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Engineering Guidelines
210
IEEE 802.1p (Layer 2 priority) uses a field in the IEEE 802.1Q tag to provide eight levels of
priority. IEEE 802.1Q is the open VLAN standard that extends the Ethernet header by adding
an additional 4 bytes to tagged packets. Because the 802.1p priority is part of the VLAN header,
ports that need to convey multiple VLANs/802.1p priorities must use tagging. This includes
ports used between LAN switches and ports connected to dual-port phones.
With dual-port phones, it is important to configure the LAN switch to use tagging for the voice
VLAN and no tagging for the default VLAN, to ensure that voice packets are properly prioritized
over data applications from the PC.
There is potential in the VLAN specification to interpret the standard, with respect to VLAN 0,
in different ways. This can lead to incompatibility between different vendor units. Do not use
VLAN 0.
The main requirements are
• Ports should be configurable to provide VLAN tagging to incoming untagged information
and remove this tagging when passing out of the switch. This is used by the controller and
associated applications.
• Ports should be configurable to pass all active VLANs with tagging from one switch to
another (there is no untagged information present in the connection). This is used between
LAN switches and maintains priority information between units.
• Ports should be configurable to accept untagged information, to pass this on to a specified
VLAN, as well as to accept tagged information. On egress, the port strips off tagging for
data from a specific VLAN, but does not strip data from other VLANs. This is used when
connecting the dual-port phones and PCs to the network, so that tagged data goes to the
phone and untagged data to the PC.
Some other VLAN guidelines for use with voice are:
• Additional bandwidth is always good.
• Use full duplex wherever possible.
• Do not use VLAN 0.
• Set Priority to value 6 for voice. (Value of 5 used in Cisco networks)
• Set Priority to value 3 for signalling. (Value of 3 used in Cisco networks)