Default QoS Cos on untrust port and L2 network protocol

[helenio]

Hello, if I enable QoS and the port is in default non trust state (not trusted, default Cos=0, cos averride disabled):

Switch#sh mls qos inter fa 1/0/7
FastEthernet1/0/7
trust state: not trusted
trust mode: not trusted
trust enabled flag: ena
COS override: dis
default COS: 0
DSCP Mutation Map: Default DSCP Mutation Map
Trust device: none
qos mode: port-based

To ports in access mode the internal COS=0 and DSCP=0.

What about port in trunk mode ? I gess it will be put to 0 isn't ?

..

Let say I have a switch2 connected to the Switch1 where the QoS is enabled and the port in in default mode (untrust).

If statement of tagged and untageed will be CoS=0 this mean that also L2 protocol like spannig tree etc will be have Cos=0. Will this impact the internal priority of process treatment ? i.e in the 3750 models ? do you see potential issues ?

[Eugene Lau]

G'day,

Heres a good reference which describes the QoS characteristics of various switches (CAMPUS QOS)

http://www.cisco.com/en/US/docs/solutions/Enterprise/WAN_and_MAN/QoS_SRND/QoS-SRND-Book.html

(email web-help@cisco.com if you have issues accessing)

- In access mode - no CoS as this value is written in the 802.1q header. You can have a DSCP

- in trunk mode, if untrusted it will be CoS0 and DSCP0 (according to CoS-DSCP map)

- Control traffic such as BPDU are subject to ingress QoS processing.

See 3750 config guide - General QoS Guidelines

http://www.cisco.com/en/US/docs/switches/lan/catalyst3750/software/release/12.2_55_se/configuration/guide/swqos.html#wp1426845

From a design perspective, enabling QoS then not having any trust state on inter-switch links kind of defeats the purpose of QoS!

HTH

Eugene

[helenio]

Hello, thank's for reply. The idea to untrust inter-switch link is because  there is situation where you can't manage the uplink switch or as first step of a migration fase to be sure that only traffic from QoS configured switches is tagged (DSCP & Cos) as from your marking policy and non QoS configured switches traffic as to be DSCP=0 and Cos=0. In this way in you backbone all traffic shoud be marked as from marking policy.

On the idea above the only think is that controll traffic (STP, EIGRP etcc.) from untrusted switch will be CoS=0/DSCP=0 therefore as from your reply this traffic will be enqueued as default traffic in the same ingress queue.