continues to send and receive management traffic, for example, Cisco Discovery Protocol (CDP), Port
Aggregation Protocol (PAgP), Link Aggregation Control Protocol (LACP), DTP, and VTP in VLAN 1.
If a trunk port with VLAN 1 disabled is converted to a nontrunk port, it is added to the access VLAN. If the
access VLAN is set to 1, the port will be added to VLAN 1, regardless of the switchport trunk allowed
setting. The same is true for any VLAN that has been disabled on the port.
A trunk port can become a member of a VLAN if the VLAN is enabled, if VTP knows of the VLAN, and if
the VLAN is in the allowed list for the port. When VTP detects a newly enabled VLAN and the VLAN is in
the allowed list for a trunk port, the trunk port automatically becomes a member of the enabled VLAN. When
VTP detects a new VLAN and the VLAN is not in the allowed list for a trunk port, the trunk port does not
become a member of the new VLAN.
Related Topics
Defining the Allowed VLANs on a Trunk , on page 2136
Load Sharing on Trunk Ports
Load sharing divides the bandwidth supplied by parallel trunks connecting switches. To avoid loops, STP
normally blocks all but one parallel link between switches. Using load sharing, you divide the traffic between
the links according to which VLAN the traffic belongs.
You configure load sharing on trunk ports by using STP port priorities or STP path costs. For load sharing
using STP port priorities, both load-sharing links must be connected to the same switch. For load sharing
using STP path costs, each load-sharing link can be connected to the same switch or to two different switches.
Network Load Sharing Using STP Priorities
When two ports on the same switch form a loop, the switch uses the STP port priority to decide which port
is enabled and which port is in a blocking state. You can set the priorities on a parallel trunk port so that the
port carries all the traffic for a given VLAN. The trunk port with the higher priority (lower values) for a VLAN
is forwarding traffic for that VLAN. The trunk port with the lower priority (higher values) for the same VLAN
remains in a blocking state for that VLAN. One trunk port sends or receives all traffic for the VLAN.
Related Topics
Configuring Load Sharing Using STP Port Priorities , on page 2141
Network Load Sharing Using STP Path Cost
You can configure parallel trunks to share VLAN traffic by setting different path costs on a trunk and associating
the path costs with different sets of VLANs, blocking different ports for different VLANs. The VLANs keep
the traffic separate and maintain redundancy in the event of a lost link.
Related Topics
Configuring Load Sharing Using STP Path Cost , on page 2145
Feature Interactions
Trunking interacts with other features in these ways:
•
A trunk port cannot be a secure port.
Consolidated Platform Configuration Guide, Cisco IOS Release 15.2(4)E (Catalyst 2960-X Switches)
2132
Information About VLAN Trunks
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