Re: mls qos & switchport priority extend

From: Petr Lapukhov (petr@internetworkexpert.com)
Date: Tue Jul 15 2008 - 13:35:21 ART

David,
Those "switchport priority" commads come from the old 3500XL/2900XL Layer 2
switches. To understand their behvaior, imagine a switch with an IP Phone
connected. Suppose next, that a PC also connects to the other port in the IP
Phone (via the buil-in three-port switch).

[Switch]--802.1q--[IP_Phone]----[PC]

The link between the switch and the IP phone is usually 802.1q trunk. Now,
the "switchport priority extend" commands configure the switch to command
the IP Phone - they don't have any impact on the switch itself. The
instructions to the IP phone are delivered via CDP protocol.

When you configure "extend trust", the IP Phone will trust 802.1p marking
(if any) that comes from the PC, and will pass the frames with CoS field
unmodified.

When you configure "extend cos x" the IP phone will forcefully set CoS value
of "x" for frames forwarded from the connected PC.

Both the commands require the switch to trust "CoS" in order to be
effective, since the commands are purely L2. Obviously, the command will
only have effect when the connection between the switch and the IP Phone is
a 802.1q trunk, or uses so-called 802.1p Voice VLAN option - since those are
the only ways to carry CoS bits from the IP Phone to the switch.

HTH 

-- 
Petr Lapukhov, CCIE #16379 (R&S/Security/SP/Voice)
petr@internetworkexpert.com

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2008/7/15 David Lonnie <david.lonnie@gmail.com>:

> Hi,experts:
> What's the differences between "mls qos trust cos" & "switchport priority
> extend trust", and "mls qos cos 1" & "switchport priority extend cos 1"?
>
> I am confused when i read the docCD, looks like the same ,but i am not
> sure.
> if somebody can explain for me, i will be very graceful.
>
>
> Thanks in advance.
>
> David
>
>
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