ZyXEL Communications IES-1248-51 Uživatelská příručka stáhnout pdf (Strana 202)

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Chapter 25 Spanning Tree Protocol

IES-1248-51V User’s Guide

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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.

After a bridge determines the lowest cost-spanning tree with RSTP, 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 RSTP. Network packets are therefore

only forwarded between enabled ports, eliminating any possible network loops.

Figure 94 STP Root Ports and Designated Ports

RSTP-aware devices exchange Bridge Protocol Data Units (BPDUs) periodically.

When the bridged LAN topology changes, a new spanning tree is constructed.

In RSTP, the devices send BPDUs every Hello Time. If an RSTP-aware device does

not get a Hello BPDU after three Hello Times pass (or the Max Age), the device

assumes that the link to the neighboring bridge is down. This device then initiates

negotiations with other devices to reconfigure the network to re-establish a valid

network topology.

In STP, once a stable network topology has been established, all devices listen for

Hello BPDUs transmitted from the root bridge. If an STP-aware device does not

get a Hello BPDU after a predefined interval (Max Age), the device assumes that

the link to the root bridge is down. This device then initiates negotiations with

other devices to reconfigure the network to re-establish a valid network topology.

Root Bridge

Bridge 1

Bridge 2

Bridge 3

Bridge 4

Bridge 5

Root Port

Discarding

Designated Port

for Bridge 1

Designated Port

for Bridge 2

Designated Port

for Bridge 4

Designated Port

for Bridge 3

Designated Port

for Bridge 5

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