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Trunk configuration
A trunk port on an Ethernet switch is one that is capable of forwarding Ethernet frames on
multiple VLANs through the mechanism of VLAN tagging. IEEE 802.1Q specifies the standard
method for VLAN tagging. Cisco also uses a proprietary method called ISL. Avaya products do
not interoperate with ISL.
A trunk link is a connection between two devices across trunk ports. This connection can be
between a router and a switch, between two switches, or between a switch and an IP
Telephone. Some form of trunking or forwarding multiple VLANs must be enabled to permit the
IP Telephone and the attached personal computer to appear on separate VLANs. The
commands in
Table 69
Table 69: Administration commands for VLAN trunking
Avaya P33xT v3.2.8 and later
set trunk <mod/port> dot1q
By default, only the port VLAN or
the native VLAN is enabled on the
trunk port. Another set of
commands is required to specify
other allowed VLANs.
Note that Cisco can remove VLANs from a trunk port. This is a highly desirable feature because
only two VLANs at most should appear on a trunk port that is connected to an IP Telephone.
That is, broadcasts from nonessential VLANs should not be permitted to bog down the link to
the IP Telephone. The pre-3.2 version of P330 code did not have the capability to clear off
unwanted VLANs. Enabling 802.1Q trunking enabled all the VLANs. Newer versions of P330
code can limit the VLANs on a trunk, but in doing so they alter the previous trunking behavior.
VLAN binding feature (P330 v3.2.8)
With both Cisco and the pre-3.2 P330 code, the default behavior of trunking is to permit all
VLANs. With the new P330 code, the default behavior is to permit only the port VLAN or the
native VLAN, and to block all other VLANs. Additional VLANs are added to a port using the
VLAN binding feature. In addition, the port does not need to be a trunk at all to forward multiple
VLANs. For one application, connecting to an Avaya IP Telephone, the port must not be a trunk
(do not issue the set trunk command).
enable trunking.
Cisco CatOS
set trunk <mod/port> nonegotiate dot1q
By default, all VLANs (1 to1005) are enabled
on the trunk port. VLANs can be selectively
removed with the command clear trunk
<mod/port> <vid>.
LAN issues
Issue 3.4.1 June 2005
287
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