Specifications

Network Configuration Specifics
233
Notes:
1. A DSCP value of 46 is recommended for newer installations using DSCP-aware routers. Value 46
will place the voice into the Expedited Forwarding Queue (EF).
2. Depending on certain network conditions the phone will use different DSCP default values. The
default values under specific conditions are:
• If the VLAN information was learnt via CDP, signalling will use a DSCP value of ‘46’ and voice will
use a DSCP value of ‘46’.
• In situations where VLAN information cannot be learnt from either CDP or DHCP, the phone will
use a DSCP value of ‘0’ for both signalling and voice.
Potential issues with using LLDP-MED in Cisco environments
The Issue:
Erroneous VoIP QoS values have been noted when using LLDP-MED with the following Cisco
IOS software releases:
• IOS 12.2(37)
• IOS 12.2(40)
Cisco switches running the above operating systems with LLDP-MED enabled will issue the
phones these LAN Policy values:
• Valid VLAN ID
• L2 (802.1p) = 0 (Incorrect value)
• L3 (DSCP) = 0 (Incorrect value)
Table 70: Priority levels for the Various Sources of L2/L3 QoS Settings
Source of L2 & L3
QoS Settings
Priority
Level
Notes
Manual Entry (Static) 5 Programmed by installer
DHCP 4 The first time a phone receives DHCP information it must contain an IP
address for the RTC and the TFTP server. This is also true for the double
DHCP fetch mechanism.
If the phone fetches DHCP information a second time, this information will
over write the previous values.
DHCP can be used to provide separate L2 and L3 QoS values for both
signalling and media.
If DHCP has only been programmed with one value, the phone will use this
value for both signalling and media.
LLDP-MED 3 Information obtained from L2 switch
CDP 2 CDP does not provide values, however if CDP is detected on the LAN the
phones will use Cisco ‘inferred’ values.
L2 Value = 5, L3 DSCP Value = 46
Default Values 1 L2 Default Value = 6, L3 DSCP Value = 46. See additional Notes below.