[c-nsp] Preserving CoS with xconnect on ME3600X

[Pshem Kowalczyk]

Hi,


On 7 November 2012 08:04, Lobo <lobotiger at gmail.com> wrote:

> Hi everyone.  I was hoping I could get some help with an EoMPLS config I'm
> working on between an ME3600X and an ME6524 and being able to preserve the
> CoS field values.
>
> The xconnect pseudowire it actually up and able to pass traffic so that's
> not a problem.  The issue is that any CoS values I assign to the data
> packets seem to get overwritten on the ME3600X in the service instance
> config.  This is a rough topology of what I'm using:
>
> Traffic generator --- 3750 --- ME3600X ---pseudowire over mpls cloud---
> ME6524 --- 3750G --- Traffic generator
>
> What I see happening (going from left to right) is that the 3 different
> CoS values (5,3,0) all get overwritten to zero as soon as the traffic goes
> over the xconnect.  LDP labels are also sent as exp zero.
>
> In the opposite direction where the xconnect starts on the ME6524, the CoS
> values are copied over to the exp values all the way to the ME3600X until
> it passes through the service instance config.  Once I inspect the traffic
> on the 3750 switch, I see all my traffic arriving with CoS 0.  Note that
> DSCP is preserved in both directions.
>
> I'm curious if there is some global command for the ME3600X or some
> additional config that I need to do on the service instance in order to
> somehow copy the CoS value to EXP bit.  Or do I have to do more inspection
> and writing of values via MQC?
>
> Here is the config I have for the xconnect on the two platforms.
>
> ME3600X
>
> interface GigabitEthernet0/8
>  switchport trunk allowed vlan none
>  switchport mode trunk
>  load-interval 30
>  speed 100
>  duplex full
>  service instance 3801 ethernet
>   encapsulation dot1q 3801
>   rewrite ingress tag pop 1 symmetric
>   xconnect 172.28.110.2 666 encapsulation mpls
>  !
>
> ME6524:
>
> interface GigabitEthernet1/1.3801
>  encapsulation dot1Q 3801
>  xconnect 172.28.110.2 666 encapsulation mpls
> !
>

Your problem is caused by the following line in 3600x config:

rewrite ingress tag pop 1 symmetric

that line removes the vlan tag and the COS bits with it, so when the frame
hits the PWE3 there is no information about what class the traffic belonged
to originally, just plain ethernet frame.  One way to make it work is to
create a set of policies on both 3600x and 6524 that inspect the incoming
packets and copy the COS bits into EXP/QoS bits of the MPLS label and then
have a symmetric set of policies on egress to copy bits from EXP/QoS back
to COS (AFAIK 65xx does it automatically, if not configured otherwise). I
believe you have to use qos-groups on egress, as MPLS markings do not
survive till the egress policy processing.

Between two 3600x you can make it also work if you do not pop the VLAN tags
(then PWE3 carries ethernet frame with VLAN tags).

kind regards
Pshem