Understanding Loop Guard; Configuring Optional Spanning-Tree Features; Default Optional Spanning-Tree Configuration - Cisco WS-C3020 Software Configuration Manual

Chapter 18

Configuring Optional Spanning-Tree Features

Understanding Loop Guard

You can use loop guard to prevent alternate or root ports from becoming designated ports because of a
failure that leads to a unidirectional link. This feature is most effective when it is enabled on the entire
switched network. Loop guard prevents alternate and root ports from becoming designated ports, and
spanning tree does not send BPDUs on root or alternate ports.
You can enable this feature by using the spanning-tree loopguard default global configuration
command.
When the switch is operating in PVST+ or rapid-PVST+ mode, loop guard prevents alternate and root
ports from becoming designated ports, and spanning tree does not send BPDUs on root or alternate ports.
When the switch is operating in MST mode, BPDUs are not sent on nonboundary ports only if the
interface is blocked by loop guard in all MST instances. On a boundary port, loop guard blocks the
interface in all MST instances.
Configuring Optional Spanning-Tree Features
These sections contain this configuration information:
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Default Optional Spanning-Tree Configuration

Table 18-1
Table 18-1

Default Optional Spanning-Tree Configuration

Feature
Port Fast, BPDU filtering, BPDU guard
UplinkFast
BackboneFast
EtherChannel guard
Root guard
Loop guard
OL-8915-03
Default Optional Spanning-Tree Configuration, page 18-9
Optional Spanning-Tree Configuration Guidelines, page 18-10
Enabling Port Fast, page 18-10
Enabling BPDU Guard, page 18-11
Enabling BPDU Filtering, page 18-12
Enabling UplinkFast for Use with Redundant Links, page 18-13
Enabling BackboneFast, page 18-13
Enabling EtherChannel Guard, page 18-14
Enabling Root Guard, page 18-15
Enabling Loop Guard, page 18-15
shows the default optional spanning-tree configuration.
(optional)
(optional)
(optional)
(optional)
(optional)
(optional)
(optional)
Default Setting
Globally disabled (unless they are individually configured
per interface).
Globally disabled.
Globally disabled.
Globally enabled.
Disabled on all interfaces.
Disabled on all interfaces.
Cisco Catalyst Blade Switch 3020 for HP Software Configuration Guide
Configuring Optional Spanning-Tree Features
(optional)
18-9