Question about using the Bandwidth statement

[andrewmorris]

Hi All,

I have a specific question relating to using the bandwidth statement.

As an example, the remote end has a point to point link using PPP encapsulation and it has full access to use all the channels off an E1 controller.

However, at the core end the bandwidth is set to 1984k yet on the remote end, the bandwidth is set to 64k.

My question is. Will this difference in bandwidth statements alone cause a response problem?

I think it will, other colleagues of mine think that the bandwidth statement is irrelevent and useful only in metric calculation.

Could anyone please settle this for us?

Many Thanks

Andrew Morris

[marikakis]

The bandwith specified with the bandwidth command

is there to be used by some routing protocols (OSPF, EIGRP, IGRP)

to help them calculate best routes.

It is a high level setting, and does not affect transmission rates.

It can only affect link selection to route traffic and how much traffic it will pass,

according to how the routing protocol sees it.

You might see less traffic traversing than you would imagine

because the routing protocol thinks the link in one direction

is not capable of carrying the traffic you would expect.

(The above could be considered a response problem.)

The link itself however is capable if you have configured

clocking and other low level settings properly.

The bandwidth command doesn't alter these low level capabilities.

It is usually best to match the settings to the true transmission capabilities,

unless you have an unstable link and want to influence path selection

or some other issue.

The bandwidth setting is also commonly used by

network management applications to help them

calculate interface/subinterface utilization.

M.

[andrewmorris]

Thanks M,

Great explanation. Much appreciated.

Regards

Andrew Morris

[marikakis]

I want to clarify one point.

If the only route available to reach a destination is through this link, it will be used.

The forwading process doesn't impose some restriction

on the rate of traffic that will traverse your link,

and you can easily see your link saturated

if you do not have enough transmission capability to handle the offered traffic.

If you have 2 paths however, this is a slightly different case.

Some protocols do only equal-cost load balancing.

In that case, you will not be able to properly load-balance

if you haven't configured bandwidth correctly.

In the case of unequal-cost load balancing,

traffic is distributed proportionaly between the links.

For example, say out of every 4 packets,

1 packet goes through one link and 3 through the other.

This proportion is affected by the configured bandwidth,

but not the total rate.

M.