A common misunderstanding with configuring Cisco routers is the meaning of the 'bandwidth' command. Following a recent discussion we had with a client we thought it appropriate to add some specifics to our Tech Tips area.

Let's say you have a serial interface between two Cisco routers and the interface is up and running with IP addresses properly configured on both routers.

If you then add the command:

bandwidth 128000

The incorrect assumption here is that the interface is running at 128kb/sec. The particular serial interface was connected to a T1 CSU providing a network interface speed of 1.544Mb/sec. When we pointed out the incorrect assumption, our client then said "Well right - the line speed is running at 1.544 MB/sec, but the bandwidth command is limiting it to 128kb/sec. We advised this was also incorrect!

The purpose of the bandwidth command in Cisco IOS is to communicate the speed of the interface to the control plane (routing) protocols like OSPF, EIGRP, IGRP or BGP. This information is then used to select the best route and/or perform load balancing calculations and it overrides whatever the line speed of the interface is.

OSPF default bandwidth is the interface speed divided by 100Mb (rounded). So if the interface is a 1.544 Mb T1, the answer is a cost of 64. If you set the bandwidth command to 10,000,000 (10 Mb) then the OSPF metric cost will be 10.

The other thing that will be affected by the bandwidth command is TCP. Based on the bandwidth command, TCP will adjust its session parameters (MSS, retransmission, etc.) based on this command.

If you do not set bandwidth, the Cisco routers will use the interface bandwidth by default. You can always view this as follows:

cell_ce03#show int serial 0
Serial0 is up, line protocol is up
Hardware is HD64570
Internet address is 142.0.0.1/16
MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec, rely 255/255, load 1/255
Encapsulation HDLC, loopback not set, keepalive set (10 sec)

The output above was redacted for brevity. You can see that this is a 1.544 Mb default. But imagine if this was actually connected to a CSU/DSU that had only a Nx64kbit or fractional T1 speed. This default would be incorrect, and the bandwidth command would correct the situation.

If you do not set bandwidth, the Cisco routers will use the interface bandwidth by default.

We hope this helps with clarifying the proper use of the bandwidth command.

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