Bandwidth command on Cisco Routers

Bandwidth is the maximum amount of data that an object can transfer within a given amount of time. It is object-specific. Two different objects may have similar or different bandwidths. It depends on many factors, such as the object's capacity, environment, and configuration.

Some objects allow you to configure their bandwidths. If an object supports bandwidth customization, you can use it to match the object's bandwidth to another object's. Two objects with similar bandwidths work more effectively. Bandwidth is not the speed or guarantee of the transmission. It is the maximum transmission capacity an object can achieve. For example, if a cable's bandwidth is 100 Mbps, it cannot transfer more than 100 Mbps of data per second.

The bandwidth command

The bandwidth command configures the interface bandwidth on the router. Interface bandwidth is the maximum amount of data an interface can transfer over the attached media within a given time. For example, if an interface can transfer 100 MB of data over the connected media in 1 second, its bandwidth is 100 Mbps.

Some protocols use the interface's bandwidth for various functions. For example, TCP and UDP use the interface's bandwidth to decide the size of a segment. EIGRP and OSPF use the interface's bandwidth to calculate routing metrics. Protocols read the interface's bandwidth from the running configuration. The bandwidth command configures the interface's bandwidth in the running configuration.

Since some protocols use the interface's bandwidth for various functions, Cisco assigns a default bandwidth to each interface. If you do not configure the interface's bandwidth, protocols will use the default bandwidth. If you configure the interface's bandwidth, protocols will use the configured bandwidth. The bandwidth command sets the interface's bandwidth in the running configuration. It does not change the interface's actual or physical bandwidth. You can use the bandwidth command for the following purposes.

• To match an interface's bandwidth to the bandwidth of the connected media.
• To influence the route metric when the routing protocol uses bandwidth as a metric component.

Different media types have different bandwidths. The bandwidth command allows you to set the interface's bandwidth to match the media's bandwidth. Media bandwidth is the amount of data that a medium can transfer within a given time. For example, if a cable can transfer 1 GB of data per second, then its bandwidth is 1 Gbps.

To utilize the cable's maximum capacity or bandwidth, configure the interface's bandwidth to match the cable's bandwidth. For example, if the cable's bandwidth is 1544 Kbps, configure the interface's bandwidth to 1544 Kbps. If you configure the interface's bandwidth to be less than or greater than the cable's bandwidth, network performance will decrease.

Let's take an example. You connect a switch to a router's interface using a 100 Mbps cable. If you configure the interface's bandwidth to 10 Mbps, the upper-level protocols will assume the bandwidth of the connected interface is 10 Mbps. They will encapsulate data packets for a 10Mbps link. Although the cable can carry 100 Mbps, the interface will only load 10 Mbps. With this configuration, you will waste 90% of the cable's bandwidth.

If you configure the interface's bandwidth to 200 Mbps, the upper-layer protocols will assume the bandwidth of the connected interface is 200 Mbps. They will encapsulate data packets for a 200 Mbps link. Since the size of data packets exceeds the cable's maximum capacity, the cable cannot carry them. This configuration will generate many errors.

If you configure the interface's bandwidth to 100 Mbps, the upper-layer protocols will encapsulate data packets for a 100 Mbps link. This configuration utilizes the full capacity of the cable and interface.

Some routing protocols, such as EIGRP and OSPF, use an interface's bandwidth to calculate each route's metric. You can use the bandwidth command to influence the calculation of their metric. For example, EIGRP uses the interface's bandwidth in its metric calculation. By changing an interface's bandwidth, you can force EIGRP to select the desired route for a particular destination without changing the network's physical layout.

Packet Tracer Example

In the following network, Router0 has two routes to reach Router2. Both routes have two serial links. Serial links or cables connect to serial interfaces. The default bandwidth of a serial interface is 1544Kbps.

You can download this network topology from the following link.

Packet Tracer Lab for the bandwidth command

If you do not change the default bandwidth of any serial interface on both routes, both routes have equal costs. To verify this, use the 'show ip route' command on router Router0.

As the above output shows, the cost of both routes is 2684416.

By default, EIGRP adds only one best route to each destination network in the routing table. To select the best route, EIGRP compares the cost of each route. It chooses the route that has the lowest cost. If there is a tie between two routes, EIGRP adds both routes to the routing table. This feature is known as the load balancing between equal-cost routes.

Change the default bandwidth of a serial link and run the "show ip route" command again. You can use the "show ip route eigrp" command to view only EIGRP routes.

The following points explain the above exercise.

Serial 0/0/0 connects Router0 to Router2 via Route1. Serial 0/0/1 connects Router0 to Router2 via Route2. With the default bandwidth on both interfaces, the cost of both routes is 2684416.

Later, you changed the bandwidth of Serial 0/0/0 to 64. As soon as you changed the bandwidth of Serial 0/0/0, EIGRP recalculated the cost of both routes. Since you reduced the bandwidth of a link on Route1, the cost of Route1 increased.

Since EIGRP keeps only the route with the lowest cost in the routing table, it excludes Route1. However, it keeps Route1 in the Topology table for backup. To verify this and view the new cost of Route1, use the 'show ip eigrp topology' command. The following image shows the output of this command.

This way, you can easily force EIGRP to select a specific route without changing the physical layout.

Conclusion

Configuring interface bandwidth on Cisco routers is crucial for optimizing network performance and ensuring proper communication between devices. By using the bandwidth command, you can align the interface's bandwidth with the capacity of the connected media. It enhances efficiency and prevents wasted resources. Additionally, you can use this command to influence the route selection process of routing protocols (such as OSPF and EIGRP) that use bandwidth as a metric.