802.1D Spanning Tree Protocol

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802.1D Spanning Tree Protocol
Although many CCIE candidates already know STP well, the details are easily forgotten. For
instance, you can install a campus LAN, possibly turn on a few STP optimizations and security
features out of habit, and have a working LAN using STP—without ever really contemplating
how STP does what it does. And in a network that makes good use of Layer 3 switching, each STP
instance might span only three to four switches, making the STP issues much more manageable—
but more forgettable in terms of helping you remember things you need to know for the exam.
This chapter reviews the details of IEEE 802.1D STP, and then goes on to related topics—802.1w
RSTP, multiple spanning trees, STP optimizations, and STP security features.
STP uses messaging between switches to stabilize the network into a logical, loop-free topology.
To do so, STP causes some interfaces (popularly called ports when discussing STP) to simply not
forward or receive traffic—in other words, the ports are in a blocking state. The remaining ports,
in an STP forwarding state, together provide a loop-free path to every Ethernet segment in the
network.