Bandwidth statement on a FR sub interface

[thomuff]

Scenario:

Why would you use the bandwidth statement on Frame Relay subinterfaces and what does it affect? Routing protocol, interface speed????

Router C connects to two main routers.

Router C has a bandwidth statement on its subinterfaces.

int s0.100

bandwidth 128

int s0.200

bandwidth 16

int s0

bandwidth 384

Running OSPF for routing protocol.

[pkhatri]

The OSPF link cost for an interface is worked out as 10^8/bandwidth, where bandwidth is the interface bandwidth as configured by the bandwidth command. If you use the 'ip ospf cost' command on an interface, it overrides the default cost determined using the bandwidth.

HTH.

Paresh.

[Richard Burts · Accepted solution]

To elaborate on the response from Paresh the bandwidth command will certainly not impact the speed of the interface. It will affect the calculation of load on the interface and it will impact the calculation of the routing metric on the subinterface. Since OSPF is designed to make routing decisions that reflect the differing capabilities of the interfaces over which it runs, appling a more accurate bandwidth than assuming the default value will assist OSPF in making more valid and accurate routing decisions. (and specifying OSPF cost on the interface is an even more direct way of achieving this objective)

HTH

Rick

[peterledwidge]

Just to add, it's also good to have accurate information about your WAN connections for good documentation purposes.

[jroyster]

Just to add...

It is almost imperative to use the correct bandwidth statements on all interfaces.

Otherwise you can wind up sending traffic halfway around the world rather than across the street. Or some rather complicated routing.

[thomuff]

Thank you for input. I failed to mention that the interfaces already have the ip ospf cost statement applied. We are taking over responsibility of this network and are trying to decipher the previous owner's encryptic redundancies.