From: Pun, Alec CL (Alec.CL.Pun@pccw.com)
Date: Thu Nov 27 2003 - 13:09:45 GMT-3
Just a minor advice :
If dot1p is used, that implies voice traffic is using the same VLAN as data
traffic.
regards,
alec
-----Original Message-----
From: William Chen [mailto:kwchen@netvigator.com]
Sent: Thursday, November 27, 2003 11:39 PM
To: Ken.Farrington@barclayscapital.com
Cc: ccielab@groupstudy.com
Subject: Re: Voice VLAN (oh no, not again......)
Hi Ken,
See Below.
----- Original Message -----
From: <Ken.Farrington@barclayscapital.com>
To: <ccielab@groupstudy.com>
Sent: Thursday, November 27, 2003 8:29 PM
Subject: Voice VLAN (oh no, not again......)
> Guys, At the risk of repeating questions on this forum, (I did read some
of
> them) could someone just check out my comments/questions:-
>
> IP Phone (voice) configuration on c3550.
> ----------------------------------------------------------
>
> The first thing to remember is that you always configure the following two
> commands:-
>
> From the Cisco config guide :-
> Before you enable voice VLAN, we recommend that you enable QoS on the
switch
> by
> entering the "mls qos" global configuration command and configure the port
> trust state
> to trust by entering the "mls qos trust cos" interface configuration
> command.
>
>
> Config 1 (trunked)
>
> mls qos
> !
> interface FastEthernet0/2
> switchport access vlan 500
> switchport trunk encapsulation dot1q
> switchport trunk native vlan 500
> switchport mode trunk
> switchport voice vlan 501
> switchport qos trust cos
> no ip address
> spanning-tree portfast
> !
>
> Config 2 (non-trunked)
>
> mls qos
> !
> interface FastEthernet0/2
> switchport access vlan 500
> switchport mode access
> switchport voice vlan 501
> switchport qos trust cos
> no ip address
> spanning-tree portfast
> !
>
First you don't have to set up trunk at the access port, the trunk will
be automatically setup run CDP between the cisco switch and the Cisco IP
Phone (Don't know if you connect different IP phone with Cisco switch). This
special trunk is created and only allow the Voice VLAN and the access VLAN.
The packets from PC is untagged as normally, but the packets from the IP
phone can be either tagged or untagged. If the packets from the IP Phone is
untagged ("switchport voice vlan none", which is the default), it is unable
to distinguish the packets from those sent by the PC and therefore poor in
QoS. The packets from the IP phone can also be tagged (a special trunk will
be formed in this case) in three different ways,
1. "switchport voice vlan <id>". This is the most common setting, the IP
phone is asked by CDP to send the packets tagged by the VLAN <id> to
distinguish from PC packets.
2. "switchport voice vlan dot1p (NOT dot1q). This is a tagged packets
with VLAN ID = 0.
3. "switchport voice vlan untagged". Although it is stated as untagged,
the packets is actually tagged with VLAN ID = 1025".
In case 2 or 3, I think the switch will just accept it even it is a
non-trunking port. It is because the packets is tagged with special VLAN ID.
(I would like to be confirmed if anyone know there is a document taking
about it.)
>
> Questions
>
> What is the difference between these commands
> switchport voice vlan 501
> or
> switchport voice vlan dot1q
"switchport voice vlan 501" states the IP Phone packets should be tagged
with VLAN ID=501 and "switchport voice vlan dot1p (NOT dot1q) states the IP
Phone packets should be tagged with VLAN ID=0. Packets tagged with VLAN ID=0
is treated as the native VLAN's packets.
Best Regards,
William Chen
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