3Com Switch 7750 Series
Command Reference Guide – MSTP
Chapter 1 MSTP Configuration Commands
1-42
A switch removes MAC address entries and ARP entries upon receiving TC-BPDUs. If
a malicious user sends a large amount of TC-BPDUs to a switch in a short period, the
switch may busy itself in removing MAC address entries and ARP entries, which may
decreases the performance and stability of the switch.
With the TC-BPDU prevention function enabled, a switch performs only one removing
operation in a specified period (it is 10 seconds by default) after it receives a TC-BPDU.
The switch also checks to see if other TC-BPDUs arrive in this period and performs
another removing operation in the next period if a TC-BPDU is received. Such a
mechanism prevents a switch from being busying itself in performing removing
operations.
Example
# Enable the TC-BPDU prevention function on the switch.
<3Com> system-view
System View: return to User View with Ctrl+Z.
[3Com] stp tc-protection enable
1.1.40 stp timer forward-delay
Syntax
stp timer forward-delay
centi-seconds
undo stp
timer forward-delay
View
System view
Parameter
centi-seconds
: Forward delay in centiseconds to be set. This argument ranges from
400 to 3,000 and defaults to 1,500.
Description
Use the
stp timer forward-delay
command to set the Forward delay of the switch.
Use the
undo stp timer forward-delay
command to revert to the default Forward
delay.
To prevent the occurrence of temporary loops, when a port changes its state from
discarding to forwarding, it undergoes an intermediate state and waits for a specific
period to synchronize with the remote switches. This state transition period is
determined by the Forward delay configured on the root bridge.
The Forward delay setting configured on a root bridge applies to all switches operating
in the same spanning tree instance.
