11-15
Optimizing Port Usage Through Traffic Control and Port Trunking
Port Trunking
Ports: Traffic Control
and Trunking
Spanning Tree Protocol (STP): STP operates as a global setting on the switch (one instance of STP per switch). However,
you can adjust STP parameters on a per-port basis. A static trunk of any type appears in the STP configuration d isplay,
and you can configure STP parameters for a static trunk in the same way that you would configure STP parameters on
a non-trunked port. (Note that the switch lists the trunk by name—such as Trk1—and does not list the individual ports
in the trunk.) For example, if ports C1 and C2 are conf igured as a static trunk named Trk1, they are listed in the STP disp lay
as TRK1 and do not appear as individual ports in the STP displays.
When Spanning Tree forwards on a trunk, all ports in t he trunk will be forwarding . Conversely, when Spanning Tree blocks
a trunk, all ports in the trunk are blocked.
Note: A dynamic LACP trunk operates only with the de fault STP settings and does not appear in the STP configurati on
display or show ip igmp listing.
If you remove a port from a static trunk, the port retains the same STP settings that were configured for the trunk.
IP Multicast Protocol (IGMP): A static trunk of any type appears in the IGMP configuration display, and you can configu re
IGMP for a static trunk in the same way that you would configure IGMP on a non-trunked port. (Note tha t the switch lists
the trunk by name—such as Trk1—and does not list the individual ports in the trunk.) Also, creating a new trunk
automatically places the trunk in IGMP Auto status if IGMP is enabled for the default VLAN. A dynamic LAC P trunk
operates only with the default IGMP settings and does not appear in the IGMP configuration display o r show ip igmp
listing.
VLANs: Creating a new trunk automatically plac es the trunk in the DEFAULT_VLAN, regardless of whether the ports in
the trunk were in another VLAN. Similarly, removing a port from a trunk group au tomatically places the port in the default
VLAN. You can configure a static trunk in the same way that you configure a port for memb ership in any VLAN.
Note: For a dynamic trunk to operate in a VLAN other than the default VL AN (DEFAULT_VLAN), GVRP must be enabled.
See “Trunk Group Operation Using LACP” on page 11-25.
Port Security: Trunk groups (and their individual ports) cannot be configured for port secu rity, and the switch excludes
trunked ports from the show port-security listing. If you configure non-default port secur ity settings for a port, then
subsequently place the port in a trunk, the port security for that port returns to the default settings. If you remove a port
from a trunk, the port security settings for that port are returned to their default values.
Monitor Port:
Note: A trunk cannot be a monitor port. A monitor port can moni tor a static trunk but cannot monitor a dy namic LACP
trunk.
In this example showing
part of the show spanning-
tree listing, ports C1 and C2
are members of TRK1 and
do not appear as individual
ports in the port
configuration part of the
listing.