802.1Q truck & set-cos command

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May 26th, 2008
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Hello All,

I need my router (2851) to mark VoIP packets on the egress I/F (Gi0/0) before sending them to my core switch (4006) with a COS value of 5. I created a policy map to do this but when I try to apply the policy map to the egress I/F I get this message:

"Set cos supported only with 802.1Q/ISL interfaces". My questions, How do I create a 802.1Q I/F on my router and what are the ramifications of doing this?

Thanks, Mitchell

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Jon Marshall Tue, 05/27/2008 - 05:45
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The Cos markings are contained within the 802.1Q tag so without a trunk link you cannot set Cos markings. To create a trunk on your 2851

int fa0/0.10

encapsulation dot1q 10

ip address

where this interface is for vlan 10. Note that the line

"encapsulation dot1q 10" you must use the same number ie. 10 as the vlan number.

If you make this link a trunk it can carry multiple vlans across it. The ramifications are difficult to say without knowing your existing configuration between the 2851 and the 4006.


mitchell.smith Tue, 05/27/2008 - 08:20
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Hi Jon,

Thanks for your post. What I am trying to do is take packets that have a dscp value of ef sent from the router to the switch and have the switch give them priority, which right now it is not. I believe the switch cannot recognize dscp as this is a layer 3 deal, is this correct? So I want the router to take these ef packets and mark the cos value before they get sent to the switch so the switch's QOS will prioritize them. Am I on the right track?

About the config you recommend, if I create this sub-interface for the vlan, can I still pass data through the primary interface? Do I need to create a matching trunking vlan on the switch?

Thanks for your help, this stuff is a bit new to me.

Jon Marshall Tue, 05/27/2008 - 20:03
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Which supervisor do you have in the 4006 and what IOS version are you running.

mitchell.smith Tue, 05/27/2008 - 21:19
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Hi Jon,

Here is show ver:

cat4006> (enable) show ver

WS-C4006 Software, Version NmpSW: 8.1(3)

Copyright (c) 1995-2003 by Cisco Systems, Inc.

NMP S/W compiled on Oct 10 2003, 13:04:11

GSP S/W compiled on Oct 09 2003, 16:28:08

System Bootstrap Version: 6.1(5)

Hardware Version: 6.0 Model: WS-C4006 Serial #: FOX0734047T

Mod Port Model Serial # Versions

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

1 2 WS-X4013 JAE07381WTZ Hw : 6.0

Gsp: 8.1(3.0)

Nmp: 8.1(3)

2 48 WS-X4548-GB-RJ45 JAE073417X1 Hw : 1.0

3 48 WS-X4548-GB-RJ45 JAE073413UB Hw : 1.0

4 6 WS-X4306-GB JAE061102YQ Hw : 2.2

Thank You!


bmcginn Tue, 05/27/2008 - 22:14
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Does your version of software know auto qos? Can you run the 'auto qos voip trust' command on the switch uplink interface? I think it auto-maps the DSCP to CoS bits and sets the queues to appropriate values etc. I think you may also need to tell the switch to trust the markings with something like 'mls qos trust' cos on the interface.

I'm pretty sure it works on IOS, but am unsure if its the same in CatOS.

my 2c

Brad :)

mitchell.smith Wed, 05/28/2008 - 05:41
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Hi Glen,

Thanks for your input, I am aware that the CatOS does not support "auto-qos trust" etc., thats why I need to set the COS value on the packets at the router egress I/F before they reach the switch. You can see from my original post what went wrong when I tried to do this on the router and why. Any ideas on the best way to do this?



mitchell.smith Wed, 05/28/2008 - 05:38
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Hi Brad,

Thanks for your input, but the CatOS does not support these features, thats why I need to set the COS value of the packet at the router egress I/F before the packets arrive at the switch.

Any ideas?



jimmyc_2 Thu, 10/16/2008 - 06:28
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We have a shop that uses both isl and dot1q. when we go to VoIP, should I rebuild everything to dot1q only? thanks


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