ZyXEL ES-2024A User Guide - Page 116

Chapter 13 Spanning Tree Protocol Path cost is the cost of transmitting a frame onto a LAN through that port. The recommended cost is assigned according to the speed of the link to which a port is attached. The slower the media, the higher the cost. Table 23 STP Path Costs LINK SPEED RECOMMENDED VALUE Path 4Mbps 250 Cost Path Cost 10Mbps 100 Path Cost 16Mbps 62 Path Cost 100Mbps 19 Path 1Gbps 4 Cost Path 10Gbps 2 Cost RECOMMENDED RANGE 100 to 1000 50 to 600 40 to 400 10 to 60 3 to 10 1 to 5 ALLOWED RANGE 1 to 65535 1 to 65535 1 to 65535 1 to 65535 1 to 65535 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. 13.1.2 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. 116 ES-2024 Series User's Guide