ZyXEL Communications MGS-3712, User Manual, Page: 102 / 308

Chapter 11 Spanning Tree Protocol
MGS-3712/MGS-3012F User’s Guide
102
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.
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.
Table 22 STP Path Costs
LINK
SPEED
RECOMMENDED
VALUE
RECOMMENDED
RANGE
ALLOWED
RANGE
Path
Cost
4Mbps 250 100 to 1000 1 to 65535
Path
Cost
10Mbps 100 50 to 600 1 to 65535
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