Cisco Systems IE 2000 Load Sharing Using STP Port Priorities, Load Sharing Using STP Path Cost

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Cisco IE 2000 Switch Software Configuration Guide
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Chapter 17 Configuring VLANs
VLANs
Load Sharing Using STP Port 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 po rt 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.
Figure 17-2 shows two trunks connecting supported switches. In this example, the switches are
configured as follows:
• 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.
In this way, 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 po rt.
Figure 17-2 Load Sharing by Using STP Port Priorities
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.
In Figure 17-3, Trunk ports 1 and 2 are configured as 100BASE-T ports. These VLAN path costs are
assigned:
• VLANs 2 through 4 are assigned a path cost of 30 on Trunk port 1 .
• VLANs 8 through 10 retain the default 100BASE-T path cost on Trunk port 1 of 19.
• VLANs 8 through 10 are assigned a path cost of 30 on Trunk port 2.
• VLANs 2 through 4 retain the default 100BASE-T path cost on Trunk port 2 of 19 .
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Switch A
Switch B
Trunk 2
VLANs 3 – 6 (priority 16)
VLANs 8 – 10 (priority 128)
Trunk 1
VLANs 8 – 10 (priority 16)
VLANs 3 – 6 (priority 128)