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Chapter (3) Networking Media and Data Link Layers Protocols

host that is added increases the amount of potential traffic on the network. Because Layer 1
devices pass on everything that is sent on the medium, the more traffic that is transmitted within
a collision domain, the greater the chances of collisions are. The final result is diminished
network performance, which is even more pronounced if all the computers on that network
demand large amounts of bandwidth.

3.8.1 Broadcast Domains

A broadcast domain is a grouping of collision domains that are connected by Layer 2
devices. Breaking up a LAN into multiple collision domains improves network efficiency by
allowing multiple transmissions of data simultaneously on separate collision domains. But
broadcasts travel across the Layer 2 devices and, if excessive, can reduce the efficiency of the
overall LAN. Broadcasts must be controlled at Layer 3 because Layer 1 and 2 devices have no
way of controlling them. The total size of a broadcast domain can be identified by looking at all
of the collision domains that the same broadcast frame is processed by.

In other words, all the nodes are a part of that network segment bounded by a Layer 3
device. Broadcast domains are controlled at Layer 3 because routers do not forward broadcasts.
https://www.youtube.com/watch?v=elx49XNGUgk

3.9 Introduction to the Spanning Tree Protocol

When multiple switches are connected, there is a possibility of creating a loop where
there is no clear path from source to destination. If switches are arranged in a simple hierarchical
tree, no loops will occur. To counteract the possibility of loops, switches are provided with a
protocol for them to talk with each other to resolve the condition. A switch sends special
messages called bridge protocol data units (BPDUs) out all its ports to let other switches know
of its existence, as shown in Figure 3-22 and Figure 3-23.
The switches use a spanning tree algorithm (STA) to resolve and shut down the
redundant paths. The process of shutting down a port is called blocking. The result of resolving
and eliminating the loops is a logical hierarchical tree created with no loops. However, the
alternate paths are still available, in case they are needed. The protocol used to resolve and
eliminate loops is known as the Spanning Tree Protocol (STP). This creates another switch and
bridge operation mode known as loop-avoidance mode.

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