Understanding How Portfast Works; Understanding How Bpdu Guard Works; Understanding How Portfast Bpdu Filtering Works - Cisco 6500 Series Software Configuration Manual

Understanding How PortFast Works

Understanding How PortFast Works
STP PortFast causes a Layer 2 LAN interface configured as an access port to enter the forwarding state
immediately, bypassing the listening and learning states. You can use PortFast on Layer 2 access ports
connected to a single workstation or server to allow those devices to connect to the network immediately,
instead of waiting for STP to converge. Interfaces connected to a single workstation or server should not
receive bridge protocol data units (BPDUs). When configured for PortFast, a port is still running the
spanning tree protocol. A PortFast enabled port can immediately transition to the blocking state if
necessary (this could happen on receipt of a superior BPDU).
With Release 12.1(11b)E:
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Because the purpose of PortFast is to minimize the time that access ports must wait for STP to converge,
Caution
it should only be used on access ports. If you enable PortFast on a port connected to a switch, you might
create a temporary bridging loop.

Understanding How BPDU Guard Works

When enabled on a port, BPDU Guard shuts down a port that receives a BPDU. When configured
globally, BPDU Guard is only effective on ports in the operational PortFast state. In a valid
configuration, PortFast Layer 2 LAN interfaces do not receive BPDUs. Reception of a BPDU by a
PortFast Layer 2 LAN interface signals an invalid configuration, such as connection of an unauthorized
device. BPDU Guard provides a secure response to invalid configurations, because the administrator
must manually put the Layer 2 LAN interface back in service. With release 12.1(11b)E, BPDU Guard
can also be configured at the interface level. When configured at the interface level, BPDU Guard shuts
the port down as soon as the port receives a BPDU, regardless of the PortFast configuration.
When enabled globally, BPDU Guard applies to all interfaces that are in an operational PortFast state.
Note

Understanding How PortFast BPDU Filtering Works

Release 12.1(13)E and later releases support PortFast BPDU filtering, which allows the administrator to
prevent the system from sending or even receiving BPDUs on specified ports.
When configured globally, PortFast BPDU filtering applies to all operational PortFast ports. Ports in an
operational PortFast state are supposed to be connected to hosts, that typically drop BPDUs. If an
operational PortFast port receives a BPDU, it immediately loses its operational PortFast status. In that
case, PortFast BPDU filtering is disabled on this port and STP resumes sending BPDUs on this port.
PortFast BPDU filtering can also be configured on a per-port basis. When PortFast BPDU filtering is
explicitly configured on a port, it does not send any BPDUs and drops all BPDUs it receives.
Explicate configuring PortFast BPDU filtering on a port that is not connected to a host can result in
Caution
bridging loops as the port will ignore any BPDU it receives and go to forwarding.
Catalyst 6500 Series Switch Cisco IOS Software Configuration Guide—Release 12.1 E
16-2
PortFast can be enabled on trunk ports
PortFast can have an operational value that is different from the configured value.
Chapter 16 Configuring Optional STP Features
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