Switches connect to both network nodes and dissimilar network segments of LANs. Switches can be described as a multiport bridge where each port is a bridge, and each connected device receives a dedicated channel, hence easing congestion in traffic. Most switches have an internal processor, OS, memory, and ports. They are mostly costly although they make good use of a limited bandwidth as compared to bridges.

Multiport

They offer better security by isolating traffic and separate channels from routers. Data can likely be lost because at times switches can be overwhelmed by continuous heavy traffic. Traffic can come to a stop because of collusions of protocols that have lost the operation and ability to detect data loss and restoration. It is therefore important for switch placement to match backbone capacity and traffic patterns.

Cut-through Switching mode

They read frames header and chose where to forward data before receiving the whole packet. Where a frame became corrupt packet, runts, or is erroneously shortened, it will await to determine the integrity of the packet. They do not stop to read the packet in its entirety. It is suitable to small working groups, has limited speed with a low number of device support and is good for larger LANs and mixed speed environments.

Routers

Apart from keeping track of certain nodes on the network as switches do, routers connect dissimilar networks e.g. LANs and WANs running at different transmission speeds and using variety of protocols. Used in specialized applications e.g. large internet nodes or digitized telephone cells.

Functions of routers

• Connect dissimilar networks
• Addressing of information interpretation.
• Determine the best path for data to follow from node A to node B
• Reroute traffic if primary path is down but another path is available

  

                                                                 Courtesy: Image of a Router

Other Functions of Routers

• Filter out broadcast transmissions to alleviate network congestion
• Prevent certain types of traffic from getting to a network, enabling customized segregation and security
• Support simultaneous local and remote connectivity
• Provide high network fault tolerance through redundant components such as power suppliers or network interfaces
• Monitor network traffic and report statistics.
• Diagnose internal or other connectivity problems and trigger alarms

Categories of Routers

Interior router

Directs data between autonomous LAN of an organization e.g. from supervisor's node to employee node.

Exterior router

Directs data between nodes external to a given autonomous LAN e.g. routers on internet backbone.

Border routers

(Gateway routers) connect autonomous LAN with a business LAN (partnership) over the WAN.

Routing Modes

Static routing

They Use specific programmed router by network administrations between nodes

• Not optimal: It does not account for congestions in networks and connects that are
• They can be used to indicate the last resort router with dynamic routing. The last resort router accepts all unrouteable packets.
• They are less efficient, less accurate and would require administrative human intervention.

Dynamic Packet Flow Problem

Dynamic routing

Dynamic routing automatically calculates the best possible path between 2 nodes and accumulatively puts this information in a routing table. Routing table has three categories of information namely:

• Network ID: It details the IP address of the router destination.
• Cost: It gives the number of routers or links packets that go through to the router destination.
• Next Hop: This is the address of the station that follows next unto which the packet is to be relayed on its way to to the final destination. It can easily make detections of possible congestions, failures and re-route traffic appropriately.

Gateways

Gateways are the configuration of hardware to be able to connect 2 dissimilar networks that have different formatting and communication protocols or architecture. They interpret information to be read by a different system and must:

• Be able to Operate at all levels of OSI
• Be able to Communicate with applications.
• Be able to establish and manage sessions.
• Be able to Translate encoded data
• Be able to Interpret both logical and physical addressing of data

They also reside on a server, a computer or a connectivity device. Gateways are however much slower than both the bridges or routers

Network Design Consideration

• The “5-4-3” rule of thumb is often considered for efficient and trouble-free network.
• There should be no more than 5 segments repeated.
• There should be no more than four repeaters or hubs between two nodes
• There should not be more than three populated segments
• Allows up to seven bridges in a system

Additional reading https://en.wikipedia.org/wiki/Router_(computing)