Protection
Function
Scenario
Configuration Impact
Loop
protection
A root port or an alternate
port will age if link
congestion or a one-way
link failure occurs. After
the root port ages, a
switching device may re-
select a root port
incorrectly. After the
alternate port ages, the port
enters the Forwarding
state. Loops may occur in
such a situation.
After loop protection is configured, if the
root port or alternate port does not receive
RST BPDUs from the upstream switching
device for a long time, the switching device
notifies the NMS that the port enters the
Discarding state. The blocked port remains
in the Blocked state and no longer forwards
packets. This function helps prevent loops
on the network. The root port transitions to
the Forwarding state after receiving new
BPDUs.
7.3 Configuring Basic STP/RSTP Functions
STP/RSTP is used to block redundant links on Layer 2 networks and trim a network into a loop-
free tree topology.
STP/RSTP is commonly configured on switching devices to trim a ring network into a loop-free
network. Devices start spanning tree calculation after the STP/RSTP working mode is set and
STP/RST is enabled. Use any of the following methods if you need to intervene in the spanning
tree calculation:
l
Set a priority for a switching device: The lower the numerical value, the higher the priority
of the switching device and the more likely the switching device becomes a root bridge;
the higher the numerical value, the lower the priority of the switching device and the less
likely that the switching device becomes a root bridge.
l
Set a path cost for a port: With the same calculation method, the lower the numerical value,
the smaller the cost of the path from the port to the root bridge and the more likely the port
becomes a root port; the higher the numerical value, the larger the cost of the path from the
port to the root bridge and the less likely that the port becomes a root port.
l
Set a priority for a port: The lower the numerical value, the more likely the port becomes
a designated port; the higher the numerical value, the less likely that the port becomes a
designated port.
7.3.1 Establishing the Configuration Task
Before configuring basic STP/RSTP functions, familiarize yourself with the applicable
environment, complete the pre-configuration tasks, and obtain the required data. This will help
you complete the configuration task quickly and accurately.
Applicable Environment
Network designers tend to deploy multiple physical links between two devices (one link is the
master and the others are backups) to fulfill network redundancy requirements. Loops are bound
to occur on such types of complex networks.
Loops will cause broadcast storms, thereby exhausting network resources and paralyzing the
network. Loops also cause MAC address flapping that damages MAC address entries.
Huawei AR3200 Series Enterprise Routers
Configuration Guide - LAN
7 STP/RSTP Configuration
Issue 02 (2012-03-30)
Huawei Proprietary and Confidential
Copyright © Huawei Technologies Co., Ltd.
188
