Protection Function Configuration; Introduction - Huawei Quidway S3100 Series Operation Manual

Operation Manual – MSTP
Quidway S3100 Series Ethernet Switches

1.5 Protection Function Configuration

1.5.1 Introduction

The following protection functions are provided on MSTP-enabled switches: BPDU
protection, root protection, loop prevention, and TC-BPDU attack prevention.
I. BPDU protection
Normally, the access ports of the devices operating on the access layer directly connect
to terminals (such as PCs) or file servers. These ports are usually configured as edge
ports to achieve rapid transition. But they resume non-edge ports automatically upon
receiving configuration BPDUs, which causes spanning tree regeneration and network
topology jitter.
Normally, no configuration BPDU will reach edge ports. But malicious users can attack
a network by sending configuration BPDUs deliberately to edge ports to cause network
jitter. You can prevent this type of attacks by utilizing the BPDU protection function.
With this function enabled on a switch, the switch shuts down the edge ports that
receive configuration BPDUs and then reports these cases to the administrator. If a port
is shut down, only the administrator can restore it.
II. Root protection
A root bridge and its secondary root bridges must reside in the same region. A CIST
and its secondary root bridges are usually located in the high-bandwidth core region.
Configuration errors or attacks may result in configuration BPDUs with their priorities
higher than that of a root bridge, which causes new root bridge to be elected and
network topology jitter to occur. In this case, flows that should travel along high-speed
links may be led to low-speed links, and network congestion may occur.
You can avoid this by utilizing the root protection function. Ports with this function
enabled can only be kept as designated ports in all spanning tree instances. When a
port of this type receives configuration BPDUs with higher priorities, it changes to
discarding state (rather than becomes a non-designated port) and stops forwarding
packets (as if it is disconnected from the link). It resumes the normal state if it does not
receive any configuration BPDUs with higher priorities for a specified period.
III. Loop prevention
A switch maintains the states of the root port and other blocked ports by receiving and
processing BPDUs from the upstream switch. These BPDUs may get lost because of
network congestions and link failures. If a switch does not receive BPDUs from the
upstream switch for certain period, the switch selects a new root port; the original root
port becomes a designated port; and the blocked ports transit to forwarding state. This
may cause loops in the network.
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Chapter 1 MSTP Configuration