layer 3 qos on layer 2 switches

Unanswered Question
Nov 19th, 2007

Hi all, i went on a course last week, he said the layer 2 switches are layer 3 aware for qos? what does that mean, does that mean they can mark the dscp value in the packet, or would they still mark the 802.1p value, also what value do phones mark dscp or 802.1p ?

I have this problem too.
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guruprasadr Mon, 11/19/2007 - 03:58

HI Carl, [Pls Rate if HELPS]

Phones means IP Phones ?

DSCP Chart:

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

7 - Reserved

6 - Reserved

5 - Voice

4 - Videoconferencing

3 - Call signaling

2 - High-priority data

1 - Medium-priority data

0 - Best-effort data

Pls Rate if Helps

Best Regards,

Guru Prasad R

andrew.butterworth Mon, 11/19/2007 - 04:25

What the instructor said is essentially correct, but only applies to more recent Layer-2 switches. For example a Catalyst 2950 running the EI features supports Layer-3 QoS - that is it can read and re-write the DSCP values in the ToS field on ingress ports (the 2950EI doesn't support all 64 DSCP values, only 0, 8, 10, 16, 18, 24, 26, 32, 34, 40, 46, 48, and 56).

The 3550 (I know it is layer-3) and all later switches support all DSCP values - I am thinking more specifically about the 2960 here. Obviously what the switch can do with these values depends on the platform - generally the DSCP value will map to an egress queue (or possibly back to it's CoS value and then to a queue). There are lots of platform limitations between the various Catalyst platforms so read the documentation to understand what you can and can't do.

HTH

Andy

carl_townshend Mon, 11/19/2007 - 08:41

hi there

so do most switches now use the dscp value rather than the 802.1p value ?, where do i apply this policy , on ingress and egress ports ?

andrew.butterworth Mon, 11/19/2007 - 08:52

Most switches support either 802.1p CoS, IP Precedence or DSCP.

With regards to policy - what are you trying to achieve? You don't state what you want to do?

The documentation on CCO is very clear, it's probably more beneficial if you read up on the subject.

Andy

carl_townshend Tue, 11/20/2007 - 08:07

is there a flow chart of some sort that tells you the steps in setting it up?

also do many people use ip precedence, does dscp use the tos field like ip precedence?

I am trying to provide quality of service to my phones which mark the dscp value on 46, what should i do on my switches?

andrew.butterworth Tue, 11/20/2007 - 08:44

Carl, please read the documentation on CCO. Regardless of the switch platform the explanations at the beginning of the QoS section in the documentation for each switch is generic. Read this to understand what QoS is and what it's uses are:

http://www.cisco.com/en/US/products/hw/switches/ps5023/products_configuration_guide_chapter09186a00808c73d4.html

You seem to have initiated lots of questions on here that are all easily answered by reading the documentation. Please refer to the documentation before posting.

Andy

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