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Cisco 3750 Qos markings bleached

scottmanzie1
Level 1
Level 1

Hi

Got a 3750 that is bleaching QOS markings - 3750G - System image file is "flash:/c3750-ipbasek9-mz.122-58.SE2.bin

Fairly simple topology,  4500--->3750 with issue--->video conferencing unit used for testing

The uplink to the core 4500 is

interface GigabitEthernet1/0/49
 description XXXXXXXXXXXXXXX
 switchport trunk encapsulation dot1q
 switchport trunk native vlan 998
 switchport trunk allowed vlan 1-997,999-4092,4094
 switchport mode trunk
 srr-queue bandwidth share 10 10 60 20
 srr-queue bandwidth shape 10 0 0 0
 queue-set 2
 priority-queue out
 udld port aggressive
 mls qos trust cos
 auto qos voip trust

Link to the VC unit is

interface GigabitEthernet4/0/47
 description XXXXXXXXXXXX
 switchport access vlan 622
 srr-queue bandwidth share 10 10 60 20
 srr-queue bandwidth shape 10 0 0 0
 queue-set 2
 mls qos trust dscp
 spanning-tree portfast
 spanning-tree bpduguard enable
 spanning-tree guard root

A conference was going between a remote VC unit and this one, while a Wireshark was run on both key ports on the 3750

The traffic is marked CS4 when it comes Gig1/0/49, but is default by the time it goes out on Gig4/0/47

The only map config on the switch is

mls qos map cos-dscp 0 8 16 26 32 46 48 56

These 2 lines from the uplink stick out to me, but I am a little rusty on thus

 mls qos trust cos - trusting DSCP on every other 3750 with no issues reported
 auto qos voip trust  - don't see this used anywhere else

Any ideas?

Thanks

1 Accepted Solution

Accepted Solutions

Ye that's strange I would have thought if it was going to remove tag it would remove it both ways if it wasn't working correctly

is the VC unit tagging as DSCP or COS at the source before it hits the switch ?

Only other thing could think is something irregular with the IOS version could be bug

can you post the local interface of the VC unit show queuing int g4/0/47

You could also try stop it rewriting DSCP locally on the switch no mls qos rewrite ip dscp to stop it removing the tag on switch side before it hits VC unit

View solution in original post

3 Replies 3

Mark Malone
VIP Alumni
VIP Alumni

Hi Scott

the 3750 should be able to mark direct by dscp and if your device supports it I would try set that instead of cos , on some platforms if you enable auto qos voip trust it actually adds the cos command in with it, but for voice once there Cisco all you need is trust dscp as the phone/vc unit is marking at the source anyway , port will trust the dscp marking and carries it to the uplink

There are multiple ways to do layer 2 qos but if your having issues with cos and its mapping I would just change it to dscp at the source as it always preferred to use that at access ports than cos as it covers l2/l3 headers and removes the requirement for cos to map to dscp

cos was more for legacy where devices did not understand the dscp markings or set them , I would test with dscp if it was me and see how it goes in wireshark if the markings carry correctly without changing

Mark

Thanks, changing to DSCP on the link to the core 4500 was my first thought, but I still find it peculiar the switch is changing to default

Just to clarify, traffic coming into the switch on its trunk port to the core shows as CS4, when the same traffic exits the access port to the VC unit, its default - DSCP0

Don't think this is going to cause a performance issue, as its happening at the last device in the path, but its still odd.

Another point to note, the reverse traffic is fine. From this VC unit, to ones at another location, the CS4 is preserved end to end


Thanks

Ye that's strange I would have thought if it was going to remove tag it would remove it both ways if it wasn't working correctly

is the VC unit tagging as DSCP or COS at the source before it hits the switch ?

Only other thing could think is something irregular with the IOS version could be bug

can you post the local interface of the VC unit show queuing int g4/0/47

You could also try stop it rewriting DSCP locally on the switch no mls qos rewrite ip dscp to stop it removing the tag on switch side before it hits VC unit

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