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MPLS QoS on Multilayer 65xx/76xx

I can't understant very well which is the mechanism of marking the Internal DSCP of the Multilayer swicth in the PE MPLS. Take a look of theese two cases and the questions related:


FIRST CASE: upstream

R1 is a 6500 PE device enabled qos and the IP packet goes in the ingress interface (configured to eas trust cos) dscp=ef and Cos=2. In this case the cos should be used in order to build the Internal-dscp. Does then the Internal-dscp used to mark the EXP of the out going packet leaving the original dscp end may be also the CoS?

EXAMPLE:

             (ACCESS) INCOMING  IP PACKET ------------------- CATALYST 6500 (PE)------------------ OUTGOING MPLS PACKET (2 labels) (BACKBONE)

  • Incoming pachet: dscp=ef, cos=2
  • Incoming interface: trust cos
  • default mapping cos-internal dscp:

CoS

0

1

2

3

4

5

6

7

DSCP

0

8

16

24

32

40

48

56

default mapping internal dscp-exp

DSCP

0-7

8-15

16-23

24-31

32-39

40-47

48-55

56-63

EXP

0

1

2

3

4

5

6

7

  • Internal dscp= 16
  • Outgoing MPLS packet: dscp=ef, exp=2, exp=2, cos=2 Is this correct or not? In this case the dscp is not remarked and still remain ef?


SECOND CASE: Downstream

R1 is the same 6500 PE device as before. The MPLS packet is returning now coming into another interface (configured for trusting exp).  The MPLS packet has just the transport label and the QoS is: dscp=ef, exp=3 and Cos=2. In this case the exp should be used in order to build the Internal-dscp. Does then the Internal-dscp used to mark the cos and maybe also the dscp of the outgoing packet toward teh access.?

EXAMPLE:

           (BACKBONE) INCOMING  MPLS PACKET (1 label) ------------------- CATALYST 6500 (PE)------------------ OUTGOING IP PACKET (ACCESS)

  • Incoming pachet: dscp=ef, exp=3, cos=2
  • Incoming interface: trust exp
  • default mapping exp-internal dscp:

EXP

0

1

2

3

4

5

6

7

DSCP

0

8

16

24

32

40

48

56

default mapping internal dscp-cos

DSCP

0-7

8-15

16-23

24-31

32-39

40-47

48-55

56-63

COS

0

1

2

3

4

5

6

7

  • Internal dscp= 24
  • Outgoing MPLS packet: dscp=cs3, cos=3 Is this correct or not? In this case the incoming dscp is rewritted from ef to cs3??

Thanks

2 REPLIES
Cisco Employee

Re: MPLS QoS on Multilayer 65xx/76xx

Hi,

PFC preserves the IP ToS field for any MPLS operation (IP2MPLS, MPLS2MPLS, MPLS2IP)

http://www.cisco.com/en/US/docs/routers/7600/ios/12.2SR/configuration/guide/mplsqos.html

So your first case analysis is correct and for the second case the DSCP field will be preserved as well. One exception is for aggregate label. In this case, the IP ToS will be set to the internal DSCP during the MPLS2IP operation.

HTH

Laurent.

Cisco Employee

Re: MPLS QoS on Multilayer 65xx/76xx

Hello,

Just a small addition to Laurent's answer for the 2-nd case.

On 65xx76xx at egress PE it is possible to implement MPLS_EXP-> DSCP propagation during MPLS2IP,

using "mpls propagate-cos" at the customer-facing interface,

as described at the example at the last section of the above document

(Configuring the Egress PE Router—>Customer Facing Interface). 

Thanks,

Sergey

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