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SRR internal DSCP

From CCIE written exam guide:

The Cisco 3560 uses a relatively simple classification scheme, assuming you consider only what

happens when the forwarding decision has been made. These switches make most internal QoS

decisions based on an internal DSCP setting. The internal DSCP is determined when the frame is

forwarded.

What internal DSCP setting means?

5 Replies 5

Hello

sh mls qos maps dscp-input-q

   Dscp-inputq-threshold map:

       d1 :d2   0     1       2        3       4       5       6       7       8     9

     ------------------------------------------------------------

      0 :    01-01 01-01 01-01 01-01 01-01 01-01 01-01 01-01 01-01 01-01

     1 :    01-01 01-01 01-01 01-01 01-01 01-01 01-01 01-01 01-01 01-01

      2 :    01-01 01-01 01-01 01-01 01-01 01-01 01-01 01-01 01-01 01-01

      3 :    01-01 01-01 01-01 01-01 01-01 01-01 01-01 01-01 01-01 01-01

      4   02-01 02-01 02-01 02-01 02-01 02-01 02-01 02-01 01-01 01-01

      5 :    01-01 01-01 01-01 01-01 01-01 01-01 01-01 01-01 01-01 01-01

      6 :    01-01 01-01 01-01 01-01

D1 column represents the most significant diit of the dscp value

D2 column represents the least significant digit of the dscp value

The body of the display represents QQ-TT

QQ = is the queue

TT = is the threshold

above you can see DSCP1 maps to queue1 & threshold 1

by default DSCP 40 to 47 are mapped to queue 2 & threshold 1

res

Paul

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Kind Regards
Paul

Ok))

I continue the guide:

So, when a frame has been assigned an internal DSCP and an egress interface, the

following logic determines into which of the four interface output queues the frame is placed:

1. The frame’s internal DSCP is compared to a global DSCP-to-CoS map to determine a CoS

value.

2. The per-interface CoS-to-queue map determines the queue for a frame based on the

assigned CoS.

Why we need to map into CoS if we just can do mapping between CoS and egress queue??? And what internal DSCP means? That DSCP is rewritten inside switch or marked inside switch?

Hello

Not sure what you are saying here?

Cos- dscp or dscp-cos either will default to the same ingress queue?

#sh mls qos maps cos-dscp

   Cos-dscp map:

        cos:   0  1  2  3  4 5 6  7

     --------------------------------

       dscp:   0  8 16 24 32 40 48 56

Sw1#sh mls qos maps cos-input-q

   Cos-inputq-threshold map:

              cos:  0   1   2   3   4   5   6   7

              ------------------------------------

  queue-threshold: 1-1 1-1 1-1 1-1 1-1 2-1 1-1 1-1

sh mls qos maps dscp-cos

   Dscp-cos map:

     d1 :  d2 0  1  2  3  4  5  6  7  8  9

     ---------------------------------------

      0 :    00 00 00 00 00 00 00 00 01 01

      1 :    01 01 01 01 01 01 02 02 02 02

      2 :    02 02 02 02 03 03 03 03 03 03

      3 :    03 03 04 04 04 04 04 04 04 04

      4 :   05 05 05 05 05 05 05 05 06 06

      5 :    06 06 06 06 06 06 07 07 07 07

      6 :    07 07 07 07

sh mls qos maps dscp-input-q

   Dscp-inputq-threshold map:

     d1 :d2    0     1     2     3     4     5     6     7     8     9

     ------------------------------------------------------------

      0 :    01-01 01-01 01-01 01-01 01-01 01-01 01-01 01-01 01-01 01-01

      1 :    01-01 01-01 01-01 01-01 01-01 01-01 01-01 01-01 01-01 01-01

      2 :    01-01 01-01 01-01 01-01 01-01 01-01 01-01 01-01 01-01 01-01

      3 :    01-01 01-01 01-01 01-01 01-01 01-01 01-01 01-01 01-01 01-01

      4 :    02-01 02-01 02-01 02-01 02-01 02-01 02-01 02-01 01-01 01-01

      5 :    01-01 01-01 01-01 01-01 01-01 01-01 01-01 01-01 01-01 01-01

      6 :    01-01 01-01 01-01 01-01

res

Paul

Please don't forget to rate any posts that have been helpful.

Thanks.


Please rate and mark as an accepted solution if you have found any of the information provided useful.
This then could assist others on these forums to find a valuable answer and broadens the community’s global network.

Kind Regards
Paul

From the book:

he Cisco 3560 uses a relatively simple classification scheme, assuming you consider only what

happens when the forwarding decision has been made. These switches make most internal QoS

decisions based on an internal DSCP setting. The internal DSCP is determined when the frame is

forwarded. So, when a frame has been assigned an internal DSCP and an egress interface, the

following logic determines into which of the four interface output queues the frame is placed:

1. The frame’s internal DSCP is compared to a global DSCP-to-CoS map to determine a CoS

value.

2. The per-interface CoS-to-queue map determines the queue for a frame based on the

assigned CoS.

What means internal DSCP?Why we need to map DSCP to CoS and then to queue if we can map DCSP to queue?

Is question clear?

This issue is also resolved. Everything depends on incoming interface config. If it trusts CoS, DSCP of packet is rewritten (or not depending on pass-through regime) according to CoS-DSCP internal map. All internal QoS in switch is done by this DSCP value (for example egress policer). If incoming interface trust DSCP, CoS is derived by internal CoS-DSCP map and this CoS is mapped to egress hardware queues of specific switch. If incoming port is untrusted and QoS is turned on, CoS and DSCP are set to its default values of 0 (or configured values).

It is my own explaination. I may make mistakes since this topic is a little bit tricky and dapands on switch platform.

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