JUNOS Enterprise Switching

Chapter 6. Spanning Tree Protocol

Switches have been used in networking for years and at times seem like a perfect solution to your problems. However, switches do have some limitations, and that is that they do exactly what they are told. (Don’t you wish everybody did?) This can lead to some undesirable results and often can lead to network meltdowns. To avoid this, protocols such as Spanning Tree Protocol (STP) were developed to add some intelligence to the physical switching architecture—in other words, “Do what you are told, unless what I am telling you is incorrect.”

The following topics are examined in this chapter:

• Switches and loop issues

• STP

• Rapid Spanning Tree Protocol (RSTP)

• Configuring and monitoring the spanning tree

• Multiple Spanning Tree Protocol (MSTP)

• Redundant Trunk Groups (RTGs)

Feeling a Little Loopy

Before diving into the problem, let’s recap how a switch works:

• It uses Media Access Control (MAC) addresses to forward traffic toward its destinations.

• It “learns” station locations based on the Source MAC (SMAC) addresses.

• If a destination is not known, it will send the frame out all ports in that broadcast domain.

With these fundamental concepts in mind, consider the network in Figure 6-1. At first glance, it may seem like a very redundant, well-designed network. Let’s examine what happens when unicast, multicast, and broadcast frames are sent from Station A to Station C.

First, the unicast packet is sent from A to C, both switches on the LAN. They will perform a Destination ...