3Com 4007, Implementation Manual, Page: 268 / 684

268 C
HAPTER
14: V
IRTUAL
LAN
S
(VLAN
S
)
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 a module’s 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 on a module
and you subsequently define a trunk for ports in one of the other
VLANs, the module removes those ports from that 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, on a 12-port Multilayer Switching Module:
■
If you have the default VLAN as well as other VLANs on a module and
you subsequently modify an existing trunk that has ports in one of the
VLANs, any port that is removed from the trunk is removed from the
VLAN and placed in the default VLAN. For example, on a 12-port
Multilayer Switching Module:
Trunking with the
default VLAN
removed
If you remove the default VLAN, there is no place to return ports altered
by trunking, as discussed in these examples:
■
If you have VLANs (but no default VLAN) and you then define a trunk
for ports in one of the VLANs, those ports are removed from that
VLAN and are not assigned to any other VLAN. If you later remove the
trunk, these ports are not reassigned to the VLAN; they no longer
have a VLAN associated with them. For example, on a 12-port
Multilayer Switching Module:
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
becomes new anchor port.)