mls QoS 3750

[tedauction]

Hello, I see that by default, a 3750 switch only allocates 4% of output bandwidth for queue 1 i.e. EF traffic (see attached image).

I am just interested as to why this percentage is so low compared to the other 3 queues which have 25%.

Thank you.

[Mark Malone]

Default Scheduler Configuration:

The priority queue is disabled. Both the shaped and shared mode are configured for the SRR. Shaped mode weights override the shared mode  value. Therefore, the net result is queue 1 is serviced in shaped mode and  queues 2, 3, and 4 are serviced in shared mode. This means queue 1 is serviced  with an       absolute value that is (1/25) percent, or four percent, of the  bandwidth.  Queues 2, 3 and 4 are serviced at 25 percent of the bandwidth. If the  bandwidth is available, then queues 2, 3 and 4 can be serviced at more than 25  percent of   the bandwidth.

[Joseph W. Doherty]

Hmm, I don't recall that being the actual (basic) default for 3750 QoS. It might be a default for some variant of auto QoS.

As to why Q1 would be limited to 4%, that would likely be done if PQ is enabled too, which on a 3750 doesn't have an implicit rate limiter. I.e. PQ, but itself, could stave all your other queues of bandwidth. As to also why 4%, that just some assumption that your EF traffic would not need more than that.