Reading CCIE Practical Studies Volume II on Safari I thought I’d check out mls qos on the 3550 I have in my CCIE lab. Being familiar with the QoS mapping on a 6500 (sup720) and a 4500 (sup IV?), I was surprised to find something I’d not seen before:

• Policed-dscp map

So I started searching for what it’s for and I quickly found the following information.

QOS Policing and Marking with Catalyst 4000/4500 IOS based Supervisor Engines – Configuring and Monitoring Marking:

class cl_test11
! police and mark down all other ip precedence 3 traffic
__police 1 mbps 1000 exceed-action policed-dscp-transmit

The 3550 can remark dscp when policing and it uses a special qos-map to do so. Unlike routers where the remark can be stated as:

policy-map dscp-remark
_class test
__police cir 128000
____conform-action transmit
____exceed-action set-dscp-transmit cs2
____violate-action drop

I must admit the 3550 way can be quite handy as the stated router config will just plainly overwrite the existing marking while the qos-map will look at the dscp value to map from.

Here is another useful link about QoS on a 3550

Categories: CCIE R&S, Main blog
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