[cisco-voip] General QOS question

[Bell, Joe]

Actually, layer 2 QoS (a.k.a CoS settings) are in the 802.1q header.
Layer 3, (DSCP or PHB bits), are the 5 in the IP Precedence field.  If
your not using 802.1q, your not using layer 2 CoS.

 

The reason you see switch configs that are not set for full trunking and
instead have command sets like:

 

Interface fa0/1

 switchport  dynamic desirable  <-- negotiates a quasi-trunk

 switchport  voice vlan 100

 switchport  access vlan 200

 spanning-tree portfast

 

 

Is because the newer voice enabled switches are smart enough to
understand the voice vlan information and respect the tags coming down
from the phone, and there are still 802.1q tags, even though you have
not configured a true trunk.  As I said, the phone does insert an 802.1q
header and tags appropriately, but the port is not in a true trunking
mode.  The reason for this, is the new config keeps a lot of trunking
management protocols off the port and frees it up for voip traffic =
efficiency.

 

Older switches were configured as full trunks and you might still see
that out there.

 

interface FastEthernet0/1   <-- true trunking config

 switchport trunk encapsulation dot1q

 switchport trunk native vlan 100  <-- data and untagged vlan

 switchport mode trunk

 switchport voice vlan 200  <-- tagged 802.1q header with the phone
inserting layer 2 CoS and layer 3 DSCP tags

 spanning-tree portfast

 

These configs accomplish the same thing, but the "newer" switches config
is much more efficient.

 

Joe

 

 

 

 

________________________________

From: cisco-voip-bounces at puck.nether.net
[mailto:cisco-voip-bounces at puck.nether.net] On Behalf Of ash AD
Sent: Tuesday, June 06, 2006 5:07 PM
To: Wydra, Jason; cisco-voip at puck.nether.net
Subject: Re: [cisco-voip] General QOS question

 

Jason, 

 

    Trunking is not required to set QoS DSCP or COS settings. Those
markings are not found in the ISL or 802.1q header; they are found in
the ToS byte of the IP header. When Cisco IP phones encapsulate the
voice bearer traffic within the IP packet they set the ToS byte by
default to the EF QoS value. Most people allow Cisco switches to trust
this value and not remark the VoIP packet. 

 

Peter

"Wydra, Jason" <jwydra at Burwood.com> wrote:

	If you don't turn on trunking for an IP phone switch interface,
then how is COS set and/or recognized on ingress?

	 

	Jason Wydra

	Consultant

	burwood group, inc.

	Phone: (312) 327-4677

	Cell: (312) 493-2834

	jwydra at burwood.com

	 

	 

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	https://puck.nether.net/mailman/listinfo/cisco-voip

 

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