Comparing spanning tree options

Without spanning tree, having more than one active path between a pair of nodes causes loops in the network, which can result in duplication of messages, leading to a “broadcast storm” that can bring down the network.

The 802.1D spanning tree protocol operates without regard to a network's VLAN configuration, and maintains one common spanning tree throughout a bridged network. This protocol maps one loop-free, logical topology on a given physical topology. This results in the least optimal link utilization and longest convergence times.

The 802.1s multiple spanning tree protocol (MSTP) uses multiple spanning tree instances with separate forwarding topologies. Each instance is composed of one or more VLANs, which significantly improves network link utilization and the speed of reconvergence after a failure in the network’s physical topology. However, MSTP requires more configuration overhead and is more susceptible to dropped traffic due to misconfiguration.

Rapid spanning tree protocol (RSTP) requires less configuration overhead, provides faster convergence on point-to-point links, and speedier failure recovery with predetermined, alternate paths. The switches covered by this guide, use the IEEE Rapid Per-VLAN spanning tree Protocol (RPVST) standard. RPVST was introduced as an enhancement to Rapid spanning tree Protocol (RSTP) to improve the link utilization issue and require less configuration overhead. Basically, RPVST+ is RSTP operating per-VLAN in a single layer 2 domain. VLAN tagging is applied to the ports in a multi-VLAN network to enable blocking of redundant links in one VLAN while allowing forwarding over the same links for non-redundant use by another VLAN. Each RPVST+ tree can have a different root switch and therefore can span through different links. Since different VLAN traffic can take different active paths from multiple possible topologies, overall network utilization increases.

Another major advantage of RPVST+ is that it localizes topology change propagation to individual VLANs. Since there is a separate spanning tree for each VLAN, topology changes affecting a particular VLAN are propagated only inside that VLAN. The switch flushes the MAC addresses learned only on the affected VLAN and other VLAN traffic is not disturbed. This minimizes the network flooding caused by the spanning tree topology changes. This is a significant improvement in the case of a large, flat, layer 2 network. In a network having a large number of per-VLAN spanning tree instances, RPVST+ can result in an increased load on the switch's CPU.