IP phone - trunk or access port?

[rolf.fischer_2]

Good day,

there seem to be two options to configure a switchport for an IP phone, either as an access port or as a trunk. In both cases there is an additional voice vlan needed (at least I'm thinking so).

Are there reasons to configure a trunk instead of an access-port (provided that the plattform supports a voice vlan)? I'm questioning because I don't want the phones play an active role in the spanning tree and portfast won't work for a trunk.

Links to related documents are welcome!

Thank's in advance

Rolf Fischer

[spremkumar]

hi

i feel you can have a seperate VLAN for the voice and data if you are connected both ur pc and ip phone on the same port.

so that you can give the necessary qos priority to the EF marked/cos 5 marked traffic.

regds

[rolf.fischer_2]

This sounds plausible. But if the phone's data-vlan and the PC's vlan are the same a trunk wouldn't be necessary, right?

[hpoel]

Rolf,

Use the port in access mode with a voice vlan. When CDP detects a ip phone, the port will operate in a limited trunk mode that allows only 802.1Q tagged frames on the voice vlan. The port still operates mainly as an access port.

Hans van der Poel

[Aaron Harrison]

It's also worth pointing out that although phones contain a switch, they DON'T participate in spanning-tree... The 'switch' is there basically to allow remarking QoS on packets passed through from PCs and so on.

Aaron

[rolf.fischer_2]

Thank you for this information. But I think that - at least in our network - the phone switches could indirectly affect the spanning tree. We're running standard ST (802.1d) and I normally configure portfast and bpduguard at end-device ports. In case of a trunk, portfast won't work and that means link changes would cause ST topology changes. And, furthermore, missing portfast might cause dhcp problems.

[Aaron Harrison]

Hi Rolf

Try it - if you have a Cisco network you'll find that on most switches when you configure a port as an access port (switchport mode access) and enter the voice vlan command, that portfast is enabled automatically and works despite it being 'trunked' to the phone.

It is the standard configuration to have ports set up like this and it works fine for everyone else in the world!

There won't be any topology changes, and DHCP will be fine.

Aaron

[rolf.fischer_2]

Hi Aaron,

this is exactly, what I wanted to know.

Currently, our phone ports are configured like this

...

switchport mode trunk (!!)

switchport encapsulation dot1q

switchport trunk native vlan xxx (!!)

switchport voice vlan yy

...

and I was asking if there is any reason not to change it to

...

switchport mode access

switchport voice vlan yy

...

For several reasons (e.g. spanning tree issues) this seem to be the much better configuration and I don't see any advantages in the current configuration.

Rolf

[jbirk]

Hey Rolf,

What switches are you using? We have tried this on 3524 switches and it doesn't work to have the access configuration. We have some newer switches that I haven't tried this on.

[PATRICK ALAND]

If you are running any of the XL series switches (or older etherswitch modules in your routers) you need to configure as a trunk with voice vlan. If you are using 2950/3550/etc you can configure as access with a voice vlan.