line protocol is up
  Internet Address 10.10.24.2/30, Area 2, Attached via Interface Enable
  Process ID 10, Router ID 44.44.44.44, Network Type POINT_TO_POINT, Cost: 10

 

By changing the bandwidth configured under the interface Ethernet0/0 we will change the OSPF cost calculation for this interface.

R4(config)#interface Ethernet0/0
R4(config-if)#bandwidth 100000
R4(config-if)#end
R4#
*Jan 30 10:04:13.603: %SYS-5-CONFIG_I: Configured from console by console
R4#sh ip ospf int Ethernet0/0
Ethernet0/0 is up, line protocol is up
  Internet Address 10.10.14.2/30, Area 2, Attached via Interface Enable
  Process ID 10, Router ID 44.44.44.44, Network Type POINT_TO_POINT, Cost: 100

 

The route to R3 loopback is now preferred via R2:

R4#sh ip route 3.3.3.3
Routing entry for 3.3.3.3/32
  Known via "ospf 10", distance 110, metric 12, type inter area
  Last update from 10.10.24.1 on Ethernet0/1, 00:00:31 ago
  Routing Descriptor Blocks:
  * 10.10.24.1, from 22.22.22.22, 00:00:31 ago, via Ethernet0/1
      Route metric is 12, traffic share count is 1

 

So changing the bandwidth is another way to modify the cost value of an interface but I would not recommend this one as this may have effects on other things like QoS…

 

 

Thank you for reading.

 

OSPF – Implement and troubleshoot path preference – Bandwidth