Configuration manual

Understanding and Configuring Loop Protect
Fixed Switch Configuration Guide 15-31
Valid values are 0–65535 seconds. Default is 300 seconds. Setting the value to 0 will set the
timeout to forever.
Use this command to manually unlock a port that was locked by the SpanGuard function. This
overrides the specified timeout variable:
set spantree spanguardlock port-string
Monitoring SpanGuard Status and Settings
Use the commands in Table 15-9 to review SpanGuard status and settings.
Understanding and Configuring Loop Protect
What Is Loop Protect?
As described previously in the overview of “Loop Protect” on page 15-7, this feature prevents or
short circuits loop formation in your network. It does this by requiring ports to receive type 2
BPDUs (RSTP/MSTP) on point-to-point inter-switch links (ISLs) before their states are allowed to
become forwarding. Further, if a BPDU timeout occurs on a port, its state becomes non-
forwarding until a BPDU is received.
In this way, both upstream and downstream facing ports are protected. When a root or alternate
port loses its path to the root bridge due to a message age expiration, it takes on the role of
designated port and will not forward traffic until a BPDU is received.
When a port is intended to be the designated port in an ISL, it constantly proposes and will not
forward until a BPDU is received. This protects against misconfiguration and protocol failure by
the connected bridge.
How Does It Operate?
Loop Protect operates as a per port, per MST instance feature and should be set on ISLs. It
comprises several related functions, including:
• Controlling port forwarding state based on reception of agreement BPDUs
• Controlling port forwarding state based on reception of disputed BPDUs
Table 15-9 Commands for Monitoring SpanGuard
Task Command
Display the status of SpanGuard on the device. show spantree spanguard
Display the status of the SpanGuard lock function on
one or more ports.
show spantree spanguardlock [port
port-string]
Display the SpanGuard timeout setting. show spantree spanguardtimeout
Display the status of the SpanGuard trap function. show spantree spanguardtrapenable
For information about... Refer to page...
What Is Loop Protect? 15-31
How Does It Operate? 15-31
Configuring Loop Protect 15-33