Chapter 5: Using Spanning Tree with VLANs

The CCNP Exam Topics Covered in This Chapter Include the Following:

• Describe LAN segmentation with VLANs

• Describe Spanning Tree (STP), and explain the operation of common and per-VLAN STP implementations.

• Configure Spanning Tree in both Common Spanning Tree (CST) and per-VLAN modes.

• Configure Spanning Tree parameters including: port priority, VLAN priority and root bridge selection.

• Enable advanced Spanning Tree features such as BPDU guard, PortFast and UplinkFast and BackboneFast.

• Configure Fast EtherChannel and Gigabit EtherChannel on inter-switch links.

Redundancy is the ability to provide an immediate backup solution to a fault in the network that might otherwise cause a network or component service outage. When you’re building a redundant network—which is a network with redundant power, hardware, links, and other network-critical components—network loops can occur. The Spanning Tree Protocol (STP) was created to overcome the problems associated with transparent bridging at layer 2.

Unfortunately, STP is a far from optimal protocol. We can hardly blame the designers for this—all they had to work with was a forwarding system designed to transmit broadcasts out of every port, and the option of adding a little intelligence with Bridge Protocol Data Units (BPDUs). So legacy STP leaves us with suboptimal forwarding paths, unused spare links, and the possibility (probability, even) of very slow convergence after a network failure.

This chapter extends our coverage of STP by focusing on providing link redundancy by using STP and the IEEE 802.1D algorithm used to support STP on a per-VLAN basis. The Spanning Tree Protocol uses timers to make the network stable. You’ll also learn how to manage the different STP timers to maximize the efficiency of your network, and how to implement specific additions to STP to decrease convergence times.