Allowed VLANs on a Trunk
Bydefault, a trunk port sends trafficto and receives traffic from all VLANs. All VLAN IDs, 1 to 4094, are
allowedon each trunk. However, you can remove VLANs from the allowed list, preventing traffic from those
VLANsfrom passing over thetrunk.
Toreduce the risk of spanning-tree loops or storms, you can disable VLAN 1 on any individual VLAN trunk
portby removing VLAN 1 from the allowed list. When you remove VLAN 1 from a trunk port, the interface
continuesto sendand receivemanagement traffic, for example, Cisco Discovery Protocol (CDP), Port
AggregationProtocol (PAgP), Link Aggregation Control Protocol (LACP), DTP, and VTP in VLAN 1.
Ifa trunk port with VLAN 1 disabled is converted to a nontrunk port, it is added to the access VLAN. If the
accessVLAN is setto 1,the port willbe addedto VLAN 1,regardless of theswitchport trunk allowed
setting.The same is true for any VLAN that has been disabled on the port.
Atrunk port can become a member of a VLAN if the VLAN is enabled, if VTP knows of the VLAN, and if
theVLAN is in the allowed list for the port. When VTP detects a newly enabled VLAN and the VLAN is in
theallowed list for a trunk port, the trunk port automatically becomes a member of the enabled VLAN. When
VTPdetects a new VLAN and the VLAN is not in the allowed list for a trunk port, the trunk port does not
becomea member of thenew VLAN.
Load Sharing on Trunk Ports
Loadsharing divides the bandwidth supplied by parallel trunks connecting switches. To avoid loops, STP
normallyblocks all but one parallel link between switches. Using load sharing, you divide the traffic between
thelinks according to which VLAN the traffic belongs.
Youconfigure load sharing on trunk ports by using STP port priorities or STP path costs. For load sharing
usingSTP port priorities,both load-sharing links must be connected to the same switch. For load sharing
usingSTP path costs, each load-sharing link can be connected to the same switch or to two different switches.

Network Load Sharing Using STP Priorities

Whentwo ports on thesame switch form a loop, the switch uses the STP port priorityto decide which port
isenabled and which port is in a blocking state. You can set the priorities on a parallel trunk port so that the
portcarries all the traffic for a given VLAN. The trunk port with the higher priority (lower values) for a VLAN
isforwarding traffic for that VLAN. The trunk port with the lower priority (higher values) for the same VLAN
remainsin a blocking state for thatVLAN. One trunkport sends orreceives all traffic for the VLAN.
Thisfigure shows two trunks connecting supported switches.
•VLANs8 through 10 are assigned a port priority of 16 on Trunk 1.
•VLANs3 through 6 retain the default port priority of 128 on Trunk 1.
•VLANs3 through 6 are assigned a port priority of 16 on Trunk 2.
•VLANs8 through 10 retain the default port priority of 128 on Trunk 2.
Catalyst 2960-XR Switch VLAN Configuration Guide, Cisco IOS Release 15.0(2)EX1
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Configuring VLAN Trunks
Allowed VLANs on a Trunk