I have 3750 in between two Router ( MPLS enabled)as below:
R1-3750 - 802.1q trunk
R2-3750 - 802.1q trunk
I have R1 sending MPLS labelled packet with layer vlan tagging and cos value.
Now, on 3750 i want to implemnet Qos with trst cos value as 3750 doesn't understand MPLS.
So i trust cos value and do qos mapping Cos to specific Queue and do SRR share on egress.
Now, 3750 map incomming cos to dscp ( using cos-dscp map table) and can modify dscp at egree.
But question is , incomming packet will be MPLS packet so for 3750 all packet will be no ip packet ( as don't understand MPLS), will that just do qos on cos rightway and send packet with unchanged cos and dscp value to R2 ??
By default, the precedence bits (or the first 3 bits of the DSCP field) of the IP header are copied to the EXP field(experimental - MPLS QoS bits) of the MPLS label header. When the label is stripped off, the IP header remains unchanged.
The problem is this 3750 is not metro switch and doen't understand MPLS.
So it will see incomming MPLS packets as a non-IP Packet.
Hence, I will have to Qos based on Cos so that R1 will set Cos value based on EXP and 3750 can do Qos at egress on Cos.
The question is , when i trust cos , 3750 will use internal dscp ( from cos-dscp mapping) and then do Qos at egress and again set cos and dscp value. I am not sure will that change cos and dscp value at egress ? i guess at least dscp not since it can't read MPLS.
You can ensure the 3750 disables egress ToS re-write by using the command no mls qos rewrite ip dscp. This preserves the layer 3 ToS byte.
However, having said that, I should point out that unless you are actually changing the egress ToS value via a policy map or something, then the final internal DSCP value (which would normally be mapped to the egress) will be the same as the initial internal DSCP value.
As the MPLS label sits between the frame header and the IP header, the switch may just see it as a jumbo frame, and/or it may not even try to re-write the layer 3 ToS byte; I don't know.
You could do a span session and see what the packets look like as they leave an interface.
Good luck and I wish I could have given you better help!
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