How Stp Works; Stp Port States; Table 24 Stp Path Costs; Table 25 Stp Port States - ZyXEL Communications XGS-4528F User Manual

Chapter 11 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 24 STP Path Costs

Path
Cost
Path
Cost
Path
Cost
Path
Cost
Path
Cost
Path
Cost
On each bridge, the bridge communicates with the root through the root port. The root port 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.

Table 25 STP Port States

PORT STATE DESCRIPTION
Disabled
Blocking
108
LINK RECOMMENDED
SPEED VALUE
4Mbps 250
10Mbps 100
16Mbps 62
100Mbps 19
1Gbps 4
10Gbps 2
STP is disabled (default).
Only configuration and management BPDUs are received and processed.
RECOMMENDED
RANGE
100 to 1000
50 to 600
40 to 400
10 to 60
3 to 10
1 to 5
XGS-4528F User's Guide
ALLOWED
RANGE
1 to 65535
1 to 65535
1 to 65535
1 to 65535
1 to 65535
1 to 65535