Load Sharing On Trunk Ports; Network Load Sharing Using Stp Priorities - Cisco Catalyst 2960 series Configuration Manual

Load Sharing on Trunk Ports

Related Topics
Defining the Allowed VLANs on a Trunk, on page 67
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.
This figure shows two trunks connecting supported switches.
• VLANs 8 through 10 are assigned a port priority of 16 on Trunk 1.
• VLANs 3 through 6 retain the default port priority of 128 on Trunk 1.
• VLANs 3 through 6 are assigned a port priority of 16 on Trunk 2.
• VLANs 8 through 10 retain the default port priority of 128 on Trunk 2.
Figure 4: Load Sharing by Using STP Port Priorities
Trunk 1 carries traffic for VLANs 8 through 10, and Trunk 2 carries traffic for VLANs 3 through 6. If the
active trunk fails, the trunk with the lower priority takes over and carries the traffic for all of the VLANs. No
duplication of traffic occurs over any trunk port.
Related Topics
Configuring Load Sharing Using STP Port Priorities, on page 71
Catalyst 2960-X Switch VLAN Configuration Guide, Cisco IOS Release 15.0(2)EX
62
Configuring VLAN Trunks
OL-29065