Basic Qos Topology question

Answered Question
Mar 10th, 2009

My topology consists of 3750's as access switches, 6509 as the core, and 3845 as a WAN router. We connect to multiple remote sites over MPLS from this router.

On the access layer switches, I have enabled auto qos cisco phone on the phone ports, and trust on the uplink to the 6509. I plan on doing modular Qos policy on the 3845 WAN router, but I am unsure what I need to configure on the 6509 to pass the QoS markings from the access layer switches.

Thanks,

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Correct Answer by Nicholas Matthews about 7 years 8 months ago

Hi,

There are two modes of operations for switches:

1) 'mls qos' is not configured. In this case, every packet that comes in leaves with the same marking. This is the default.

2) 'mls qos' is configured. Now, every time a packet comes in an interface it checks for 'mls qos trust' commands. If you're trusting cos it uses the cos-dscp map, if you're trusting ToS is checks the tos-dscp map, and if you're trusting DSCP it will check if there is a DSCP-mutation map.

If there is no 'mls qos trust' command on the interface, then all packets will be marked 0.

What you need to do is find out if mls qos is enabled, and if so, the best solution will most likely be 'mls qos trust dscp' on all trunk ports on the 6500.

It should be noted that the marking/trust only happens on INBOUND ports, and packets do not get remarked outbound.

hth,

nick

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Correct Answer
Nicholas Matthews Tue, 03/10/2009 - 14:58

Hi,

There are two modes of operations for switches:

1) 'mls qos' is not configured. In this case, every packet that comes in leaves with the same marking. This is the default.

2) 'mls qos' is configured. Now, every time a packet comes in an interface it checks for 'mls qos trust' commands. If you're trusting cos it uses the cos-dscp map, if you're trusting ToS is checks the tos-dscp map, and if you're trusting DSCP it will check if there is a DSCP-mutation map.

If there is no 'mls qos trust' command on the interface, then all packets will be marked 0.

What you need to do is find out if mls qos is enabled, and if so, the best solution will most likely be 'mls qos trust dscp' on all trunk ports on the 6500.

It should be noted that the marking/trust only happens on INBOUND ports, and packets do not get remarked outbound.

hth,

nick

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