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Tip of the Day
Language: Web Development
Expertise: Beginner
Mar 18, 1997

Bridging vs Routing

Question:
What are the differences between bridging and routing?

Answer:

A bridge is a device that connects two or more network segments. A router is also a device that connects two or more network segments, so what is the difference?

When a bridge is installed between two networks, it gathers the packets from one network and repeats them at the other and vice-versa. This way, nodes on one network can talk to nodes on the other. Bridges also have such features as broadcast filtering to allow for greater efficiency and reduce traffic. Therefore, in its most simple form, a bridge is a two-port network device that connects two network segments. Some advanced implementation of bridging can monitor traffic and determine which nodes are on which segment and later use this information while forwarding packets. A main factor to note is the fact that bridging takes place in the Data-link Layer of the OSI reference model.

Routers, on the other hand, can be classified as packet exchanges or switches. They deal with packets of information. Most importantly, routers operate in the Network Layer of the OSI model. They interconnect network segments, LANs and WANs. In their advanced forms, they can provide various schemes of filtering, path control and traffic control functions.

Routers can handle one or more protocols such as TCP/IP, IPX, etc. By operating in the Network Layer, router allow networks to be segmented by creating uniquely addressable networks. Each segment can have its own network number and this information used during packet addressing/delivery.

When a packet arrives at a router, its checksum information is verified to ensure accuracy. The physical and data-link layer (layers below the network layer) information is stripped. The network layer information is then deciphered. If the packet is meant for the router that received it, the rest of the information is processed. On the other hand, if the packet was addressed to another destination on the same network, the router simply forwards the packet. Filtering controls are implemented at this point if implemented and packet might be discarded if the situation demands. If the destination address belongs to a remote network, the router consults its routing table before forwarding the packet. The router maintains a routing table internally to keep a picture of the network and the various paths in effect. Sometime, the packet itself might contain source routing information and if this is the case, it is utilized by the router during delivery or forwarding. If the destination cannot be found or the hop count is exceeded (such a feature is sometimes used in protocols such as TCP/IP), the packet is discarded and an error message is sent back to the source of the packet.

When two or more network segments are connected by bridges and a node sends out a packet, a typical bridge would repeat the packet on all the segments on the segments that are connected to it. Contrast this with routing which is discussed below. A router would only copy the packet to the network that hosts the intended destination rather than send packets on all links.

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