The All-New Switch Book: The Complete Guide to LAN Switching Technology, Second Edition

12.6. VLANs and the Spanning Tree

As discussed in Chapter 2, "Transparent Bridges," and Chapter 6, "Source Routing," it is critical that active loops not be present in the catenet. Loops in the topology can cause catastrophic proliferation of multicast traffic and will prevent the filtering database from converging to a steady state. As a consequence, we impose a tree structure on the topology of the catenet, using the Spanning Tree Protocol to determine and maintain the tree automatically.

VLANs are no different from physical LANs in this regard. Because the operation of a switch within a given VLAN context is essentially the same as that of a VLAN-unaware bridge in a traditional catenet, we need to ensure that no loops are present in the active topology of the VLAN. To this end, 802.1Q specifies the use of the identical Spanning Tree Protocol as that of 802.1D. In fact, 802.1Q simply points implementers to the relevant section of 802.1D, and does not restate the protocol itself.

The Spanning Tree Protocol defines a single tree structure interconnecting all physical LANs in the catenet. Because a VLAN constitutes a proper subset of the catenet, traffic for a given VLAN will only propagate along a subset of the spanning tree — the precise set of links needed to reach all members of the VLAN, as depicted in Figure 12-15. In the figure, it can be seen that the loop in the topology has been resolved by disabling one of the links (the dotted line); all traffic flows along the solid-line ...