Trunking And The Default Vlan - 3Com 4007 Implementation Manual

268 C 14: V
HAPTER IRTUAL
Trunking with the
default VLAN intact
Trunking with the
default VLAN
removed
LAN (VLAN )
S 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 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:
Ports Before Action
default VLAN: ports 1-4
ipvlan1: ports 5–11
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:
Ports Before Action
default VLAN: ports 1–4
ipvlan1: ports 5–11 (Ports
5-8 are trunk ports.)
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:
Trunking Action
Define a trunk with ports
7, 8.
Trunking Action
Modify existing trunk to
have ports 6–8. (Remove
port 5, the anchor port.)
Ports After Action
default VLAN: ports 1–4, 7, 8
ipvlan1: ports 5, 6, 9-11
Ports After Action
default VLAN: ports 1–5
ipvlan1: ports 6–11 (Port 6
becomes new anchor port.)