Cisco IE 4000, Software Configuration Manual

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Configuring Optional Spanning-Tree Features
Information About Configuring the Optional Spanning-Tree Features
Caution: Configure Port Fast only on ports that connect to end stations; otherwise, an accidental topology loop
could cause a data packet loop and disrupt switch and network operation.
You also can use the spanning-tree bpduguard enable interface configuration command to enable BPDU guard on any
port without also enabling the Port Fast feature. When the port receives a BPDU, it is put it in the error-disabled state.
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 PortFast-enabled interfaces by using the spanning-tree portfast
bpdufilter default global configuration command. This command prevents interfaces that are in a PortFast-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 PortFast-enabled interface, the interface loses its PortFast-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 PortFast 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.
Enabling BPDU Filtering
When you globally enable BPDU filtering on Port Fast-enabled interfaces, it 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.
Caution: Configure Port Fast only on interfaces that connect to end stations; otherwise, an accidental topology loop
could cause a data packet loop and disrupt switch and network operation.
You can also use the spanning-tree bpdufilter enable interface configuration command to enable BPDU filtering on any
interface 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.
UplinkFast
Switches in hierarchical networks can be grouped into backbone switches, distribution switches, and access switches.
Figure 45 on page 356 shows a complex network where distribution switches and access switches each have at least
one redundant link that spanning tree blocks to prevent loops.