ZyXEL Communications MES-2110 11.1.2 How STP Works

Chapter 11 Bridge
MES-2110 User’s Guide
90
On each bridge, the root port is the port through which this bridge communicates
with th e root. It is the port on this swi tch with the lowest path cost to the root (the
root path cost). If there is no root port, t hen this switch has be en 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.
11.1.2 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 frames 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.
Path
Cost 16Mbps 62 40 to 400 1 to 65535
Path
Cost 100Mbps 19 10 to 60 1 to 65535
Path
Cost 1Gbps 4 3 to 10 1 to 65535
Path
Cost 10Gbps 2 1 to 5 1 to 65535
Table 23 STP Path Costs
LINK
SPEED RECOMMENDED VALUE RECOMMENDED RANGE ALLOWED RANGE