How Stp Works; Table 14 Stp Path Costs - ZyXEL Communications 1 User Manual

NetAtlas Enterprise 1.00 User's Guide
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 14 STP Path Costs

LINK SPEED
4Mbps
10Mbps
16Mbps
100Mbps
1Gbps
10Gbps
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.

5.2.2 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.
63
RECOMMENDED VALUE RECOMMENDED RANGE
250
100
62
19
4
2
100 to 1000
50 to 600
40 to 400
10 to 60
3 to 10
1 to 5
ALLOWED RANGE
1 to 65535
1 to 65535
1 to 65535
1 to 65535
1 to 65535
1to 65535
Chapter 5 Status