ZyXEL Communications ZyXEL Dimension ES-2724, User Manual, Page: 104 / 341

ES-2724 User’s Guide
102 Chapter 11 Spanning Tree Protocol
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.
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 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.
11.1.3 STP Port States
STP assigns five port states to eliminate packet looping. A bridge port is not allowed to go
directly from blocking state to forwarding state so as to eliminate transient loops.
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 22 STP Path Costs
LINK SPEED RECOMMENDED VALUE
RECOMMENDED
RANGE
ALLOWED RANGE
Table 23 STP Port States
PORT STATE DESCRIPTION
Disabled STP is disabled (default).
Blocking Only configuration and management BPDUs are received and processed.
Listening All BPDUs are received and processed.