Mapping PRE to COS

Unanswered Question
Jul 22nd, 2009


Could someone confirm if Mapping from EXP to PRE to COS is automatic in any router? in my case I am ussing an 7600 with SIP-400. In other case I am ussing the same 7600 with a WS card. I am worried about the mapping between PRE to COS. Tranks.

I have this problem too.
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Roman Rodichev Wed, 07/22/2009 - 14:24

PRE to COS will require "mls qos trust ip-precedence" on the next-hop ingress port (if it's a catalyst)

EXP to PRE is not automatic. IP Packets are left alone when VPN label is removed. You'll need to configure policy-map that classifies based on EXP and sets PREC, and I think you could set COS there too

amit.bhagat Wed, 07/22/2009 - 17:21


There are 5 rules when it comes to MPLS QoS. They are-

1) By default, the IP precedence bits (or first three bits of DSCP field) of IP header are copied to the EXP bits of all imposed labels at the ingress LSR.

2) By default, the EXP bits of the incoming label are copied onto the swapped outgoing label and to any label pushed onto that packet.

3) By default, the EXP bits are not copied to the EXP bits of the newly exposed label when the incoming label is popped.

4) By default, the EXP bits are not copied to the IP precedence or DSCP field of the IP header when the incoming label is popped and the packet becomes unlabeled.

5) When you change the EXP bits through manual configuration (using MQC), the value of the EXP bits in the labels other than the top label, the swapped label, or the imposed labels and the IP precedence bits or DSCP bits in the IP header remain unchanged.

Hope this helps.


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