STP Varieties
Several varieties of STP have emerged after the original IEEE 802.1D:
- STP: The original specification of STP, defined in 802.1D, provides a loop-free topology in a network with redundant links. STP is sometimes referred to as Common Spanning Tree (CST) because it assumes one spanning tree instance for the entire bridged network, regardless of the number of VLANs.
- PVST+: Per-VLAN Spanning Tree Plus is a Cisco enhancement of STP that provides a separate 802.1D spanning tree instance for each VLAN configured in the network.
- RSTP: Rapid STP, or IEEE 802.1w, is an evolution of STP that provides faster convergence than STP. However, RSTP still only provides for a single instance of STP.
- Rapid PVST+: Cisco enhancement of RSTP that uses PVST+. Rapid PVST+ provides a separate instance of 802.1w per VLAN.
- Multiple Spanning Tree Protocol: MSTP is an IEEE standard inspired by the earlier Cisco proprietary Multiple Instance STP (MISTP) implementation. MSTP maps multiple VLANs into the same spanning tree instance. The Cisco implementation of MSTP is MST, which provides up to 16 instances of RSTP and combines many VLANs with the same physical and logical topology into a common RSTP instance.
Part of your switch administration skill set is the ability to decide which type of STP to implement. Table 31-3 summarizes the features of each STP flavor.
Table 31-3 Features of STP Varieties
|
Protocol |
Standard |
Resources Needed |
Convergence |
Tree Calculation |
STP |
802.1D |
Low |
Slow |
All VLANs |
PVST+ |
Cisco |
High |
Slow |
Per VLAN |
RSTP |
802.1w |
Medium |
Fast |
All VLANs |
Rapid PVST+ |
Cisco |
Very high |
Fast |
Per VLAN |
MSTP |
802.1s, Cisco |
Medium or high |
Fast |
Per instance |
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