MSTP
Protection
Scenario
Configuration Impact
Root
protection
Due to incorrect
configurations or malicious
attacks on the network, a
root bridge may receive
BPDUs with a higher
priority than its own priority.
Consequently, the legitimate
root bridge is no longer able
to serve as the root bridge
and the network topology is
changed, triggering
spanning tree recalculation.
This may transfer traffic
from high-speed links to
low-speed links, causing
traffic congestion.
To address this issue, the root protection
function can be configured to protect the root
bridge by preserving the role of the
designated port. With this function, when the
designated port receives RST BPDUs with a
higher priority, the port enters the Discarding
state and does not forward the BPDUs. If the
port does not receive any RST BPDUs with a
higher priority for a certain period (double the
Forward Delay), the port transitions to the
Forwarding state.
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 and after the
alternate port ages, the port
enters the Forwarding state.
Loops may occur in such a
situation.
The loop protection function can be used to
prevent such network loops. If the root port
or alternate port cannot receive RST BPDUs
from the upstream switching device, the root
port is blocked and 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.
8.3 Configuring Basic MSTP Functions
MSTP based on the basic STP/RSTP function divides a switching network into multiple regions,
each of which has multiple spanning trees that are independent of each other. MSTP isolates
user traffic and service traffic, and load-balances VLAN traffic.
MSTP is commonly configured on switching devices to trim a ring network to a loop-free
network. Devices start spanning tree calculation after the working mode is set and MSTP 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 in an MSTI: 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 in an MSTI: 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
Huawei AR3200 Series Enterprise Routers
Configuration Guide - LAN
8 MSTP Configuration
Issue 02 (2012-03-30)
Huawei Proprietary and Confidential
Copyright © Huawei Technologies Co., Ltd.
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