Stp Operation; Stp Overview; Implementing Stp; Designated Switch And Designated Port - 3Com 7700 Configuration Manual

8

STP Overview

Implementing STP

Designated Switch and
Designated Port
STP O
PERATION
Spanning Tree Protocol (STP) is applied in a loop network to block some
undesirable redundant paths with certain algorithms and prune the network into a
loop-free tree, thereby avoiding the proliferation and infinite cycling of a packet in
the loop network.
The fundamental of STP is that the switches exchange packets called configuration
Bridge Protocol Data Units, or BPDU, to decide the topology of the network. The
configuration BPDU contains the information that ensures that switches can
compute the spanning tree.
The configuration BPDU mainly contains the following information:
The root ID consisting of root priority and MAC address
■
The cost of the shortest path to the root
■
Designated switch ID consisting of designated switch priority and MAC address
■
Designated port ID consisting of port priority and port number
■
The age of the configuration BPDU: MessageAge
■
The maximum age of the configuration BPDU: MaxAge
■
Configuration BPDU interval: HelloTime
■
Forward delay of the port: ForwardDelay.
■
A designated switch is a switch in charge of forwarding packets to the local switch
by a port called the designated port. For a LAN, the designated switch is a switch
that forwards packets to the network segment by a port called the designated
port.
As illustrated in Figure 1, Switch A forwards data to Switch B through Ethernet
port 1/0/1. So to Switch B, the designated switch is Switch A and the designated
port is Ethernet 1/0/1 of Switch A. Also, Switch B and Switch C are connected to
the LAN and Switch B forwards packets to the LAN. So the designated switch of
LAN is Switch B and the designated port is Ethernet 1/0/4 of Switch B.