How Stp Works; Stp Port States - ZyXEL Communications OLT2406 User Manual

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 233 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 OLT with the lowest path cost to the root (the root path cost). If there is no root port, then this OLT
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.

63.2.2 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.

63.2.3 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 234 STP Port States
PORT STATE
Disabled
Blocking
Listening
Chapter 63 Spanning Tree Protocol
RECOMMENDED
LINK SPEED
VALUE
4 Mbps 250
10 Mbps 100
16 Mbps 62
100 Mbps 19
1 Gbps 4
10 Gbps 2
DESCRIPTION
STP is disabled (default).
Only configuration and management BPDUs are received and processed.
All BPDUs are received and processed.
Note: The listening state does not exist in RSTP.
OLT2406 User's Guide
RECOMMENDED
RANGE
100 to 1000
50 to 600
40 to 400
10 to 60
3 to 10
1 to 5
470
ALLOWED RANGE
1 to 65535
1 to 65535
1 to 65535
1 to 65535
1 to 65535
1 to 65535