ZyXEL Communications OLT2406 - How STP Works; STP Port States

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