Polycom SoundPoint IP 300 Supplementary Manual page 3

The first two bytes of the tag are statically defined to the Ethertype for tags, so that tagged frames
can be distinguished from untagged frames. Twelve bits optionally encode a VLAN number (0-4095),
and three bits encode the QoS value (0-7). Each Ethernet link has a "native VLAN" which is assumed
for all frames which do not have a tag or which have a VLAN number of 0 in the tag.
General Operation of VLANs
All Polycom SoundPoint IP models support the optional use of 802.1q/p tags both for VLAN
separation as well as QoS.
By default, the phone will tag all traffic coming from the phone with a null 802.1q VLAN ID. If a
VLAN ID value is configured or learned as described below, it will be inserted into the tag of all
packets generated by the phone. A different QoS value may be configured, but this is not
recommended.
If a PC is connected to the phone, all packets generated by the PC will be passed through
unmodified, regardless of the presence of an 802.1q/p tag or its contents. Since PCs do not typically
tag frames, this means they will be on the native VLAN.
Manual Configuration of VLAN ID
In the local Polycom SoundPoint IP's Network Settings menu, there is a field called "VLAN ID"; this
allows configuration of the particular VLAN that the phone should be a member of. It defaults to
null, which means that the phone will be on the native VLAN specified in the Ethernet switch.
Because this setting must be known before the phone can obtain an IP address or retrieve its
configuration files from its boot server, there is no way to specify the VLAN ID in either those
configuration files or via the web configuration interface.
Automatic Configuration of VLAN ID
In the Network Settings menu, there is another field called "CDP"; this controls whether the phone
supports the Cisco Discovery Protocol. Among many other things, CDP allows the Ethernet switch to
inform an IP phone what VLAN ID it should use without requiring manual configuration of each
phone.
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