so advan- tages to minimizing the number of technologies that need to be managed in the network. Protection Mechanisms The mechanism employed in Ethernet networks to ensure that loops are removed when one or more paths exist between endpoints is the construction of a spanning tree using a spanning tree protocol. Even a small loop can cause significant disruption, and considerable work has been carried out by the IEEE to avoid loops. In many packet technologies the effect of loops is mitigated by means of a Time-to-Live (TTL) field associated with each packet, but Ethernet has no such mechanism and as such it is necessary to create a loop-free topology-in the form of a tree-using a spanning tree protocol. Chapter 14 Next Generation Ethernet Forwarding Path Fig. 14.7 Spanning tree and topology. Standby Path To establish a spanning tree, the bridges in the network distribute messages between themselves in the form of Bridge Protocol Data Units (BPDUs). During this exchange of messages, a single bridge is elected as the root bridge and then the spanning tree algorithm calculates the shortest path from each of the other bridges back to the root bridge. When this tree structure has been created only those paths associated with the tree are active and support traffic; all other paths between the bridges are blocked (their associated ports are not part of the spanning tree). This is illustrated in Figure 14.7. A good description of how the spanning tree protocol works can be found in Perlman (2000). It can be seen from the figure that two paths have been blocked to prevent the creation of forwarding loops and are therefore not used in normal operation. Clearly, if a link in the spanning tree should fail, it is necessary to provide some form of resiliency by recalculating the spanning tree. This is achieved by an aging mechanism, which is triggered when no new BPDUs are received on a port within a fixed period of time, at which point a recalcula- tion occurs. Although the active topology is recalculated, it is evident tha