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Voice VLAN in Cisco SG300 Switches

Hi,

I have done some more reading of the Cisco Small Business 300 1.1 Series Managed Switch CLI Administration Guide, and I am a little confused about the voice VLAN functionality on this platform. In IOS-based (and Cat-OS based) Cisco switches, setting a voice vlan on an interface programs the Cisco phone attached to the port to use the specified VLAN ID as its voice VLAN. The commands used on the switch port are:

Switch#(config)#interface Gi0

Switch#(config-if)#switchport mode access

Switch#(config-if)#switchport access vlan 10

Switch#(config-if)#switchport voice vlan 20

This configuration does the following:

1. It creates a special-case trunk port, which only allows tagged traffic with VLAN ID 20 and uses VLAN ID 10 as the native VLAN on this trunk port.

2. It uses CDP to instruct the attached Cisco IP phone to tag voice traffic with VLAN ID 20.

3. It places all untagged traffic received on this port into VLAN 10. This means that all untagged broadcasts and unknown unicasts received on this port are flooded only out of ports assigned to VLAN 10 and out of trunk ports. This also means that any traffic coming into this port untagged and forwarded out of a trunk port is tagged with VLAN ID 10.

The alternative to the above configuraiton is:

Switch#(config)#interface Gi0

Switch#(config-if)#switchport mode access

Switch#(config-if)#switchport access vlan 10

Switch#(config-if)#switchport voice vlan dot1p

This configuration does the following:

1. It creates a special-case trunk port, which only allows tagged traffic with VLAN ID 0 and uses VLAN 10 as the Native VLAN.

2. It uses CDP to instruct the attached Cisco IP phone to tag voice traffic with VLAN ID 0.

3. It places all received traffic on this port ( tagged traffic with VLAN ID 0 and untagged traffic) into VLAN 10. This means that all broadcasts and unknown unicasts  received on this port are flooded only out of ports assigned to VLAN 10 and out of trunk  ports. This also means that any traffic coming into this port and forwarded out of a trunk port is tagged with VLAN ID 10.

What's the point of this configuration? It's created for QoS purposes. In this case, voice traffic is not separated from data traffic into a separate VLAN once the traffic enters the switch port, but different COS markings can be assigned to voice and data traffic entering the same switch port. Cisco IP phone will mark voice traffic (COS 5 for RTP voice, COS 3 for SCCP or SIP signaling), and the switch port will assign QoS markings to the data traffic (either COS or DSCP), based on the switch model and its capabilities to process internal QoS markigns (COS or DSCP). The reason to create VLAN ID 0 between the IP phone and the switch port is to allow the IP phone to place COS markings, which live inside the 802.1Q header. So, slapping an 802.1Q header with the VLAN ID 0 is done so that the IP phone can place its COS markings inside of that header.

Now I am looking at how Cisco SG300 handle voice VLAN assignment, and I see that the port itself can be placed in the voice VLAN based on different criteria, one of them being OUI of the phone. It appears that in order for an attached Cisco IP phone's traffic to be placed in a voice VLAN (based on CDP information), the voice vlan state auto-triggered command should be configured. Then, the manual says that:

The operational state is auto-triggered only if:

- A static local configured voice VLAN ID, CoS/802.1p, and/or DSCP that

is not factory default is configured.

- A CDP voice VLAN advertisement is received from a neighboring

switch. A Cisco UC device that advertises itself as a switch router is not

considered to be a switch.

- Voice Service Discovery Protocol (VSDP) is a Cisco Small Business

proprietary protocol for SF and SG series managed switches. A Voice

Service Discovery Protocol (VSDP) message was received from a

neighbor.

So, it appears that if the voice vlan state auto-triggered command is configured, and a CDP packet is received on the port, the port will be placed in a voice VLAN. This brings up some questions.

1. Is there a way to specify the voice VLAN ID for each individual port instead of using the global voice vlan id <id_tag>command, which only allows one voice VLAN across the entire switch?

2. If the Cisco IP phone connected to the switch port has a computer plugged into its (IP phone's) LAN port, which VLAN will the data traffic be placed into by the switch?

3. Does the SG300 switch instructs the Cisco IP phone connected to one of its ports to tag vioce traffic with the VLAN ID specified in the SG300 configuration? In IOS switches, CDP instructs the IP phone which VLAN ID to use for tagging its voice traffic.

Thanks!

1 REPLY 1
Participant

Voice VLAN in Cisco SG300 Switches

Hello telecastle,

You may assign multiple vlans to one port or different vlans to different ports. However, the Sx300 only supports one Auto voice vlan.

When a switchport has both a phone and computer connected to it, the data will be part of whatever vlan is untagged on that port. The phone will be part of the voice vlan you have assigned to the phone.

For your last question, the answer is yes if configured to do so. Here is part of the admin guide which better describes what I belive you are looking for.

" A phone/endpoint may obtain the voice VLAN information from CDP and

LLDP-MED advertisements it receives from their neighbor voice systems

and switches. "

I hope this helps answer the questions you have.