Bridge & Switch Facts

Bridge Facts

A bridge is a data forwarding device that provides data transfer. You should understand the following concepts relating to the operation of bridges.

• Bridges connect two media segments that use the same protocol.
• Bridges examine the source address to determine the media segment of network devices.
• Bridges operate at the Data Link layer of the OSI model.
• Bridges maintain a table of device addresses and their corresponding segments.
• Each segment connected by a bridge can have the same network address.
• Messages within a media segment are prevented from crossing over to another segment.

Bridges offer the following advantages:

• Bridges prevent wasted bandwidth by eliminating unnecessary traffic between segments.
• Bridges increase the maximum network length.
• Bridges forward packets for multiple upper-layer protocols.
• Bridges can link segments with dissimilar transmission media and media access methods.

Bridges have the following limitations:

• Bridges cannot link multiple architectures because different frame types are used.
• Bridges cannot translate upper-layer protocols.
• Bridges cannot forward packets to different networks based on the network address.
• Bridges do not filter broadcast packets.

Use bridges to isolate traffic to a segment, or to prevent unwanted traffic from crossing over to other segments, or to slow WAN links. When designing the placement of bridges on the network, follow the 80/20 rule.

• At least 80% of network traffic should stay within a segment.
• No more than 20% of network traffic should pass through the bridge to another segment.

Switch Facts

A switch is a multiport bridge. It provides the same functionality, but with a higher port density. In addition, switches provide features that cannot be found in bridges.

• Switches are associated with the Data Link layer of the OSI Model.
• Switches build a forwarding database in a manner similar to bridges. Switches examine the source and destination Data Link address in each packet to build the database and make forwarding decisions.
• Switches connect multiple segments or devices and forward packets to only one specific port.
• You can connect a single device to a switch port or multiple devices to a switch port by using a hub.

Switches offer the following advantages over a non-switched network.

• Switches create separate collision domains.
• Switches provide guaranteed bandwidth between devices, if dedicated ports are used.
• Switches can be used to provide collision-free networking, if only one device is connected to each switch port.
• Switches enable full-duplex communication.
• Switches induce less latency than other segmentation solutions.
• Switches can simultaneously switch multiple messages.
• Switches can mix 10 Mbps- and 100 Mbps-capable devices, if the switch is a 100 Mbps switch.
• Ethernet switches can be implemented without re-cabling.

Switches have replaced bridges in most network applications.