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New Member

Voice VLAN / Trunking Question

I am in the process of a designing the configs for a large network upgrade. All access switches are Catalyst 3550-24-PWRs. We will be getting a handful of Cisco IP phones shortly. I've seen several posts stating that ports connecting to IP phones (which in turn connect to PCs) must be trunks. Logic says that this must be the case. However, in the guides covering voice VLAN configuration, there is no mention of trunking. Do the guides assume that a trunk will be negotiated between the switch and the phone? If I put a "switchport mode access" line in the interface config, will the data traffic be dropped (even if the access VLAN and voice VLANs are defined)? Can anybody point a black and white reference? Thanks in advance.

10 REPLIES
New Member

Re: Voice VLAN / Trunking Question

Hi,

Yes the recoomedation is to use seperate vlan for voice and data and the connection to the IP phone from the switch would be a trunk port in DOT1Q because it is.

1. Used to do QOS and vlan tagging

2 QoS required when there is a possibility of congestion in buffers

3. Using the dot1Q trunk creats Multiple Queues and they are the only way to “Guarantee” Voice Quality

4.You can't have two Vlans on same port without it being in trunk mode.

If you use switchport mode access, you can only bind one vlan to that swicth, with no way to do QOS and the PC port on the phone can be considered inactive as there is no way to forward the traffic to its intended data vlan

Even if documentation does not mention, make is a must to use seperate vlan for voice and data and use dot1Q trunks to phone with voice vlan specified or auxilliary vlan is using Native IOS

Hope it helps

Tola.

New Member

Re: Voice VLAN / Trunking Question

Great, thanks for the quick response.

Gold

Re: Voice VLAN / Trunking Question

Check my earlier post, which details interface configuration for a Cisco IP Phone switch port:

http://forum.cisco.com/eforum/servlet/NetProf?page=netprof&CommCmd=MB%3Fcmd%3Dpass_through%26location%3Doutline%40%5E1%40%40.ee96e6b/0#selected_message

I configure all the ports on a PWR switch as VLAN trunk ports by default, so I can plug an IP Phone into any one of them and have it come up on the voice VLAN, with priority. Computers plugged directly into the PWR switch, or into IP Phones that plug into the switch, come up on the native VLAN assigned to the port. Sometimes the native VLAN (a.k.a. "data" VLAN) on a switch may need to be different, port by port; but I find that it works best for me when the voice VLAN is the same on every port of a switch, by building.

Hope this helps.

Re: Voice VLAN / Trunking Question

Hi,

We do it the same way, but you have to listen carefully if the customer mentions performance problems.

Most of the cases it poses no problems.

However, there are some NICs which have difficulties handling the tagged packets and slow down a lot. You'll have session breaks, packet drops - it can get pretty ugly....

We had at least 2 cases where customers reported problems after configuring every port like this.

Cheers,

Martin

New Member

Re: Voice VLAN / Trunking Question

Hi all,

I have some confusing regarding "trunk connection to the IP phone from the switch", which Vlan's traffic is tagged through the trunk? voice vlan only? or voice and date(native) both?

And even data(native) and voice vlan configured, still have management vlan 1 working for CDP...? or all management traffic goes through native vlan?

thanks,

David

Gold

Re: Voice VLAN / Trunking Question

Voice traffic is tagged; workstation traffic is left untagged. Therefore, workstation traffic appears in that port's native VLAN.

I don't think CDP is VLAN-tagged under any circumstance. CDP is purely local to the link and shouldn't need to be tagged or trunked.

New Member

Re: Voice VLAN / Trunking Question

On a related topic ...

What about the trunk between switches? The most common switch configuration I've used so far in small VOIP projects is a 3550-12 as the hub and 3550-24 PWR as the spokes. So there's a trunk between the 3500-12 and the 3550-24 . Any thoughts on configuring the trunks between switches? 8021.Q Trunk? Dynamic Access?

Re: Voice VLAN / Trunking Question

For switch to switch I've just carried on with the dot1Q as I couldn't see anypoint in running different trunking between switches to that running on the ports.

Just only allow the VLANs you require rather than all.

Paul

New Member

Re: Voice VLAN / Trunking Question

So stick with dot1Q ...

What trunk setting do you use for the inter-switch trunk connection if you're using 802.1Q encapsulation because of Voice VLANs? The GUI gives about 5+ options. Don't have one here to double-check the options. As I recall, the default in the GUI is dynamic access for GBIC slots. Do you use this dynamic access setting? Or change to something else like the 802.1Q trunk?

New Member

Re: Voice VLAN / Trunking Question

For switch to switch I use the basic:

switchport trunk encapsulation dot1q

switchport mode trunk

This is on our 3508, 3524, 3548 and 3550-24's. This allows vlans to propagate to all switches. Hope this helps

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