How to divide bandwidth using cisco routers
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.
You configure the Cisco IOS bandwidth command on interfaces. Here's an example:
ip address 188.8.131.52 255.255.255.0
This command has only one option—the bandwidth, in kilobits, of the interface.
Router(config-if)# bandwidth ?
<1-10000000> Bandwidth in kilobits
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.
on Apr 24, 2010 | Cisco 2610XM Router