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Routers
March 31, 2008
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Routers
A router is a device that connects multiple networks and routes packets from one network to another. A router may be used to inter network similar or dissimilar networks (e.g. Ethernet, or token ring). An inter-network is composed of subnets (sub-networks). The main feature of a router include:
- Routers work at the network layer. They are able to identify source and destination network addresses within packets.
- Routers are able to keep track of multiple active paths between any given source and destination network.
- Routers provide excellent traffic management using sophisticated path selection, they select the best routes based on traffic loads, line speeds, number of hops or administrator preset cost. The parameters used for determining routes for packets is generally known as metrics.
- Routers can share status and routing information with other routers, and can listen to the network and identify which connections are busiest or not working. They rate network traffic avoiding slow or malfunctioning connections.
- Routers do not forward any information that does not have a correct network address. They do not forward bad data, they also filter broadcast traffic by not routing broadcast pockets.
Note:
A router may be a dedicated box with a port to each of the networks, or it may be a NOs server with multiple interface cards. (This is known as multi-timed).
Routers often support multiple protocols (e.g. TCP/IP, IPX/SPX), but not all protocols are routable e.g. NetBeul and DLC.
Routable protocols differ from non-routable protocols in that they contain information in each packet relating to the network address of the source and destination routes.
Choosing a routing Path:
A routing algorithm is used to build a routing table for forwarding packets. There are 2 types of algorithms used.
(a) Non Adaptive:
The choice of route is normally configured into the router. This is run as static routing.
b) Adaptive:
Routing decisions are based on traffic levels, connection speeds and a number of hops, or administrator preset costs. Routing information is obtained from other routers. This is known as dynamic routing.
The routing table contains the following information.
- Address of all known networks
- Interface of the router used to forward packets to the network.
- Next router in the path to the network.
- Metric or cost of using this path. If multiple paths exist, use the path with the lowest metric.
Static and Dynamic Routers:Static Routers:
Static routers require the administrator to manually configure routes through each network (The routers do not communicate amongst themselves).
This configuration is only possible with a small number of routers and does not provide the flexibility of dynamic routing. Its advantage is that complete control remains with the network administrator.
Dynamic Routers:
These routers automatically discover routes by communicating with each other. They require minimal configuration since their routing table are built and modified through these communications. It’s high flexible and can reach to changes in the internetwork e.g. route
Dynamic routers use routing protocols to manage information.
- Open shortest path first (OSPF) uses a link state algorithm to calculate routes based on the number of hops, line speed, traffic and cost.
- Network link state protocol (NLSP): This is the equivalent of OSPF for network environment.
- Routing Information Protocol (RIP): This method uses distance vector algorithm to determine routes. This is less efficient than link state algorithm because
The entire routing table is broadcasted instead of just the changes which result in large and often multiple packets (there is a maximum of 25 entries per R.P. packet).
The entire routing table is broadcasted at regular interval (every 30 seconds) resulting in considerable network traffic.
The routing table are slow to stabilize when a change in the internetwork occurs.
Brouters
These are routers that can also bridge. A router routes any routable protocol - supported, but bridges any other frames. These devices combines the best of both a bridge and a router.
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