Note

Starting in 12.0 IOS, the routers started using a subfield boundary that allows the Port Number subfield to be between 9 and 10 bits in length. This was done to support high-density bridging routers such as the 8540. The new scheme still allows Port Priority to be specified as an 8-bit value (0-255) and then the value is divided by either two (9-bit Port Number) or four (10-bit Port Number) to scale the value to the appropriate size.

How does all this bit-twiddling cause traffic to flow across multiple paths? Figure 716 redraws the VLANs originally presented in Figure 7-13 to locate the Root Bridge for both VLANs on Cat-B.

Figure 7-16 Back-to-Back Switches: The Root Bridge for

Both VLANs Is on Cat-B

Figure 7-16 Back-to-Back Switches: The Root Bridge for

Both VLANs Is on Cat-B

As was the case with Part A of Figure 7-13, the default Configuration BPDUs received on Port 1/1 of Cat-A contains a Port ID of 0x8001, but Port 1/2 receives the value 0x8002. Because 0x8001 is lower, Port 1/1 becames the Root Port for all VLANs by default. However, if you lower VLAN 3's Port Priority to 31 on Cat-B:Port-1/2, it lowers the Port ID that Cat-A:Port-1/2 receives for VLAN 3 to 0x7C01. Because

0x7C01 is less than 0x8001, Cat-A now elects Port 1/2 as the Root Port for VLAN 3 and sends traffic over this link. The syntax to implement this change is as follows:

Cat-B (enable) set spantree portvlanpri 1/2 31 3

Voila, you have load balancing—VLAN 2 is using the left link and VLAN 3 is using the right link.

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