How does the Router know the link is congested and the QoS can kick in?

[Yonglu Jian]

Dear All,

After studying the "bandwidth" and "priority" commands, we know that during the "link congestion" situation, the traffic volume which defined in "bandwidth" or "priority" will be preserved for the defined class. However, how does the router know the link is congested?

Normally the WAN link bandwidth is much smaller than the physical interface bandwidth. For example, the WAN link is 1024k. Yes, I can configure "bandwidth 1024" on the interface level. But this command is information only. It will not actually limit the outgoing traffic to be 1024k. So how does the router know when the link is congested and the bandwidth preservation for the particular class will be taken?

[John Blakley]

Yes, I can configure "bandwidth 1024" on the interface level. But this   command is information only. It will not actually limit the outgoing   traffic to be 1024k.

It takes into account the available bandwidth on an interface versus how much data is trying to pass through the device. The bandwidth command is what the calculation is based on. If you have a 10Mb ethernet circuit and you have a bandwidth statement of 1024, the qos policy that you have applied will base it on the 1024 that you have specified. If you were to apply a bandwidth command to an interface and do a "show int " You'd see on the second line that BW: = the amount that you have specifed on the interface.

That being said, by default the router only allows 75% of the assigned bandwidth to be used when congestion happens. The reason for this is to allow for routing updates and control plane traffic to have at least 25% of the rest of the bandwidth. You can change this behavior with a "max-reserved-bandwidth " command.

HTH,

John

[Joseph W. Doherty]

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Packets sitting in a queue or queues (i.e. waiting to be transmitted) is how the router "knows" it's dealing with congestion.