Figure 1118 CatB Is the Spanning Tree Root Bridge

This causes Spanning Tree to reconverge to a logical topology where one of the ports on Segment 2 is Blocking. This allows the traffic from Host-A to the router to follow the same path as in Figure 11-17. Both Cat-A and Cat-B recognize the first packet as a candidate packet and create a partial shortcut entry. However, the traffic flowing from the router to Host-B cannot use Segment 2 because it is blocked. Instead, the traffic flows back through Cat-B and uses Segment 1 and Segment 3. Notice that this causes both Cat-A and Cat-B to see the Enable Packet and complete the shortcut entry.

When the second packet is sent from Host-A to Host-B, Cat-B uses its shortcut entry to Layer 3 switch the packet directly onto Segment 3, bypassing the router. Because Cat-A does not see any traffic for the shortcut entry it created, the entry ages out in 256 seconds by default. Although this allows MLS to function (in fact, it creates a more efficient flow in this case), it can be disconcerting to see the shortcut switching operation move from Cat-A to Cat-B only because of Spanning Tree. Obviously, the interaction between MLS and Spanning Tree can get very complex in large and very flat campus networks (yet one more reason to avoid the flat earth approach to campus design; see Chapters 14 and 15 for more information).

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