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LAN issues
Issue 6 January 2008 291
VLANs
Virtual Local Area Networks (VLANs) are an often-misunderstood concept. This section begins
by defining VLANs, and then addresses configurations that require the Avaya IP Telephone to
connect to an Ethernet switch port that is configured for multiple VLANs. The IP Telephone is on
one VLAN, and a personal computer that is connected to the telephone is on a separate VLAN.
Three sets of configurations are given: Avaya C360, Cisco CatOS, and some Cisco IOS.
VLAN defined
With simple Ethernet switches, the entire switch is one Layer 2 broadcast domain that usually
contains one IP subnetwork (Layer 3 broadcast domain). Think of a single VLAN (on a
VLAN-capable Ethernet switch) as being equivalent to a simple Ethernet switch. A VLAN is a
logical Layer 2 broadcast domain that typically contains one IP subnetwork. Therefore, multiple
VLANs contain logically separated subnetworks. This arrangement is analogous to multiple
switches being physically separated subnetworks. A Layer 3 routing process is required to route
between VLANs, just as one is required to route between subnetworks. This routing process
can take place on a connected router or a router module within a Layer 2/Layer 3 Ethernet
switch. If no routing process is associated with a VLAN, devices on that VLAN can only
communicate with other devices on the same VLAN.
The port or native VLAN
Port VLAN and native VLAN are synonymous terms. The IEEE 802.1Q standard and most
Avaya switches use the term port VLAN, but Cisco switches use the term native VLAN. Issue
the show trunk command on C360s and CatOS Catalysts to see which term is used in the
display output.
Every port has a port VLAN or a native VLAN. Unless otherwise configured, it is VLAN 1 by
default. It can be configured on a per-port basis with the commands in Table 50
.
All untagged Ethernet frames (with no 802.1Q tag, for example, from a personal computer) are
forwarded on the port VLAN or the native VLAN. This is true even if the Ethernet switch port is
configured as an 802.1Q trunk, or otherwise configured for multiple VLANs. For more
information, see VLAN binding feature (
C360).
Table 50: Commands to configure a port VLAN or a native VLAN
Avaya C360 Cisco CatOS
set port vlan <id> <mod/port> set vlan <id> <mod/port>