Cisco Support Community
cancel
Showing results for 
Search instead for 
Did you mean: 
Announcements

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. And see here for current known issues.

New Member

QoS clarification

All,

My QoS is a bit rusty, was looking for some help. My understanding is a switch classifies traffic w/ the dscp map. Then the router 'marks or tags' based on the info given from the switch. The router does not change the classification. So if, my dscp map is bad on my switch, my tags are probably bad on the router. Is this correct?

Thanks in advance,

rjp

4 REPLIES

Re: QoS clarification

Hi rjp,

The DSCP/CoS markings are set originally at endpoint. The access layer port is generally where the decision is made to trust, not trust (set everything to 0), or do some type of mapping.

The router can mark, but will not by default. This is done with the 'set dscp'/'set tos' command within a class in a policy map.

You can match traffic for a class-map on many other things than DSCP, so depending on what you're trying to match you can get around the problem as well.

hth,

nick

New Member

Re: QoS clarification

Nick,

First, thanks for the quick response. To clarify, if I'm not using the 'set dscp or set tos' commands in my policy map, my router is not changing tags received from the switch. Is this correct?

Thanks again,

-rjp

Re: QoS clarification

Hi rjp,

This is correct. The only time a router will change the DSCP byte is when you tell it to via a route map or policy map.

-nick

New Member

Re: QoS clarification

Nick,

Thank you so much for your help.

Best Regards,

-rich

101
Views
0
Helpful
4
Replies