IEEE 802.1D Spanning Tree Protocol | L2 Network Availability Protocols

In TelePresence campus networks, redundant paths are encouraged within the network design; however, redundant paths might cause Layer 2 loops and, thus, recursive forwarding and packet drops.

The IEEE 802.1D Spanning Tree Protocol (STP) prevents loops from being formed when switches are interconnected through multiple paths. SPT implements the Spanning Tree Algorithm by exchanging Bridge Protocol Data Unit (BPDU) messages with other switches to detect loops and then removes the loop by blocking selected switch interfaces. This algorithm guarantees that there is one—and only one—active path between two network devices, as illustrated in Figure 1.

 
Figure 1: STP-based redundant topology

STP prevents a loop in the topology by transitioning all (STP-enabled) ports through four STP states:

• Blocking: The port does not participate in frame forwarding. STP can take up to 20 seconds (by default) to transition a port from Blocking to Listening.
• Listening: The port transitional state after the Blocking state when the spanning tree determines that the interface should participate in frame forwarding. STP takes 15 seconds (by default) to transition between Listening and Learning.
• Learning: The port prepares to participate in frame forwarding. STP takes 15 seconds (by default) to transition from Learning to Forwarding (provided such a transition does not cause a loop; otherwise, the port will be set to Blocking).
• Forwarding: The port forwards frames.

Figure 2 illustrates the STP states, including the disabled state.

 
Figure 2: STP port states

You can enable STP globally on a per-VLAN basis (referred to as Per-VLAN Spanning Tree [PVST]) by entering the following command:

Switch(config)# spanning-tree vlan 100

The two main availability limitations for STP follow:

• To prevent loops, redundant ports are placed in a Blocking state and as such are not used to forward frames and packets. This significantly reduces the advantages of redundant network design, especially for network capacity and load-sharing.
• Adding up all the times required for STP port-state transitions shows that STP can take up to 50 seconds to converge on a loop-free topology. Although this might have been acceptable when the protocol was first designed, it is certainly unacceptable today.

Both limitations are addressable using additional technologies. The first limitation can be addressed by using the Cisco Virtual Switching System, and the second limitation can be addressed by various enhancements that Cisco developed for STP.