ZyXEL Communications ONU-2024 Series, Руководство пользователя

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Chapter 11 Spanning Tree Protocol
ONU User’s Guide
96
11.1.2 What You Need to Know About the STP Screens
The following terms and concepts may help as you read through this chapter.
STP Terminology
The root bridge is the base of the spanning tree.
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 root port is the port through which this bridge communicates with the root.
It is the port on this ONU with the lowest path cost to the root (the root path cost). If there is no
root port, then this ONU 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.
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.
Table 21 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