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LAN Switching SRR/WTD on 3750s

Hi,

Trying my level best to understand how the LAN QoS works for the configuration of WTD, SRR and Queuing on 3750s.

Is there any good doc explaining that, any help would be helpful.

Regards

Wilson SAmuel

8 REPLIES
Super Bronze

Re: LAN Switching SRR/WTD on 3750s

The basics (e.g. WTD, SRR) are, I think, fairly explained in the configuration guides QoS chapter. Have you read your IOS verison's QoS chapter?

You might find this document helpful too: http://www.cisco.com/en/US/products/hw/switches/ps5023/products_tech_note09186a0080883f9e.shtml

Hall of Fame Super Blue

Re: LAN Switching SRR/WTD on 3750s

Wilson

The configuration guides give a good overview of how WTD/SRR work -

http://www.cisco.com/en/US/docs/switches/lan/catalyst3750/software/release/12.2_46_se/configuration/guide/swqos.html#wp1284809

or was there something more specific you were looking for ?

Jon

Re: LAN Switching SRR/WTD on 3750s

Thanks Joe/Jon,

Actually I was trying to interpret the cos-dscp table, which wen over the head.

I tried reading (deciphering) the Config Guide, and thats where it all started.

Any simplistic answer to read/interpret the maps?

Regards

Wilson Samuel

Hall of Fame Super Blue

Re: LAN Switching SRR/WTD on 3750s

Wilson

"Actually I was trying to interpret the cos-dscp table, which wen over the head"

Could you clarify what you mean ie. is it the CoS and DSCP markings you are having trouble with or reading the output of the map on the switch.

If it's the output can you post a sample as i don't have access to a switch at present.

Jon

Super Bronze

Re: LAN Switching SRR/WTD on 3750s

If you mean this map:

3750#sh mls qos maps

.

.

Cos-dscp map:

cos: 0 1 2 3 4 5 6 7

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

dscp: 0 8 16 24 32 40 48 56

.

.

It shows what CoS values set various DSCP ToS values. This can be useful if you receive a CoS tagged frame and then want to have the IP packet reflect the L2 CoS value in the IP ToS. The default translation map changes the 8 CoS values into their (more or less) equivalent DSCP (or IP Prec) values, but you can change the translations, if you so desire.

The common reason for such a translation, many pure L2 switches that support QoS only can work with the CoS value, not the IP ToS. Further, CoS comes and goes with VLAN tags. Given these two issues, it can be convenient to be able to set a CoS value based on ToS or the converse.

Re: LAN Switching SRR/WTD on 3750s

Gentlemen,

Apologies, its the other way around I cant find myself comfortable, ie DSCP-COS

FordAnnex-MDF-01#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

FordAnnex-MDF-01#

(Included the output to make sure that I'm not missing anything this time :-)

Any suitable easy understanding trick for this?

Regards

Super Bronze

Re: LAN Switching SRR/WTD on 3750s

What this map does it translate any DSCP value to a CoS value. Keep in mind there are 64 DSCP values but only 8 CoS values.

What the default map does, is translate DSCP 0..7 to CoS 1, DSCP 8..15 to CoS 2. etc.

The map matrix represents each DSCP value, the values in the matrix represent the CoS value.

e.g.

d1 = means ten values

d2 = means one values

So, DSCP 16 (d1 = 1 and d2 = 6) would become CoS value (by default) 2

Hall of Fame Super Blue

Re: LAN Switching SRR/WTD on 3750s

Wilson

There are 64 possible DSCP values. If you look at the above table you will see that their are 64 entries. So it's simply a matter of mapping the DSCP value to this table eg.

DSCP 46 =

row beginning with 4: ie.

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

so it's on the above row. Then starting from 0 simply count along the row so

40 = 1st entry

41 = 2nd

42 = 3rd

..

46 = 7th entry

the 7th entry on this row has the value 5 so DSCP 46 maps to CoS value 5.

Jon

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