Chapter 4. Spanning Tree Protocol

Terms you'll need to understand:

• Spanning Tree Protocol (STP) and 802.1D

• Bridge Protocol Data Unit (BPDU)

• Bridge identifier

• Port and path costs

• Hello, maximum age, and forward delay timers

• Root and designated ports

• Common Spanning Tree (CST) and Per-VLAN Spanning Tree (PVST)

Techniques you'll need to master:

• Understand the three main functions of a transparent bridge

• Know what BPDUs are used for

• Understand how the root switch is elected and how the root and designated ports are chosen

• Know the states a port goes through when STP is running and how long convergence can take

• Compare and contrast CST and PVST

• Configuring and verifying STP on your Catalyst switch

The need for high reliability and availability in today's networks is extremely important. However, this redundancy can create problems in switched networks specifically, Layer 2 loops. The problem that loops create is that a local broadcast or multicast is automatically forwarded by a switch. If a loop exists, the broadcast would circle around the loop forever. Some solution is needed to deal with this problem. The remainder of this chapter will focus on the Spanning Tree Protocol (STP): its components, its operation, its configuration, and troubleshooting.