Trunks Interacting With Other Features - Cisco WS-C2955T-12 Software Manual

Chapter 8 Configuring VLANs

Trunks Interacting with Other Features

ISL, IEEE 802.1Q, and ATM trunking interacts with other switch features as described in
Table 8-11 Trunks Interacting with Other Features
Switch Feature
Port monitoring
Network port
Secure ports
Blocking unicast and
multicast packets on a trunk
Port grouping
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Trunk Port Interaction
A trunk port cannot be a monitor port. A static-access port can monitor
the traffic of its VLAN on a trunk port.
When configured as a network port, a trunk port serves as the network
port for all VLANs associated with the port. A network port receives
all unknown unicast traffic on a VLAN.
A trunk port cannot be a secure port.
The port block interface configuration command can be used to block
the forwarding of unknown unicast and multicast packets to VLANs on
a trunk. However, if the trunk port is acting as a network port, unknown
unicast packets cannot be blocked.
ISL and 802.1Q trunks can be grouped into EtherChannel port groups,
but all trunks in the group must have the same configuration. ATM
ports are always trunk ports but cannot be part of an EtherChannel port
group.
When a group is first created, all ports follow the parameters set for the
first port to be added to the group. If you change the configuration of
one of these parameters, the switch propagates the setting that you
entered to all ports in the group:
• Allowed-VLAN list.
• STP path cost for each VLAN.
• STP port priority for each VLAN.
• STP Port Fast setting.
Trunk status: if one port in a port group ceases to be a trunk, all
•
ports cease to be trunks.
Catalyst 2900 Series XL and Catalyst 3500 Series XL Software Configuration Guide
How VLAN Trunks Work
Table 8-11.
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