Solved: L2 QoS on 3560 and 6509 switches

i.va · ‎08-18-2008

Hi!

I've got some questions concerning QoS on access layer 3560 (doing L2) and core 6509 (doing L2 and L3) native IOS switches.

3560: IOS c3560-ipbase-mz.122-35.SE5.bin

6509: IOS c6k222-psv-mz.122-18.SXD7b.bin

- are the 3560 series Switches (doing L2 only) able to trust DSCP on a normal switch/access port? I have found statements that seem to confirm this and statements mention that its only possible on a routed port. Which is true?

- The 6509 native IOS switch is doing L2 and L3 (inter-VLAN routing). I want to trust incoming L2 CoS and preserve/reassign these CoS markings after routing has taken place. Does the (internal)dscp-cos map accomplish this? Will packets leaving the switch be marked with the CoS defined by this map?

regards,

Ingo

andrew.butterworth · ‎08-19-2008

The 6500 by default has an internal DSCP-CoS mapping table (show mls qos maps) so if the packet is leaving the switch on an 802.1q trunk then the CoS value will be set according to it's internal DSCP value. If it isn't being transmistted on an 802.1q trunk then the CoS value is still used to determine the queue (On all linecards except the 10Gbps ones which use the DSCP value to determinte the queue I think?). The internal DSCP value is mapped to a CoS value and the CoS then determines the queue. Check with the command 'show queuing interface x/x'.

HTH

Andy

View solution in original post

Collin Clark · ‎08-18-2008

Ingo-

The 3560's can trust DSCP at the switchport (we do it). With the 6509 you should trust DSCP/CoS and have the switch map CoS to DCSP. You should'nt have to map DSCP back to CoS unless you have some legacy switches downstream (ie 2948G).

Hope that helps.

i.va · ‎08-19-2008

Hi!

thank you for the reply! The Cisco docs weren't quite clear on that. Since the 3560's can trust dscp at the switchport level, I will most likely rethink my QoS strategy.

However it would still interest me what happens to the CoS markings after inter-VLAN routing has taken place on the 6509...i.e. if the switch rewrites the markings using the dscp-cos map. So if anyone has info regarding that, I would be interested...

Ingo

andrew.butterworth · ‎08-19-2008

The 6500 by default has an internal DSCP-CoS mapping table (show mls qos maps) so if the packet is leaving the switch on an 802.1q trunk then the CoS value will be set according to it's internal DSCP value. If it isn't being transmistted on an 802.1q trunk then the CoS value is still used to determine the queue (On all linecards except the 10Gbps ones which use the DSCP value to determinte the queue I think?). The internal DSCP value is mapped to a CoS value and the CoS then determines the queue. Check with the command 'show queuing interface x/x'.

HTH

Andy

i.va · ‎08-19-2008

thanks for answering my question so thoroughly! This confirms my previous assumption, and is definately good to know when trying to keep the L2 markings consisten throughout the LAN!

Ingo