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This chapter is from the book

This chapter is from the book

Data Link Layer Devices

Objective: Design a simple LAN using Cisco technology

At the Data Link layer, either a bridge or a Layer 2 switch can be installed to segment a LAN. Hubs and repeaters at the Physical layer only serve to extend a network. With segmentation, switches and bridges create a separate collision domain for each connected node, which effectively reduces the number of collisions that occur on that network.

Remember from Chapter 1 that a collision domain is a group of nodes that shares the same media and are segmented by switches or bridges. A collision occurs if two nodes attempt a simultaneous transmission within the same collision domain. This reinforces the need for an increased number of collision domains. Figure 3.7 demonstrates how a bridge creates two collision domains.

Figure 3.7

Figure 3.7 Example of a bridged network.

Figure 3.8 provides an example of a situation in which a switch creates separate collision domains.

Figure 3.8

Figure 3.8 Example of a switched network.


Bridges were created to alleviate several expansion-related network issues. As networks were growing and becoming more complex, hubs and repeaters no longer provided sufficient network resources. Because they do not segment the network, all the devices connected to a hub or repeater had to share the same bandwidth. Also, if one device sent a frame it could collide with a frame from another device on that LAN. This meant that all devices on that LAN had to take turns sending frames. Again, this is not very efficient as additional devices are added to a network.

Transparent bridges were introduced and helped solve these growing pains. The word transparent is used to indicate that the other devices on a network are not aware of its existence. Bridges use a software application to forward frames.

The following are the primary tasks performed by both bridges and switches:

  • The source MAC address of every inbound frame is examined to learn its MAC address.
  • Frames may either be forwarded or filtered depending on the destination MAC address (they can also be flooded if the destination is unknown).
  • Eliminates loops that are caused by redundant connections by configuring Spanning Tree Protocol (STP).

Learned MAC addresses and their interfaces are stored in a bridge table on the bridge or switch. When a new frame arrives on that bridge or switch, the device refers to the bridge table to decide how to forward or filter the frame. If the frame’s destination MAC address is on a different segment of that LAN, the device forwards the frame to that segment. If the frame’s destination MAC address is on the same segment as the source MAC address, the device filters the frame. That frame reaches its destination without the assistance of a bridge or switch. Figure 3.9 shows a segmented LAN with the MAC addresses of each end user.

As frames are received by the bridge or switch from each end user, it updates its bridge table with their MAC addresses and the interface on which the frame came into the device. Table 3.8 shows the bridge table of this bridge.

Figure 3.9

Figure 3.9 Bridge LAN.

Table 3.8 Example Bridge Table for Figure 3.9

MAC Address










If the incoming frame destination address is...

  • Unicast—The bridge checks the bridge table first. If the destination unicast address is not in the bridge table, it forwards the frame to all interfaces except for the interface that originally sent the frame. If the destination unicast address is in the bridge table and on a different interface than the interface that originally sent the frame, it forwards the frame. If the destination unicast address is in the bridge table and on the same interface as the sender, the frame is filtered.
  • Multicast—The bridge forwards the frame to all interfaces except for the interface that originally sent the frame.
  • Broadcast—The bridge forwards the frame to all interfaces except for the interface that originally sent the frame.


Layer 2 switches are multi-port bridges; therefore, they have all the same functionality of bridges. There are differences that differentiate a switch from a bridge. For example, switches utilize hardware or Application-Specific Integrated Circuit (ASIC) chips to forward frames rather than software. Also, each port of the switch has a dedicated bandwidth. If the dedicated port on a switch is 10Mbps, the connected LAN segment has a dedicated bandwidth of 10Mbps. This works in the same manner for 100Mbps and 1000Mbps dedicated switch ports. This feature also sets a switch apart from a bridge that has a low port density.

A popular ethernet switch port is the 10/100 ethernet port, where you can set the port to pass traffic at 10Mbps or 100Mbps. Chapter 4, "IP at the Network Layer," goes into more detail regarding specific Cisco devices, including the 2950 series switches.

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