QoS problem

Unanswered Question
Oct 16th, 2008

I created a class map last night on an 871W router (at my house). It's matching packets but it doesn't seem like it's doing anything.


class-map match-any BITTORRENT

match protocol bittorrent

policy-map PROTOCOLS


bandwidth 200

int fa4

service-policy outbound PROTOCOLS

If there's errors in the above, I just typed this out; it's not copy/pasted from the router.

I get matches on the protocol, but I started to download a linux distro, and the bandwidth got up to 623.5k. Any ideas?



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Overall Rating: 5 (1 ratings)
Jon Marshall Thu, 10/16/2008 - 05:41


Does downloading a linux distro match on bittorrent then ?

Regardless your bandwidth statement does not specify a maximum amount of bandwidth that can be used rather it specifies a minimum amount of bandwidth. If there is available bandwidth not being used then even though you have specified 200 it can still use any available extra bandwidth.

If you want to restrict it to a maximum of 200 then you need to police the traffic or shape it.


John Blakley Thu, 10/16/2008 - 05:45

It did match packets on the policy-map. I'm just learning QoS. What's the difference between policing and shaping? I didn't realize the bandwidth command was a minimum amount. :-)


Jon Marshall Thu, 10/16/2008 - 05:49

Assuming that the default action is to drop packets when you police then the difference is

1) Policing drops any excess over 200

2) Shaping buffers any excess over 200 and sends it when there is space. Shaping smooths out bursty traffic.

Shaping places more demand on the router because you are buffering the packets.

Note that you don't have to drop the packets when policing you can remark them to a different value instead and then transmit.

QOS is quite a big subject so that is really a very high-level explanation.

Edit - one thing forgot to mention. If you specify a bandwidth within a Priority queue that bandwidth is automatically policed so as to make to sure other queues do not get starved of router resources.



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