From: Chris Lewis \(chrlewis\) (chrlewis@cisco.com)
Date: Thu Jun 30 2005 - 15:54:50 GMT-3
I had promised to check with a few internal folks for confirmation,
howver that was before Larry posted and he has basically answered all
the questions we had. I have replies and teh following is what I have
received for those still following this thread.
CSL to Cisco SME:
I have a question that is out of my normal work and don't have access to
a 3550 and IP phone sto test it out, but I think you probably know
straight off, so here goes :)
What is the "right" way to configure voice vlan on a 3550, I see
differing recommendations. For example in Maurilio Gorito's book, I see
the following:
In practice lab 2, configurations are shown for connecting a 7960 that
does trunking, and a 7905 that does not do trunking.
The port connecting to a 7960 is configured for trunking, and the port
connected to the 7905 is not. This is given on p96
3550 config for 7960 phone
int fa0/16
switchport access vlan 2
switchport trunk encapsulation dot1q
switchport trunk native vlan 2
switchport mode trunk
switchport voice vlan 50
no ip address
duplex full
speed 100
spanning-tree portfast
3550 config for 7905 phone
int fa0/17
switchport access vlan 50
no ip address
duplex half
speed 10
The explanation is given as follows:
The 7960 has the capability to trunk to the 3550 as it has an on-board 3
port switch and can separate the voice and data traffic
appropriately.The 7905 phone only has 10 base T and needs manual
insertion in to the voice vlan. Ensure that the port connecting to the
7960 is configured as a trunk using dot1q and that the native vlan is 2.
SME: This is an older method of configuring the Voice VLAN (VVLAN) and
the native Data VLAN (DVLAN).
However, if I search newsgroups, I see lots of recommendations that list
the following:
mls qos
! interface FastEthernet0/7
switchport access vlan 20
switchport voice vlan 10
switchport priority extend cos 0
mls qos trust cos
spanning-tree portfast
SME: Yes - this is the newer way. The "switchport voice vlan" command
was introduced in 12.1(9)EA1. Basically, it allows the config to be
shortened by a few lines, as it automatically sets up the dot1q trunk
(by default), and assosciates the DVLAN as a native VLAN, etc.
I guess my main question is if you have a 3550 connected to a 7960
phone, should the 3550 port be configured for trunking or not? If the
answer is both are valid configurations, are there any differences in
operation?
SME: Both configs will set up the VVLAN/DVLAN via dot1q trunking. It's
just an administrative preference to enter the full set of commands or
just "switchport access vlan" coupled with "switchport voice vlan".
In maurilio's example, why would the 3550 port be configured both for
accees and trunking?
SME: Because the port is configured to operate as an access-port for the
native VLAN (the DVLAN); trunking only applies to the VVLAN. Yes, it's a
bit confusing, but that's how they coded it.
In Maurilio's example, is this analysis correct?
For the 7960 phone to encapsulate in to .1q packets, it needs to know
the vlan ID to use, and you do not configure the 7960. What you do
configure is the 3550 port and that uses CDP to tell the 7960 what the
voice vlan number should be, in this case 50. Now nothing has told the
7960 what vlan header to encapsulate the data traffic in to, so it does
not encapsulate it in to anything. So if we want to get the non .1q
packets coming from the PC attached to the 7960 phone in to a specific
vlan, our only choice is to make that the native vlan, so the 3550 will
say, a hah, a non .1q packet, therefore it is in the native vlan and off
we go.
SME: This seems about right to me.
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