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LAN Switch Types

AN switch types decide how a frame is handled when it's received on

a switch port. Latency--the time it takes for a frame to be sent out an exit
port once the switch receives the frame--depends on the chosen switching
mode. There are three switching modes:

Cut-through When in this mode, the switch waits for the destination
hardware address to be received before it looks up the destination address
in the MAC filter table.

FragmentFree (modified cut-through) This is the default mode for the
Catalyst 1900 switch, and it's sometimes referred to as modified cut-
through. In FragmentFree mode, the switch checks the first 64 bytes of
a frame before forwarding it for fragmentation, thus guarding against
possible collisions.

Store-and-forward Herein, the complete data frame is received on the
switch's buffer, a CRC is run, and then the switch looks up the destination
address in the MAC filter table.

Figure 2.10 delimits the different points where the switching mode takes

place in the frame.

When should I worry about spanning tree?

If you have fewer than six switches in your internetwork, then depending on

the number of users in your network, you typically would just let STP do its

job and not worry about it.

However, if you have dozens of switches and hundreds of users in your net-

work, then it is time to consider how STP is running. If you do not set the

root switch in this larger switched network, your STP may never converge

between switches, which could bring your network down.

Setting the timers and root switch are covered in the Sybex CCNP: Switch-

ing Study Guide.

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