Layer 2 QoS

Unanswered Question
Apr 10th, 2008

Hello all,

I have a video streaming multicast server witch performs rather poorly in a LAN environment. I used IGMP snooping for multicast and that part works just fine. However some of the clients are experiencing unacceptable latency and jitter (the video app freezes).

I tried to apply some layer 2 QoS as follows:

1. on the acces port to which the server is connected I used an input service policy which sets CoS to 6 and DSCP to 48;

2. on the trunk ports between the switches - mls qos trust cos;

Then, I started a live capture of traffic from one on the clients which showed that all packets/frames came with default CoS and DSCP of 0 - therefore no QoS in place.

I use Catalyst 4500, 3550 and 2950 series switches in my network.

Any suggestions?

I have this problem too.
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noroutes4u Thu, 04/10/2008 - 06:56

The only think I can think of is that it's not marking at all. You might want to check the ACL of your interesting packets to mark. Could you post the relevant config for us?

ionutionescu Thu, 04/10/2008 - 07:13

access-list 1 remark video-server

access-list 1 permit

class-map video-server

match access-group 1

policy-map video-server-policy

class video-server

set cos 6

set ip dscp 48

police 5000000 8192 exceed-action drop

interface Fax/y

description *** to video server ***

service-policy input video-server-policy

interface Fax/z

description *** uplink ***

mls qos trust cos

wrr-queue bandwidth 1 2 3 4

wrr-queue cos-map 4 6 7

This is configured on a 3550-12T.

All uplinks on other switches are configured with "mls qos trust cos".


Joseph W. Doherty Fri, 04/11/2008 - 04:30

BTW: Multicast replication can tax the performance capabilities of switches. Also, multicast, from lack of customer usage, hasn't always "wrung out" firmware issues quite the same way unicast does. Unknown whether either is a possible issue in this case, but at least with the latter, firmware current?

ionutionescu Fri, 04/11/2008 - 10:28

Well, I don't have the latest IOS on the 3550-12T, I only have version 12.1(22)EA1a.

I searched issues related to mcast with Cisco Bug Tool and didn't find too much.

I will upgrade the IOS though and see what happens...

Now that I think about it, there might be something to this - I've also discovered that although IGMP snooping IS running properly, the interfaces attached to the server and clients report 0 multicasts packets received and a huge and ever increasing number of broadcasts.

The funny thing is that I've extended the solution to mcast routing (PIM) and that part works fine - so basically clients several hops away have no problems while some clients on the same LAN with the server have performance issues.

memosoto Thu, 04/17/2008 - 12:52


You migth need to apply the coomand qos rewrite ip dscp so the packets dscp can be rewritten.




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