Exam Essentials

Know the types of spanning tree available. STP comes in a variety of different flavors, and you need to be sure which one to configure. It's not necessarily a case of which one is best, because not every switch supports every option, but you do need to understand the different types of spanning tree and know what their limitations and benefits are. ISL is Cisco proprietary and allows for one spanning tree instance per VLAN (PVST), whereas the standards-based 802.1Q supports only Common Spanning Tree, unless you also implement 802.1s and the MST option.

Know what can be configured to reduce the delay a port must go through with a topology change. A perennial STP problem is slow convergence. The Cisco proprietary options of PortFast, BackboneFast, and UplinkFast are capable of speeding up the process, and you need to understand what they are doing and under what circumstances you can use them. At the time this book went to press, these are the main players, but the recent standardization of 802.1w- RSTP-means that they may be used less in the future.

Understand how an EtherChannel works. An EtherChannel is formed from bonding together between two and eight ports connecting the same two switches. A single command on each switch logically binds the circuits together, but only if each circuit is configured in an identical fashion.