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Summary

Store-and-Forward

Store-and-forward switching is Cisco's primary LAN switching method.
When in store-and-forward, the LAN switch copies the entire frame onto its
onboard buffers and then computes the cyclic redundancy check (CRC).
Because it copies the entire frame, latency through the switch varies with
frame length.

The frame is discarded if it contains a CRC error, if it's too short (less than

64 bytes including the CRC), or if it's too long (more than 1518 bytes includ-
ing the CRC). If the frame doesn't contain any errors, the LAN switch looks
up the destination hardware address in its forwarding or switching table to
find the correct outgoing interface. When it does, out goes the frame toward
its destination. This is the mode used by the Catalyst 5000 series switches,
and you can't modify it.

Summary

ou can think of this as the layer-2 switching background chapter

because the information I presented was designed to give you everything you
need before continuing with the rest of this book. Specifically, we covered
the following information:

Layer-2 switching and how switches differ from bridges

Address learning and how the MAC address filter table is built

Forward/filtering decisions that layer-2 switches make and how they
make them

Loop avoidance and the problems caused when loop avoidance
schemes are not used in the network

Spanning Tree Protocol and how it prevents loops

LAN switch types used on Cisco routers and how they differ

I've got to say this... if any of the subjects in this list here aren't clear to

you yet, look into the appropriate section again. These concepts are both
fundamental and progressive, and if you're a little confused at this point,
you'll probably just get frustrated if you move on to Chapter 3 and beyond.
It will only take a few minutes, and it's worth going over again to make the
rest of the course clearer and easier to understand!

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