Note

Note that this section is referring to Layer 2 Catalysts such as the 2900s, 4000s, 5000s, and 6000s. Because these devices currently have one IP address that is only assigned to a single VLAN, the selection of this VLAN can be important. On the other hand, this point generally does not apply to router-like Catalysts such as the 8500. Because these platforms generally have an IP address assigned to every VLAN, trying to pick the best VLAN for an IP address obviously becomes irrelevant. For more information on the Catalyst 8500, see Chapter 11.

In fact, this Spanning Tree problem is one of the more common issues in flat earth campus networks. The story usually goes something like this: The network is humming along fine until a burst of broadcast data in the management VLAN causes a switch to become overwhelmed to the point where is starts dropping packets. Because some of these packets are BPDUs, the switch falls behind in its Spanning Tree information and inadvertently creates a Layer 2 loop in the network. At this point, the broadcasts in the network go into a full feedback loop as discussed in Chapter 6, "Understanding Spanning Tree."

If this loop occurs in one or more VLANs other than the management VLAN, it can quickly starve out all remaining trunk bandwidth throughout the entire campus in a flat network. However, the Supervisor CPUs are insulated by the VLAN switching ASICs and continue operating normally (recall that all data forwarding is handled by ASICs in Catalyst gear).

On the other hand, if the loop occurs in the management VLAN (the VLAN where SCO is assigned), the results can be truly catastrophic. Suddenly, every switch CPU is hit with a tidal wave of broadcast traffic, completely crushing every switch in a downward spiral that virtually eliminates any chance of the network recovering from this problem. If a network is utilizing campus-wide VLANs, this problem can spread to every switch within a matter of seconds.

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