Clarifying the Cisco IOS bandwidth command

by David "Davis CCIE, MCSE+I, SCSA" | Jul 13, 2006 8:13:00 PM

Tags: Cisco IOS, Cisco Systems Inc., David Davis CCIE, MCSE+I, SCSA, bandwidth, Cisco Routers and Management Newsletter

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Takeaway: David Davis has recently noticed that there seems to be some confusion out there about Cisco's bandwidth command, and he wants to set the record straight. In this edition of Cisco Routers and Switches, find out what the bandwidth command does�and doesn't�do.

I've recently noticed that there seems to be some confusion over the Cisco IOS bandwidth statement. What does the bandwidth statement really do? Is that really your bandwidth? Why is the bandwidth statement there? In this article, I want to clear up the confusion with the Cisco IOS bandwidth command.

A common assumption

Someone recently asked me why their organization's network wasn't getting the 4 Mbps of bandwidth from its Internet connection that they had configured. When the person told me that they're using a T1 circuit connection, I clarified that it was a full T1, explaining that a T1 boasts a raw bandwidth of 1.544 Mbps, full duplex.

The person countered that the router had a set bandwidth statement of 4096, so the network should be getting 4 Mbps of bandwidth. Unfortunately, it just doesn't work that way. Let's find out why.

The bandwidth command's real purpose

First, let's discuss the real purpose of the bandwidth command. In the above scenario, the questioning administrator didn't understand the true purpose of this command, incorrectly assuming instead that the network would receive the bandwidth configured with the command.

The bandwidth command is only there to communicate the speed of the interface to higher level protocols. Most of the time, a routing protocol needs to know the speed of the interface so it can choose the best route.

In the case of routing protocols, IGRP, EIGRP, and OSPF all use the bandwidth statement. However, TCP will also adjust its initial retransmission parameters based on the bandwidth configured on the interface.

OSPF uses cost as its routing metric, which it calculates using bandwidth. For example, OSPF takes 10⁸ and divides it by the bandwidth of the interface. To calculate the cost of a full T1, OSPF divides 100,000,000 by 1,544,000, which returns an OSPF cost of 64. (Cisco routers don't use floating-point math, so they drop the numbers after the decimal.)

On the other hand, EIGRP uses the bandwidth of the link to calculate its routing metric. Here's the EIGRP metric formula:

metric = [K1*bandwidth + (K2*bandwidth)/(256 - load) 
+ K3*delay] * [K5/(reliability + K4)]

We won't try to calculate a metric in this article, but as you can see, the process definitely requires using bandwidth. In fact, due to the default K values, the only values used to calculate the EIGRP metric are bandwidth and delay.

Examples

You configure the Cisco IOS bandwidth command on interfaces. Here's an example:

interface Serial0/0
 bandwidth 128
 ip address 1.1.1.1 255.255.255.0

This command has only one option�the bandwidth, in kilobits, of the interface.

Router(config-if)# bandwidth ?
  <1-10000000>  Bandwidth in kilobits
Router(config-if)#bandwidth

There are always default bandwidth values set for each type of interface, such as the Serial interface, as shown below:

Router# show interface s0/0
Serial0/0 is administratively down, line protocol is down 
  Hardware is PowerQUICC Serial
  MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec,

In the case of a serial interface, the default bandwidth is 1,544 K (or a full T1 circuit). However, you could have a fractional T1 circuit, and the default may be incorrect.

As you can see, setting the correct bandwidth on each interface is very important when it comes to routing protocols choosing the right router. However, no matter what you set the bandwidth command to, you won't actually get faster throughput out of any interface�the two simply aren't related.

For more information, check out Cisco's bandwidth command documentation and Cisco's Setting Bandwidth reference.

How familiar are you with Cisco's bandwidth command? Has this helped clear up any confusion? Share your experiences and questions in this article's discussion.

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David Davis has worked in the IT industry for 12 years and holds several certifications, including CCIE, MCSE+I, CISSP, CCNA, CCDA, and CCNP. He currently manages a group of systems/network administrators for a privately owned retail company and performs networking/systems consulting on a part-time basis.

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