Clarifying the Cisco IOS bandwidth command
Thursday, July 20, 2006 09:00 AM
There seems to be some confusion about Cisco's bandwidth command. 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?
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.
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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Hello DAvid
I am working as a tech support for a telecom company in Houston, Texas. There had been an argument regarding the bandwidth command within the group which led me to your column.
Does the command "bandwidth" serve any other purpose as far as VOIP traffic or QoS are concerned.
Below are the configs that we have for a router sub-interfcae intended to carry voice traffic. I thought its always the map class that does BW allocation for the PVC and not the "bandwidth" command.
Its running on a satellite link with 1 meg BW full duplex. 2 PVC are configured, 1 for voice (shown below) and 1 for data. The map class for voice sub-interface was configured for the entire BW of 1 meg, cir set to 1024000. And the data PVC was also set to a CIR of 1 meg.
The questions are:
1. Does the "Bandwidth" command do BW allocation based on the configs below ?
2. Is it possible to set the CIR of both PVC (voice and data) to 1 meg when the entire link only has 1 meg.
interface Serial2/1.100 point-to-point
bandwidth 240
ip address x.x.x.x x.x.x.x
frame-relay interface-dlci 400
class 1MVoIPTraffic
load-interval 30
map-class frame-relay 1MVoIPTraffic
frame-relay cir 1024000
frame-relay bc 10240
frame-relay be 0
frame-relay mincir 1024000
service-policy output voice_voip-TX
Thanks,
Rico
Posted by anonymous on Thursday, July 27 2006 04:59 PM