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flapping. In normal cases, these ports do not receive any BPDU packets. However, someone may forge BPDU
to maliciously attack the switch and cause network flapping.
BPDU protection can be enabled in RSTP to prevent such attacks. When BPDU protection is enabled, the
switch disables an edge port that has received BPDU and notifies the network manager about it.
3.8.5. STP Configuring Examples
This section contains the following examples:
ï‚·
Configuring STP
ï‚·
Configuring MSTP
3.8.5.1. Configuring STP
This example shows a LAN with four switches. On each switch, ports 1, 2, and 3 connect to other switches, and
ports 4–20 connect to hosts (in Figure 14, each PC represents 17 host systems).
Figure 3-14: STP Example Network Diagram
Of the four switches in Figure 14, the administrator decides that Switch A is the most centrally located in the
network and is the least likely to be moved or redeployed. For these reasons, the administrator selects it as
the root bridge for the spanning tree. The administrator configures Switch A with the highest priority and
uses the default priority values for Switch B, Switch C, and Switch D.
For all switches, the administrator also configures ports 4–17 in Port Fast mode because these ports are
connected to hosts and can transition directly to the Forwarding state to speed up the connection time
between the hosts and the network.
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