4

Imagine this topology:

[PC] ------- [VOIP Phone] ------- [eth0/0 on a Switch]

And this configuration on the switch:

vlan 22
  name DATA

vlan 33
  name VOICE

There are two ways to have both the PC's traffic and the VOIP phone's traffic "arrive" on eth0/0 in distinct VLANs. 1. Use a Trunk port, 2. Use the Auxillary VLAN feature".

Option 1:

int eth0/0
  switchport mode trunk
  switchport trunk native vlan 22
  switchport trunk allowed vlan 22,33

Option 2:

int eth0/0
  switchport mode access
  switchport access vlan 22
  switchport voice vlan 33

The net effect of either of these is the same untagged traffic from the PC arrives and is accepted into VLAN 22 (the data VLAN), and tagged traffic from the VOIP phone arrives and is accepted into VLAN 33.

My question:

What are the benefits of using Option 1 or Option 2? What reasons exist that make either of these better than the other?

  • 1
    with option 2 you can use "spanning-tree portfast edge" as a global command to enable Portfast on all "switchport mode access" ports. Enabling "spanning-tree portfast trunk" with Option1 will set your edge-ports to portfast, but also the Trunks to your upstream switches which might be an unwanted option. – Andreas Schaefer Feb 1 at 18:46
2

Option 1 carries some security risk because broadcast and flooded traffic for the VoIP VLAN will be sent to the interface, even if there is no phone attached. Also, most PCs do not understand VLAN tags (some can be configured to do so, which is the security risk), and they will receive what appears to be garbage, wasting bandwidth on the link.

Option 2 is preferred because the switch interface will automatically optimize for what is, or is not, attached to it.A Cisco phone and switch will use CDP to negotiate a trunk.

In some cases with a non-Cisco phone, you may be forced to use Option 1, or you may be able to use LLDP instead of CDP to negotiate the trunk.

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