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Chapter 4
Layer 2 Switching and the Spanning Tree Protocol (STP)
F I G U R E 4 . 3
Multiple frame copies
The MAC address filter table will be confused about where a device is
located because the switch can receive the frame from more than one link. It
is possible that the switch can't forward a frame because it is constantly
updating the MAC filter table with source hardware address locations. This
is called thrashing the MAC table.
Multiple Loops
One of the biggest problems is multiple loops generating throughout an
internetwork. This means that loops can occur within other loops. If a
broadcast storm were to then occur, the network would not be able to
perform packet switching.
To solve these three problems, the Spanning Tree Protocol was developed.
Spanning Tree Operation
I
n layer 3 devices, which are typically routers, the routing protocols are
responsible for making sure routing loops do not occur in the network. What
is used to make sure network loops do not occur in layer 2 switched net-
works? This is the job of the Spanning Tree Protocol (STP).
Digital Equipment Corporation (DEC), which was purchased and is now
called Compaq, was the original creator of STP. The IEEE then created its
Segment 1
Segment 2
Unicast
Unicast
Unicast
Router C
Switch A
Switch B
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