This chapter covers
- The standard and Cisco-proprietary versions of Spanning Tree Protocol
- A comparison of the port costs, states, and roles of STP and Rapid STP
- How RSTP-enabled switches react to topology changes
- How RSTP link types affect convergence
- Optional STP features Root Guard, Loop Guard, and BPDU Filter
In this chapter, we will continue to look at Spanning Tree Protocol (STP). There’s a reason STP is enabled by default on almost any vendor’s switches: Layer 2 loops are disastrous for a LAN. However, there are some downsides to the original STP as defined by IEEE 802.1D, the main one being speed; it can take up to 50 seconds to converge and reach a new, stable state after a change in the LAN. When STP was first released, 50 seconds was an acceptable time frame, but expectations have changed by now.
The answer to the increased demand for speed in modern LANs is Rapid Spanning Tree Protocol (RSTP), the topic of this chapter. The good news is that since we covered the original STP in chapter 14, you’re already 80% of the way to understanding RSTP (from the perspective of the CCNA—there is more nuance when you dig deeper). In this chapter, we will continue from the previous chapter and finish covering exam topic 2.5: Identify basic operations of Rapid PVST+ Spanning Tree Protocol.