Specifications
Engineering Guidelines
252
1. Manual Entry at boot time
2. LLDP-MED
3. CDP
4. DHCP.
The ability to provide partial information at each stage allows these modes to be used together
to ease installation. For example, the IP phone’s IP address may be supplied manually, but the
RTC address could be picked up via DHCP. Also, CDP does not provide priority (COS)
information, so the VLAN could be picked up from CDP, but the priority (COS) provided by DHCP.
In order to obtain VLAN information via CDP, some network port settings need to change. The
ideal settings are as follows:
• Set the network port to Access (this can be static or set to dynamic for use with VMPS).
• Set the Voice VLAN or the Auxiliary_VLAN setting. (The example uses VLAN 2)
• Enter the data, or default, VLAN into the Native_VLAN setting (note that this value can
change if VMPS is active). (The example uses VLAN 100)
• In DHCP, there is no requirement to enter VLAN or Priority into the default/data VLAN
(during upgrade to 5.1 this setting may still needed).
• If the VLAN information is obtained via CDP and the default priority value of 5 is not to be
used, remember to program this value elsewhere, e.g. the Priority field in the voice VLAN
scope of DHCP.
Note: Default Priority with CDP: Where CDP provides the VLAN information, Layer 2
priority (802.1p), or COS, information is not provided. If the VLAN information is provided
via CDP then the IP phone will provide a default priority value, or COS, of 5 unless
provided by other means, e.g. manual or via DHCP. In this case, the phones will be
compatible with Layer 2 settings that might also be employed by Cisco IP Phones. This
will ease some installations by allowing certain textbook examples to be used. For a
Cisco environment, many installations use a COS value of 5, although with other vendor
equipment, a value of 6 is still preferred. DHCP can be used to override this default COS
value, allowing CDP to provide the VLAN information.
Table 78: VLAN Priority Information
VLAN Information
Priority Information
(location)
Priority (802.1 p)/COS
Value
Manual Entry Manual, DHCP 0-6
LLDP-MED Manual, LLDP-MED, DHCP 0-6
CDP Default 5
CDP DHCP 0-6
DHCP DHCP 0-6