Name: Cisco Catalyst 3750-E
Brand: Cisco
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20-3

Catalyst 3750-E and 3560-E Switch Software Configuration Guide

OL-9775-08

Chapter 20 Configuring Optional Spanning-Tree Features

Understanding Optional Spanning-Tree Features

At the interface level, you enable BPDU guard on any port by using the spanning-tree bpduguard

enable interface configuration command without also enabling the Port Fast feature. When the port

receives a BPDU, it is put in the error-disabled state.

The BPDU guard feature provides a secure response to invalid configurations because you must

anually put the interface back in service. Use the BPDU guard feature in a service-provider network

to prevent an access port from participating in the spanning tree.

Understanding BPDU Filtering

The BPDU filtering feature can be globally enabled on the switch or can be enabled per interface, but

the feature operates with some differences.

At the global level, you can enable BPDU filtering on Port Fast-enabled interfaces by using the

spanning-

tree portfast bpdufilter default global configuration command. This command prevents

interfaces that are in a Port Fast-operational state from sending or receiving BPDUs. The interfaces still

send a few BPDUs at link-up before the switch begins to filter outbound BPDUs. You should globally

enable BPDU filtering on a switch so that hosts connected to these interfaces do not receive BPDUs. If

a BPDU is received on a Port Fast-enabled interface, the interface loses its Port Fast-operational status,

and BPDU filtering is disabled.

At the interface level, you can enable BPDU filtering on any interface by using the spanning-

tree

bpdufilter enable interface configuration command without also enabling the Port Fast feature. This

command prevents the interface from sending or receiving BPDUs.

Caution Enabling BPDU filtering on an interface is the same as disabling spanning tree on it and can result in

spanning-tree loops.

You can enable the BPDU filtering feature for the entire switch or for an interface.

Understanding UplinkFast

Switches in hierarchical networks can be grouped into backbone switches, distribution switches, and

access switches. Figure 20-2 shows a complex network where distribution switches and access switches

each have at least one redundant link that spanning tree blocks to prevent loops.

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