How Stp Works - ZyXEL Communications MGS-3712F User Manual

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 24 STP Path Costs
Path
Cost
Path
Cost
Path
Cost
Path
Cost
Path
Cost
Path
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.

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.
126
LINK RECOMMENDED
SPEED VALUE
4Mbps 250
10Mbps 100
16Mbps 62
100Mbps 19
1Gbps 4
10Gbps 2
RECOMMENDED
RANGE
100 to 1000
50 to 600
40 to 400
10 to 60
3 to 10
1 to 5
MGS-3712/MGS-3712F User's Guide
ALLOWED
RANGE
1 to 65535
1 to 65535
1 to 65535
1 to 65535
1 to 65535
1 to 65535