[c-nsp] Cisco 3560 LAN QoS egress queing shaping/sharing questions

[Peter Rathlev]

On Mon, 2009-08-17 at 16:27 +0200, Roger Wiklund wrote:
> Does this then mean that per default, the egress queue 1, handling COS
> 5, EF etc, only has 25mbit on a fastethernet port. Everything above
> that gets dropped.

Yup, as you say it's actually 1/25th of the port bandwidth. But yes,
everything beyond this gets dropped. Since EF is meant to be used for
voice only the 4 mbps default is plenty IMHO.

> And also, when enabling egress prio queue, that queue qets 100% of the
> bandwith? That will starve all the other traffic. Im reading in the
> Cisco QoS book where you can have strict prio + weighted round robin.
> But it looks like thats not available on the 3560.

Correct. You should always combine a priority queue with policing so you
don't starve the other queues. Or use SRR shaping, which "elegantly"
gives you both policing and prioritisation.

Priority queue and SRR shaping are not compatible; one rules the other
out.

Beware that the classification on the 3560 prevents you from matching
via ACL _and_ DSCP; you have to choose between matching DSCP for any
incoming packet or matching by ACL and then trusting DSCP.

Regards,
Peter