Why do we need Spanning Tree within our networks?
To ensure that high availability is provided at Layer 2, redundancy of devices, modules, and links throughout the network. Network redundancy at Layer 2, however, introduces the potential for bridging loops, where packets loop endlessly between devices, crippling the network. The Spanning Tree Protocol identifies and prevents such Layer 2 loops.
Evolution of Spanning Tree Protocols
Multiple redundant paths between switches can cause loops in the network topology. If a loop exists, the potential for message duplication exists. When loops occur, some switches see stations appear on both sides of the switch. This condition confuses the forwarding algorithm and enables duplicate frames to be forwarded. To prevent loops while providing path redundancy, Spanning Tree Protocol (STP) defines a tree that spans all switches in an extended network. STP will allow only one active path and block any redundant paths.
- The first STP, called the DEC STP, was invented in 1985 by Radia Perlman at the DigitalEquipment Corporation.
- In 1990, the IEEE published the first standard for the protocol as 802.1D based on the algorithm designed by Perlman. Subsequent versions were published in 1998 and 2004 incorporating various extensions.
- Common Spanning Tree (CST) assumes one 802.1D spanning-tree instance for the entire bridged network, regardless of the number of VLANs. Because there is only one instance, the CPU and memory requirements for this version are lower than the others. However, because there is only one instance, there is only one root bridge and one tree. This means that traffic for all VLANs flows over the same path. This can lead to suboptimal traffic flows. Also the network is slow in converging after topology changes due to inherent 802.1D timing mechanisms.
- Per VLAN Spanning Tree Plus (PVST+) is a Cisco enhancement of STP that provides a separate 802.1D spanning-tree instance for each VLAN configured in the network. The separate instance supports enhancement such as PortFast, BPDU guard, BPDU filter, root guard, and loop guard. Creating an instance for each VLAN increases the CPU and memory requirements but allows for per-VLAN root bridges. This allows the STP tree to be optimized for the traffic of each VLAN. Convergence of this version is similar to 802.1D; however, convergence is per-VLAN.
- Rapid STP (RSTP), or IEEE 802.1w, is an evolution of STP that provides faster convergence of STP. This version addresses many of the convergence issues, but because it still had a single instance of STP, it did not address the suboptimal traffic flow issues. To support that faster convergence, the CPU usage and memory requirements of this version are slightly more than CST but less than PVRST+.
- Multiple Spanning Tree (MST) is an IEEE standard inspired from the earlier Cisco proprietary Multi-Instance Spanning Tree Protocol (MISTP) implementation. To reduce the number of required STP instances, MST maps multiple VLANs that have the same traffic flow requirements into the same spanning-tree instance. The Cisco implementation provides up to 16 instances of RSTP (802.1w) and combines many VLANs with the same physical and logical topology into a common RSTP instance. Each instance supports PortFast, BPDU guard, BPDU filter, root guard, and loop guard. The CPU and memory requirements of this version are less than PVRST+ but more than RSTP.
- PVRST+ is a Cisco enhancement of RSTP that is similar to PVST+. It provides a separate instance of 802.1w per VLAN. The separate instance supports PortFast, BPDU guard, BPDU filter, root guard, and loop guard. This version addressed both the convergence issues and the suboptimal traffic flow issues. To do this, this version has the largest CPU and memory requirements.
The RSTP algorithm is far superior to 802.1D STP and even PVST+ from a convergence perspective. It greatly improves the restoration times for any VLAN that requires a topology convergence due to link up, and it greatly improves the convergence time over BackboneFast for any indirect link failures.
Comparison of Spanning Tree Protocols
Protocol Standard Resources Needed Convergence
CST 802.1D Low Slow All VLANs
PVST+ Cisco High Slow Per VLAN
RSTP 802.1w Medium Fast All VLANs PVRST+ Cisco Very high Fast Per VLAN
MSTP 802.1s Medium/high Fast VLAN list