Layer 2 bridging / Layer 2 switching

Layer 2 bridging 
8.1.1 This page will discuss the operation of Layer 2 bridges.
As more nodes are added to an Ethernet segment, use of the media increases. Ethernet is a shared media, which means only one node can transmit data at a time. The addition of more nodes increases the demands on the available bandwidth and places additional loads on the media. This also increases the probability of collisions, which results in more retransmissions. A solution to the problem is to break the large segment into parts and separate it into isolated collision domains.

To accomplish this a bridge keeps a table of MAC addresses and the associated ports. The bridge then forwards or discards frames based on the table entries. The following steps illustrate the operation of a bridge:

• The bridge has just been started so the bridge table is empty. The bridge just waits for traffic on the segment. When traffic is detected, it is processed by the bridge.
• Host A pings Host B. Since the data is transmitted on the entire collision domain segment, both the bridge and Host B process the packet.
• The bridge adds the source address of the frame to its bridge table. Since the address was in the source address field and the frame was received on Port 1, the frame must be associated with Port 1 in the table.
• The destination address of the frame is checked against the bridge table. Since the address is not in the table, even though it is on the same collision domain, the frame is forwarded to the other segment. The address of Host B has not been recorded yet.
• Host B processes the ping request and transmits a ping reply back to Host A. The data is transmitted over the whole collision domain. Both Host A and the bridge receive the frame and process it.
• The bridge adds the source address of the frame to its bridge table. Since the source address was not in the bridge table and was received on Port 1, the source address of the frame must be associated with Port 1 in the table.
• The destination address of the frame is checked against the bridge table to see if its entry is there. Since the address is in the table, the port assignment is checked. The address of Host A is associated with the port the frame was received on, so the frame is not forwarded.
• Host A pings Host C. Since the data is transmitted on the entire collision domain segment, both the bridge and Host B process the frame. Host B discards the frame since it was not the intended destination.
• The bridge adds the source address of the frame to its bridge table. Since the address is already entered into the bridge table the entry is just renewed.
• The destination address of the frame is checked against the bridge table. Since the address is not in the table, the frame is forwarded to the other segment. The address of Host C has not been recorded yet.
• Host C processes the ping request and transmits a ping reply back to Host A. The data is transmitted over the whole collision domain. Both Host D and the bridge receive the frame and process it. Host D discards the frame since it is not the intended destination.
• The bridge adds the source address of the frame to its bridge table. Since the address was in the source address field and the frame was received on Port 2, the frame must be associated with Port 2 in the table.
• The destination address of the frame is checked against the bridge table to see if its entry is present. The address is in the table but it is associated with Port 1, so the frame is forwarded to the other segment.
• When Host D transmits data, its MAC address will also be recorded in the bridge table. This is how the bridge controls traffic between to collision domains.

These are the steps that a bridge uses to forward and discard frames that are received on any of its ports.

The next page will describe Layer 2 switching.

Layer 2 switching
8.1.2 This page will discuss Layer 2 switches.


Generally, a bridge has only two ports and divides a collision domain into two parts. All decisions made by a bridge are based on MAC or Layer 2 addresses and do not affect the logical or Layer 3 addresses. A bridge will divide a collision domain but has no effect on a logical or broadcast domain. If a network does not have a device that works with Layer 3 addresses, such as a router, the entire network will share the same logical broadcast address space. A bridge will create more collision domains but will not add broadcast domains.

A switch is essentially a fast, multi-port bridge that can contain dozens of ports. Each port creates its own collision domain. In a network of 20 nodes, 20 collision domains exist if each node is plugged into its own switch port. If an uplink port is included, one switch creates 21 single-node collision domains. A switch dynamically builds and maintains a content-addressable memory (CAM) table, which holds all of the necessary MAC information for each port.

The next page will explain how a switch operates