ZyXEL Communications VES-1000 Series, User Manual, Page: 43 / 227

VES-1000 Series Ethernet Switch
Getting Started Screens 5-3
Path cost is the cost of transmitting a frame onto a LAN through that port. It is assigned according to the speed of
the link to which a port is attached. The slower the media, the higher the cost - see the next table.
Table 5-2 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
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.
5.4.2 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 the 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.
Figure 5-2 Root Ports and Designated Ports
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