Summary

PortFast, UplinkFast, and BackboneFast are Cisco proprietary STP enhancement features. All are disabled by default. PortFast keeps a port in forwarding mode, where it is not included in the STP algorithm. Nonswitch and nonbridge devices should be connected to PortFast ports. To enable PortFast, use the spanning-tree portfast interface command. To prevent inadvertent switch connections to PortFast ports, you can use the BPDU Guard and filter features. BPDU Guard shuts down a PortFast port if a BPDU is received on it. If BPDU filtering detects more than 10 BPDUs on a port, it disables PortFast on the port and treats it as a normal STP port.

UplinkFast provides fast convergence for uplink ports. If the root port fails, a secondary uplink port can be immediately taken from a blocking state and placed in a forwarding state. Use the spanning-tree uplinkfast command to enable UplinkFast on your switch.

Unlike UplinkFast, BackboneFast can detect failures on connections not directly connected to a switch. BackboneFast detects this condition by looking for inferior BPDUs on blocking ports. When it sees an inferior BPDU show up on a blocking port, it starts the STP process of moving the port from blocking to listening and eventually to a forwarding state. Use the spanning-tree backbonefast command to enable BackboneFast.

IEEE enhanced the 802.1D STP into RSTP (802.1W). RSTP provides better performance than Cisco's proprietary Fast features. RSTP only has three port states: discarding, learning, and forwarding. There are two additional port roles. An alternate port is a standby port for the primary root port. A backup port is a standby port for a designated port. RSTP determines which ports are edge ports and places them into a forwarding state. An edge port is connected to a nonswitch device. This is similar to PortFast.

MST is IEEE's version of Cisco's PVST. MST is more scalable than Cisco's PVST because MST uses instances for STP, and an instance can contain multiple VLANs. Switches in an MST region have the same region name, revision number, and VLAN table contents. For backward-compatibility when connecting to a CST switch, MST has an IST, which makes the MST region look like a single virtual switch to the CST switch.

EtherChannels enable you to take up to eight FastEthernet or Gigabit Ethernet connections to supply 1.6 16Gbps of bandwidth (in a full-duplex configuration). Connections in the channel must be configured identically to be part of the EtherChannel. Channels provide an advantage in an STP environment: If one connection fails in the channel, the link still remains operational. PAgP (a Cisco protocol) and LACP (an IEEE protocol) allow channels to be dynamically formed by sharing configuration information across channel-capable connections. For PAgP, one side has to be set to desirable and the other side has to be set to auto or desirable. For LACP, one side has to be set to active and the other side has to be set to active or passive. An on mode enables channeling, but disables PAgP and LACP. Use the channel-group command to include an interface in a channel.

Root Guard enables you to force a particular port to be a designated port so that a connected switch does not become a root switch. When a violation occurs, the offending port is placed into an inconsistent state and an error message is generated. This feature is disabled by default, but can be enabled with the spanning-tree guard root command.

The UDLD feature checks to see whether any unidirectional connections exist on the switch's interfaces. If any are found, the switch disables the interfaces. Unidirectional connections can cause one-way bridging loops. To enable UDLD, use the udld enable command. UDLD is automatically enabled on fiber-optic interfaces. Loop Guard is similar to UDLD. Loop Guard typically detects STP software issues, whereas UDLD detects Layer 1 issues. To enable Loop Guard, use the spanning-tree guard loop command.

When troubleshooting STP issues, you should first have a network diagram of your network layout, including your STP setup. Use show commands to discover loops. Any loops that you discover should be broken up by disabling interfaces. Examine the status of your interfaces to determine where loops are, as well as the CPU's and interfaces' utilization. In certain cases, you might want to disable certain STP features to pinpoint a problem. The debug spanning-tree events command is a useful command when you're trouble shooting STP issues, including loops.