Specifications

Engineering Guidelines
216
Network topology with priority
The following network diagram highlights the use of the dual-port phones and the configuration
of a network including VLAN priority and also the use of DiffServ/TOS in the WAN connection.
Figure 35: Network Topology with Priority
In Figure 35, the network switch ports connected to the dual-port phones must be able to accept
both untagged and tagged information. The untagged data is translated to a data VLAN (1). In
this case, VLAN1 is also the default or native VLAN. The voice is destined for a voice VLAN (2).
In the outgoing direction, these ports must also pass information from the voice VLAN still
tagged, but traffic from the data VLAN must be sent untagged for the devices that are not able
to handle VLAN information.
The requirement to use VLAN and priority queuing becomes obvious when both data and voice
information must share a link between units within the network. It is important that the
deterministic voice information gets priority over the non-deterministic data traffic. This is where
IEEE 802.1p comes into play (IEEE 802.1p is a subset of IEEE 802.1Q).
Routers or Layer 3 switches involved in segmenting the network also need connections to the
different VLANs. Each VLAN is identified by a VLAN number and by a unique subnet address.
Routers and Layer 3 switches that are unaware of VLAN can still pass data between the VLANs.
A separate physical connection to each VLAN must exist and ports on the Layer 2 switch must
pass information only to and from one specific VLAN. At the Layer 2 port, the VLAN information
is removed on egress and added on ingress according to the port or VLAN configurations.
Some routers are VLAN-aware and are considered to include a virtual Layer 2 switch within
the unit, which then directs data according to the VLAN information. These devices are often
referred to as including virtual ports. Their advantage is that only one physical connection is
required to handle multiple VLANs.