ccna 7 book

ccna 7 book.Each spanning tree instance (switched LAN or broadcast domain) has a switch that is designated as the root bridge. The root bridge acts as a reference point for all spanning tree calculations to determine which redundant paths to block.6.2 Layer 2 Security Considerations 157The election process is used to determine which switch is elected as the root bridge.All switches in the broadcast domain participate in the election process. After a switch reboots, it begins sending BPDU frames every two seconds. These BPDU frames contain the switch BID and the root ID.When a switch forwards its BPDU frames, neighboring switches in the broadcast domain read the root ID information from the BPDU frames. If the root ID read from a received BPDU frame is less than the root ID on the receiving switch, the receiving switch updates its root ID, indicating that the adjacent switch is the root bridge. In fact, the root bridge does not have to be the adjacent switch, but could be another switch in the broadcast domain. The switch then forwards the new BPDU frame with the lower root ID to the other neighboring switch. Eventually, the switch with the lowest BID becomes the root bridge for the spanning tree instance.Each spanning tree instance needs to elect a root bridge. It is also possible to have multiple different root bridges. If all ports on all switches are members of VLAN1, there is only one spanning tree instance. The extended system ID plays a key role in how the spanning tree instance is determined.6.2.4.4 Spanning Tree Algorithm: path overheadOnce the root bridge has been elected in the spanning tree instance, the spanning tree algorithm will begin to determine the best path from all destinations in the broadcast domain to the root bridge. On the path from the destination to the root bridge, the path information is determined by adding up the respective port overheads on the path. Each "destination" is actually a switch port.The default overhead of a port is determined by the speed at which the port is running.Note: As newer, faster Ethernet technologies enter the market, the value of the path overhead may need to be changed to accommodate different speeds. To account for the ongoing changes associated with high-speed network interconnects, the Catalyst 4500 and 6500 switches can support a longer path overhead approach. For example, 10Gbit/s has a path overhead value of 2000; 100Gbit/s has a path overhead value of 200, and 1Tbit/s has a path overhead value of 20o