ZyXEL MI-7248 User Guide - Page 96

Chapter 16 Spanning Tree Protocol Path cost is the cost of transmitting a frame onto a LAN through that port. It is assigned according to the speed of the link to which a port is attached. The slower the media, the higher the cost. Table 22 STP Path Costs LINK SPEED RECOMMENDED VALUE RECOMMENDED RANGE ALLOWED RANGE Path 4 Mbps 250 Cost 100 to 1000 1 to 65535 Path Cost 10 Mbps 100 50 to 600 1 to 65535 Path Cost 16 Mbps 62 40 to 400 1 to 65535 Path Cost 100 Mbps 19 10 to 60 1 to 65535 Path 1 Gbps 4 Cost 3 to 10 1 to 65535 Path Cost 10 Gbps 2 1 to 5 1 to 65535 On each bridge, the root port is the port through which this bridge communicates with the root. It is the port on this switch with the lowest path cost to the root (the root path cost). If there is no root port, then this switch has been accepted as the root bridge of the spanning tree network. For each LAN segment, a designated bridge is selected. This bridge has the lowest cost to the root among the bridges connected to the LAN. 16.1.2 How STP Works After a bridge determines the lowest cost-spanning tree with STP, it enables the root port and the ports that are the designated ports for connected LANs, and disables all other ports that participate in STP. Network packets are therefore only forwarded between enabled ports, eliminating any possible network loops. STP-aware switches exchange Bridge Protocol Data Units (BPDUs) periodically. When the bridged LAN topology changes, a new spanning tree is constructed. Once a stable network topology has been established, all bridges listen for Hello BPDUs (Bridge Protocol Data Units) transmitted from the root bridge. If a bridge does not get a Hello BPDU after a predefined interval (Max Age), the bridge assumes that the link to the root bridge is down. This bridge then initiates negotiations with other bridges to reconfigure the network to re-establish a valid network topology. 96 MS-7206 User's Guide