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Trust CoS or DSCP on 802.1q trunk port

There's been some discussion that an "802.1q uplink or trunk" port between a distribuition 6509 and a closet or user access layer 2 switch can be configured to trust dscp.  If I have a video unit on a 3750 for example and the user access port is configured for trust dscp, can the uplink trunk ports on both sides of the uplink be configured to trust dscp? The port will see the dscp value within the packet on the trunk port at the distribution switch? Are we sure it's not going to ignore dscp and re-write the dscp value to zero?  MLS configured on distribution switch with cos-dscp and dscp-cos maps.

2 REPLIES
Hall of Fame Super Blue

Re: Trust CoS or DSCP on 802.1q trunk port

tgurney wrote:

There's been some discussion that an "802.1q uplink or trunk" port between a distribuition 6509 and a closet or user access layer 2 switch can be configured to trust dscp.  If I have a video unit on a 3750 for example and the user access port is configured for trust dscp, can the uplink trunk ports on both sides of the uplink be configured to trust dscp? The port will see the dscp value within the packet on the trunk port at the distribution switch? Are we sure it's not going to ignore dscp and re-write the dscp value to zero?  MLS configured on distribution switch with cos-dscp and dscp-cos maps.

If the packet is marked with a DSCP value you can indeed choose to trust DCSP. In fact on the 3750 that is the default ie. trust DSCP if you don't specify which marking you are trusting efg. mls qos trust.

It won't ignore the DSCP value. It will use the DSCP value received as the internal DSCP value it uses to prirotise traffic through the switch. Note in contrast, if you trusted CoS then the CoS-to-DSCP map is consulted because the switch still needs to derive an internal DSCP value to use while the packet moves through the switch.

Jon

Hall of Fame Super Bronze

Re: Trust CoS or DSCP on 802.1q trunk port

You can trust CoS or DSCP. The trust command allows you to preserve the priority information within the 802.1q frame as described here;

http://www.cisco.com/en/US/tech/tk389/tk689/technologies_tech_note09186a0080094665.shtml#topic2

If you enable 'mls qos' and the trust isn't enabled on the trunk port, this field gets rewritten to 0.

Regards,

Edison

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