1.8 Network Devices (Layer 3)

1.8 Network Devices (Layer 3)


OSI Model: Layer 3 Device
Protocol Data Unit(PDU): Packets

Where a network consists of several segments with differing protocols and architectures a bridge might be inadequate. The network needs a device that not only knows the address of each segment, but can also determine the best path for sending data and filtering broadcast traffic to the local segment. Such a device is called a router.

Routers work at the network layer of the OSI reference model. This means they can switch and route packets across multiple networks. This is achieved by exchanging protocol specific information between separate networks. Routers can read complex network addressing information in the packet and, because they function at a higher layer in the OSI reference model than bridges, they have access to additional information. Routers can provide the filtering and isolating of traffic and the connection of network segments.

Routers have access to more of the information in packets than bridges have and use this information to improve packet deliveries. Routers are used in networks because they provide better traffic management. Routers can share routing information with one another and use this information to bypass slow or broken connections.

Routers require specific addresses. They use the network number of IP Addresses to allow them to communicate with other routers. Routers do not talk to remote computers.

Because routers read only addressed network packets, they do not allow corrupted data to get onto the network. Because they do not pass corrupted data or broadcast data storms, routers put little stress on networks.

Routers do not look at the destination node address; they look only at the network address. Routers will pass information only if the network address is known. This ability to control the data passing through the router reduces the amount of traffic between networks and allows routers to use these links more efficiently than bridges.

Functions of Routers

The two major types of routers are:

  • Static Static routers require an administrator to manually set up and configure the routing table and to specify each route.
  • Dynamic Dynamic routers are designed to discover routes automatically and therefore require a minimal amount of setting up and configuration. More sophisticated than static routers, they examine information from other routers and make packet-by-packet decisions about how to send data across the network.

Routers communicate with each other to share information about available paths and directly connected routes.

A Static route can be manually programmed to create a routing table. This means that all data will follow the same route. This is also known as non-adaptive routing. Each node on the WAN has a fixed routing table. All messages for a particular destination must follow a predetermined path this is good for determining secure routes.

If a node is faulty, added or removed the tables will have to be altered manually. This method does not cope well with unplanned events such as breakdown or congestion although it does use less router processor overhead.

Most routers are dynamic with the capability of being statically configured.

Dynamic routers automatically create dynamic routing tables based on the data that is provided from other routers. Dynamic routing is designed to cope with unplanned events. Routers respond to changes in the network status by automatically updating the routing table.

How Routers Work

Routers are complex devices that are used to connect two or more separate networks. Typically they will have a number of physical interfaces to interconnect many networks regardless of the technology or platform of the networks. A router can work across different architectures, they work at the Network Layer and use network addressing; IP addresses are universal therefore a Unix network can communicate with a Windows network.

A router will match packet headers to LAN segments and perform a best effort delivery service. A router will work out the best way to send messages through other routers.

WAN messages travel from point to point over long distances passing a number of routing stations on the way. Each router will clean up the message and send it to the next link; each of the links is known as a hop. The maximum life of a message is 255 hops; this may be less depending on the routing protocol.

Routers build up a database of available routes and information about the routes. This is called a routing table.