CoS/DSCP to queue mapping in Cat 3750

[e-chuah]

Hi..

Cat 3750 supports mapping of CoS to queue as well as mapping DSCP to queue.

WOuld like to understand which one will the switch use when actually placing packets in the queue..

thanks

Eng Wee

[Roberto Salazar]

3750 uses CoS value when placing packet on egress queue. It has internal dscp-cos mapping that can be tweak and re-mapped if you wish to do so.

[pkhatri]

Hi,

It depends on which command you have used. The command are mutually exclusive so you can configure one or the other. If you have configured the 'mls qos srr-queue output dscp-map' command, the internal DSCP value will be used to map the packets to a queue. If you have configured the 'mls qos srr-queue output cos-map' command, the CoS value will be used to map the packets to a queue.

Hope that helps - pls rate the post if it does.

Paresh

[e-chuah]

Hi pkhatri,

THanks for the reply..

if u look at the configuration generated by auto-qos, 'mls qos srr-queue output dscp-map' and 'mls qos srr-queue output cos-map' are generated, that's why i am not sure which one is used..

Eng Wee

[pkhatri]

My apologies, that was not a very good explanation :-)

Here is what really happens:

- when a frame is received on a port, the switch maps the CoS or DSCP in the packet to a QoS label to distinguish one kind of traffic from another. If the port is set to trust CoS, the CoS value is used to generate the QoS label. If the port is set to trust DSCP, the DSCP value is used to generate the QoS label.

- The QoS label that is generated identifies all future QoS actions to be performed on this packet.

- when the packet is switched to the egress interface, an output queue is selected based on the QoS label. If the QoS label was based on DSCP, the DSCP-to-queue mapping is used. If the QoS label was based on CoS, the CoS-to-queue mapping is used.

Hope that helps - pls rate the post if it does.

Paresh

[e-chuah]

Paresh, thanks for the explanation. I think u r correct. Will test it out if i can get hold of a Cat 3750 switch.

[pkhatri]

No problems.. apologies for not being too clear with the first response. It's a bit hard to keep track of all the differences on these seemingly similar Catalyst platforms.

Paresh