Cisco Spanning Tree Enhancements | L2 Network Availability Protocols

The STP 50-second convergence time results in TelePresence calls being self-terminated and is, therefore, unacceptable. Thus, if STP is to be used within a TelePresence campus network, STP convergence times need to be significantly improved.

To improve on STP convergence times, Cisco has made a number of enhancements to 802.1D STP, including the following:

• PortFast (with BPDU-Guard)
• UplinkFast
• BackboneFast

STP PortFast causes a Layer 2 LAN port 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 because interfaces connected to a single workstation or server should not receive BPDUs. Because the purpose of PortFast is to minimize the time that access ports must wait for STP to converge, it should be used only on access ports. Optionally, for an additional level of security, PortFast can be enabled with BPDU-Guard, which immediately shuts down a port that has received a BPDU.

You can enable PortFast globally (along with BPDU-Guard) or on a per-interface basis by entering the following commands:

Switch(config)# spanning-tree portfast default
Switch(config)# spanning-tree portfast bpduguard default

UplinkFast provides fast convergence after a direct link failure and achieves load balancing between redundant Layer 2 links. If a switch detects a link failure on the currently active link (a direct link failure), UplinkFast unblocks the blocked port on the redundant link port and immediately transitions it to the Forwarding state without going through the Listening and Learning states, as illustrated in Figure 1. This switchover takes approximately one to five seconds.

 
Figure 1: UplinkFast recovery example after direct link failure

You can enable UplinkFast globally, as follows:

Switch(config)# spanning-tree uplinkfast

In contrast, BackboneFast provides fast convergence after an indirect link failure, as shown in Figure 2. This switchover takes approximately 30 seconds (yet improves on the default STP convergence time by 20 seconds).

 
Figure 2: BackboneFast recovery example after indirect link failure

You can enable BackboneFast globally, as follows:

Switch(config)# spanning-tree backbonefast

These Cisco-proprietary enhancements to 802.1D STP were adapted and adopted into a new standard for STP, IEEE 802.1w or Rapid Spanning Tree Protocol (RSTP)