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BCMSN Exam Cram™ 2 (Exam Cram 642-811) by Richard A. Deal

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Spanning Trees

The assumption made so far in this chapter is that all the switches are running one instance of STP for the whole switched network. This is sometimes referred to as a Common Spanning Tree (CST). IEEE 802.1Q on non-Cisco switches uses CST to remove loops.

Cisco, on the other hand, has two proprietary forms of STP implementations: Per-VLAN Spanning Tree or Shared Spanning Tree (PVST) and PVST+. By default, PVST is used on ISL trunks and 802.1Q trunks between Cisco switches, where a separate instance of STP is run for each VLAN. That means for each VLAN you have, you'll have a separate STP algorithm and database, a separate root switch and BPDUs for each VLAN. PVST+ is used in mixed trunk environments in which you have both ISL and ...

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