Bandwidth command question

binaryhat

Bandwidth command is defined as the logical speed of the connection. What does it mean by "logical speed of the connection?" The speed in theory it can operate at? Say I set the bandwidth to 100Mbps but in reality I won't get that speed, more like 85Mbps?

Thanks

JoeBirds

The bandwidth command allows the user to manipulate calculations (namely EIGRP) routers perform when choosing the best route. Think of the bandwidth command like changing the cost of a link with the spanning-tree command. You're not actually changing the physical speed of the link, but you're telling the router or switch to think that this is the speed of the link and to use this configured speed in STP or EIGRP calculations. I hope that makes sense.

NetworkVeteran

binaryhat wrote: »

Say I set the bandwidth to 100Mbps but in reality I won't get that speed, more like 85Mbps?

As JoeBirds says, the command doesn't have any impact on the actual speed of the link. If it's a 10Mbps link, you can set the "bandwidth" to 1Mbps, 10Mbps, or 100Mbps. In all case you'll actually get 10Mbps.

Think of it like a sign. The sign says "This is a X Mbps pipe!"

Some protocols use these signs to determine the best path, so make them as accurate as possible. If your 128Kbps pipe has a "10 Mbps!" sign and your 100 Mbps pipe has a "1 Mbps!" sign, you my be disappointed by your network performance.

NetworkVeteran

What does it mean by "logical speed of the connection?"

To add a bit more clarity, suppose your ISP provides you with an Ethernet cable as an access link. You have a 10/100/1000 Mbps port on your router, but you're only paying for 5Mbps. Your router has no way of knowing that your carrier will toss any traffic you send it beyond 5 Mbps. This is where the bandwidth command comes in so routing protocols know how good the link really is.

(Which is important when there are multiple paths to the same destination going across different links.)

atorven

Out of interest, what technologies do ISPs uses to actually limit customer traffic?

networker050184

Usually policing, sometimes shaping depending on the product.

deth1k

Don't forget about QoS configuration. When allocation bandwidth to queues, it's important to make sure your interface bandwidth is set to represent what you have bought from your service provider. In cases where nested policy with shaper applied is used it's not so relevant.