Читать книгу CCNP Enterprise Certification Study Guide: Implementing and Operating Cisco Enterprise Network Core Technologies - Ben Piper, David Higby Clinton - Страница 55

THE CCNP ENCOR EXAM OBJECTIVES COVERED IN THIS CHAPTER INCLUDE THE FOLLOWING:

 Domain 3.0: Infrastructure✓ 3.1 Layer 2

Unlike IP packets, Ethernet frames don't have a time-to-live (TTL) to prevent routing loops. That means if a LAN is configured in a physical loop, it may blindly forward the same frame over and over again. Switches don't keep track of frames they've already seen, so in a looped topology, a switch actually ends up multiplying the same frame in the network each time it forwards it! Eventually, this consumes all available bandwidth and pegs the CPU on each switch, causing a network meltdown.

The goal of STP is to impose a loop-free logical topology by strategically dropping certain Ethernet frames on specific interfaces (what STP calls ports) to prevent them from endlessly looping through the network. Radia Perlman began work on the original STP (802.1D) in 1984. Because this was prior to the widespread use of VLANs, 802.1D worked by simply blocking interfaces, a logical choice since the physical topology and the logical topology were the same. An interface could be a member of only a single LAN.

The advent of VLANs required a different approach. Each VLAN represented a different logical topology such that one VLAN might be configured in a loop while another VLAN would be loop-free. No longer could STP prevent loops by blocking interfaces. Instead, it had to determine the logical topology of each VLAN, and then selectively drop any Ethernet frames that might cause a loop—an approach called Per-VLAN Spanning Tree (PVST). Much of Spanning Tree's reputation for being complicated is due to this decoupling of the physical and logical topologies. It's not that Spanning Tree itself is hard to understand but that it behaves differently depending on the VLAN.

The first step to understanding and configuring Spanning Tree is to get a handle on the logical topology of each VLAN, so we'll start by looking at VLANs and trunking. After that, you'll learn about Rapid Per-VLAN Spanning Tree (RPVST+ or RSTP), which is the most common Spanning Tree protocol in use today. We'll then cover Multiple Spanning Tree Protocol (MSTP) and finally some Cisco-proprietary Spanning Tree extensions. You can classify each STP type by the approach it uses to prevent loops. There are three approaches:

 802.1D—Avoids a loop in the LAN.

 PVST—Prevents loops in a VLAN. Most of the time this is what people mean when they say “Spanning Tree.”

 MST—Prevents loops in an arbitrary group of VLANs.

Keep these differences in mind as you read through this chapter.