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edundancy 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 net-
work-critical components--network loops can occur. The Spanning Tree
Protocol (STP) was created to overcome the problems associated with trans-
parent bridging at layer 2.

This chapter will focus on providing link redundancy by using STP and

the IEEE 802.1d algorithm used to support STP. The Spanning Tree Protocol
uses timers to make the network stable. You'll learn how to manage the dif-
ferent STP timers to maximize the efficiency of your network.

Cisco and IEEE 802.1q Committee

isco and the IEEE do not see everything eye-to-eye when it comes to

using spanning tree and VLANs. Per-VLAN Spanning Tree (PVST) is a Cisco
proprietary implementation of STP. PVST uses Inter-Switch Link (ISL) rout-
ing and runs a separate instance of STP for each and every VLAN.

The IEEE uses what is called Common Spanning Tree (CST), which is

defined with IEEE 802.1q. The IEEE 802.1q defines one spanning tree
instance for all VLANs.

There is one more implementation of STP, and that is called PVST+.

Because it ends with a plus sign, it must be better, right? Well, maybe. What
it does is allow CST information to be passed into PVST. Cisco thinks it
would be easier if you just had all Cisco switches; then you wouldn't even
have to think about this issue.

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