9-3
Catalyst 6500 Series Switch Software Configuration Guide—Release 8.7
OL-8978-04
Chapter 9 Configuring Spanning-Tree PortFast, UplinkFast, BackboneFast, and Loop Guard
Understanding How PortFast BPDU Filtering Works
Understanding How PortFast BPDU Filtering Works
BPDU filtering allows you to avoid transmitting BPDUs on a port that is connected to an end system.
When you enable BPDU filtering on the switch, spanning tree places that port in the forwarding state
immediately, instead of going through the listening, learning, and forwarding states. When you enable
BPDU filtering globally and set the port configuration as the default for BPDU filtering (see the
“Configuring PortFast BPDU Filtering on the Switch” section on page 9-13
), then PortFast enables or
disables the BPDU filter.
If the port configuration is not set to default, then the PortFast configuration will not affect BPDU
filtering.
Table 9-2
lists all the possible BPDU filter combinations. The BPDU filter allows access ports
to move directly to the forwarding state as soon as the end hosts are connected.
Understanding How UplinkFast Works
UplinkFast provides fast convergence after a spanning-tree topology change and achieves load balancing
between redundant links using uplink groups. An uplink group is a set of ports (per VLAN), only one of
which is forwarding at any given time. Specifically, an uplink group consists of the root port (which is
forwarding) and a set of blocked ports. The blocked ports do not include self-looping ports. The uplink
group provides an alternate path in case the currently forwarding link fails.
Note
UplinkFast is most useful in wiring-closet switches. This feature may not be useful for other types of
applications.
Figure 9-1
shows an example topology with no link failures. Switch A, the root switch, is connected
directly to Switch B over link L1 and to Switch C over link L2. The port on Switch C that is connected
directly to Switch B is in the blocking state.
Table 9-2
BPDU Filter Port Configurations
Per-Port Configuration
Global Configuration
PortFast Operational Value
Operational BPDU Filter
Default
Enable Enable
Enable
1
1.
The port transmits at least 10 BPDUs. If this port receives any BPDUs, then the operational PortFast value is set to disable and the operational BPDU filter
is then disabled.
Default
Enable
Disable
Disable
Default
Disable
X
Disable
Disable
X
X
Disable
Enable
X
X
Enable