Effective

Intermediate Distribution Frame

Figure 7-17 is a simplified diagram of a typical (but not necessarily recommended) campus design. In this design, VLANs 2 and 3 span all switches in both buildings. Each building consists of a pair of redundant MDF (Main Distribution Frame) switches in the main wiring closet. Both MDF switches connect to the IDF (Intermediate Distribution Frame) switches sitting on every floor (only one IDF switch in each building is shown to simplify the diagram). Link costs are also indicated (the IDF links are Fast Ethernet and MDF links are Gigabit Ethernet).

Figure 7-17 is a simplified diagram of a typical (but not necessarily recommended) campus design. In this design, VLANs 2 and 3 span all switches in both buildings. Each building consists of a pair of redundant MDF (Main Distribution Frame) switches in the main wiring closet. Both MDF switches connect to the IDF (Intermediate Distribution Frame) switches sitting on every floor (only one IDF switch in each building is shown to simplify the diagram). Link costs are also indicated (the IDF links are Fast Ethernet and MDF links are Gigabit Ethernet).

To implement load balancing, you cannot use the Port Priority load balancing technique because the switches are not back-to-back. How about using Root Bridge placement? To load balance in Building 1, you could place the Root Bridge for VLAN 2 on Cat-1A while placing the Root Bridge for VLAN 3 on Cat-1B. This causes VLAN 2's traffic to use the left riser link and VLAN 3's traffic to use the riser on the right. So far so good.

But what does this do to the traffic in Building 2? The IDF switch in Building 2 (Cat-2C) has several paths that it can use to reach the Root Bridge for VLAN 2 (Cat-1A). Which of these paths does it use? Well, refer back to the four-step STP decision criteria covered earlier. The first criterion evaluated is always the Root Bridge. Because everyone is in agreement that Cat-1A is the Root Bridge for VLAN 2, Cat-2C proceeds to the second criterion—Root Path Cost. One possibility is to follow the path Cat-2C to Cat-2B to Cat-2A to Cat-1A at a Root Path Cost of 27 (19+4+4). A better option is Cat-2C to Cat-2B to Cat-1A at a cost of 23 (19+4). However, path Cat-2C to Cat-2A to Cat- 1A also has a Root Path Cost of 23 (19+4).

Because there are two paths tied for the lowest cost, Cat-2A proceeds to the third decision factor—Sending BID. Assume that both Cat-2A and Cat-2B are using the default Bridge Priority (32,768). Also assume that Cat-2A has a MAC address of AA-AA-AA-AA-AA-AA and Cat-2B has a MAC address of BB-BB-BB-BB-BB-BB. Because Cat-2A has the lower BID (32,768.AA-AA-AA-AA-AA-AA), all traffic for VLAN 2 uses the path Cat-2C to Cat-2A to Cat-1B.

OK, how about VLAN 3? Because all switches are in agreement that Cat-1B is the Root Bridge of VLAN 3, Root Path Cost is considered next. One option is to follow the path Cat-2C to Cat-2A to Cat-1A to Cat-1B at a cost of 27 (19+4+4). A better option is Cat-2C to Cat-2A to Cat-1B at a cost of 23 (19+4). However, again, there is an equal cost path along Cat-2C to Cat-2B to Cat-1B (cost =19+4=23). Cat-2C then evaluates the BIDs of Cat-2A and Cat-2B, choosing Cat-2A. VLAN 3 traffic therefore follows the path Cat-2C to Cat-2A to Cat-1B. This does provide load balancing across the campus core, but now both VLANs are using the same IDF riser cable. In other words, the load balancing in Building 1 destroyed the load balancing in Building 2.

Clearly, a new technique is required. Assuming that you want to maintain both VLANs across both buildings (I am using this assumption because it is a common design technique; however, in general, I recommend against it—see Chapter 14 for more details), there are two options:

• Bridge Priority

Of these, port/VLAN cost is almost always the better option. However, because set spantree portvlancost was not available until 3.1 Catalyst images, the Bridge Priority technique is discussed first.

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