3Com CoreBuilder 3500, Implementation Manual, Page: 177 / 592

Port-based VLANs 177
Trunking and the Default VLAN
Another benefit of maintaining the default VLAN (with any number of
ports) involves trunking. 3Com strongly recommends that you define
your trunks
before you define your VLANs.
Trunking with the
default VLAN intact
Trunking actions affect the default VLAN in the following ways:
■
If you have only the default VLAN with all ports and you define a trunk
(or subsequently remove a trunk), the ports listed in the VLAN
summary for the default VLAN do not change. In this case,
maintaining the default VLAN with all ports ensures that trunks can
come and go without causing any VLAN changes.
■
If you have the default VLAN as well as additional VLANs and you
subsequently define a trunk for ports in one of the other VLANs, the
system removes those ports from that other VLAN and places them in
the default VLAN. The same action occurs when you remove an
existing trunk from a VLAN that you created after the trunk. For
example:
■
If you have the default VLAN as well as other VLANs and you
subsequently modify an existing trunk that has ports in one of the
VLANs, any port removed from the trunk is removed from the VLAN
and placed in the default VLAN. For example:
Ports Before Action Trunking Action Ports After Action
default VLAN: ports 1-4
ipvlan1: ports 5-11
Define a trunk with
ports 7,8 default VLAN: ports 1-4, 7-8
ipvlan1: ports 5-6, 9-11
Ports Before Action Trunking Action Ports After Action
default VLAN: ports 1-4
ipvlan1: ports 5-11 (ports
5-8 are trunk ports)
Modify existing trunk
to have ports 6-8
(remove port 5, the
anchor port) default VLAN: ports 1-5
ipvlan1: ports 6-11 (port 6 is
new anchor port)