• Animesh Gupta

What is Routing?

Routing is the act of moving information across an inter-network from a source to a destination. Along the way, at least one intermediate node typically is encountered. Routing is often contrasted with bridging, which might seem to accomplish precisely the same thing to the casual observer. The primary difference between the two is that bridging occurs at Layer 2 (the link layer) of the OSI reference model, whereas routing occurs at Layer 3 (the network layer). This distinction provides routing and bridging with different information to use in the process of moving information from source to destination, so the two functions accomplish their tasks in different ways.

The topic of routing has been covered in computer science literature for more than two decades, but routing achieved commercial popularity as late as the mid-1980s. The primary reason for this time lag is that networks in the 1970s were simple, homogeneous environments. Only relatively recently has large-scale inter networking become popular.

Routers can route in a two basic ways. They can use pre-programmed static routes, or they can dynamically calculate routes using any one of a number of dynamic routing protocols. Dynamic routing protocols are used by routers to perform discover routes. Routers then mechanically forward packets (or datagrams) over those routes.  Statically programmed routers cannot discover routes; they lack any mechanism to communicate routing information with other routers. Statically programmed routers can only forward packets using routes defined by a network administrator.

Static Routing

The simplest form of routing is pre-programmed and, consequently, static routes. The tasks of discovering routes and propagating them throughout a network are left to the inter-network’s administrator(s). A router programmed for static routing forwards packets out of predetermined ports. After the relationship between a destination address and a router port is configured, there is no longer any need for routers to attempt route discovery or even communicate information about routes.

There are many benefits to using static routes. For instance, statically programmed routes can make for a more secure network. There can be only a single path into, and out of, a network connected with a statically defined route. That is, of course, unless multiple static routes are defined. Another benefit is that static routing is much more resource efficient. Static routing uses far less bandwidth across the transmission facilities, doesn’t waste any router CPU cycles trying to calculate routes, and requires far less memory. In some networks, you might even be able to use smaller, less expensive routers by using static routes. Despite these benefits, you must be aware of some inherent

Dynamic Routing

There are three broad categories of dynamic routing protocols:

  • Distance-vector

  • Link-state

  • Hybrids

1. Distance-Vector Routing

In routing based on distance-vector algorithms, also sometimes called Bellman-Ford algorithms, the algorithms periodically pass copies of their routing tables to their immediate network neighbors. Each recipient adds a distance vector—that is, its own distance “value”—to the table and forwards it on to its immediate neighbors. This process occurs in an omnidirectional manner among immediately neighboring routers. This step-by-step process results in each router learning about other routers and developing a cumulative perspective of network “distances.” The cumulative table is then used to update each router’s routing tables.

Drawbacks to Distance-Vector Routing

Under certain circumstances, distance-vector routing can actually create routing problems for Distance-vector protocols. A failure or other change in the network, for example, requires some time for the routers to converge on a new understanding of the network’s topology. During the convergence process, the network may be vulnerable to inconsistent routing, and even infinite loops. Safeguards can contain many of these risks, but the fact remains that the network’s performance is at risk during the convergence process. Therefore, older distance-vector protocols that are slow to converge may not be appropriate for large, complex WAs.

What’s Distance-Vector Routing Good For

Generally speaking, distance-vector protocols are very simple protocols that are easy to  configure, maintain, and use. Consequently, they prove quite useful in very small networks that have few, if any, redundant paths and no stringent network performance requirements.

2. Link-State Routing

Link-state routing algorithms, known cumulatively as shortest path first (SPF) protocols, maintain a complex database of the network’s topology. Unlike distance-vector protocols, link-state protocols develop and maintain a full knowledge of the network’s routers as well as how they interconnect. This is achieved via the exchange of link-state advertisements (LSAs) with other routers in a network. Each router that has exchanged LSAs constructs a topological database using all received LSAs. An SPF algorithm is then used to compute reachability to networked destinations. This information is used to update the routing table. This process can discover changes in the network topology caused by component failure or network growth.

Drawbacks to Link-State Routing

During the initial discovery process, link-state routing protocols can flood the network’s transmission facilities, and thereby significantly decrease the network’s capability to transport data. This performance degradation is temporary but can be very noticeable. Whether this flooding process will impede a network’s performance noticeably depends on two things: the amount of available bandwidth and the number of routers that must exchange routing  information.

Link-state routing is both memory and processor intensive. Consequently, more fully configured routers are required to support link-state routing than distance-vector routing. This increases the cost of the routers that are configured for link-state routing.

Advantages of Link-State Routing

The link-state approach to dynamic routing can be quite useful in networks of any size. In a well-designed network, a link-state routing protocol will enable your network to gracefully weather the effects of unexpected topological change. Using events, such as changes, to drive updates (rather than fixed-interval timers) enables convergence to begin that much more quickly after a topological change. The overheads of the frequent, time-driven updates of a distance- vector routing protocol are also avoided. This allows more bandwidth to be used for routing traffic rather than for network maintenance, provided you design your network properly.

A side benefit of the bandwidth efficiency of link-state routing protocols is that they facilitate network scalability better than either static routes or distance-vector protocols.

3. Hybridized Routing

The last form of routing discipline is hybridization. The balanced hybrid routing protocols use distance-vector metrics but emphasize more accurate metrics than conventional distance-vector protocols. They also converge more rapidly than distance-vector protocols but avoid the overheads of link-state updates. Balanced hybrids are event driven rather than periodic and thereby conserve bandwidth for real applications.

Although “open” balanced hybrid protocols exist, this form is almost exclusively associated with the proprietary creation of a single company, Cisco Systems, Inc. Its protocol, Enhanced Interior Gateway Routing Protocol (EIGRP), was designed to combine the best aspects of distance-vector and link-state routing protocols without incurring any of their performance limitations or penalties.

4. Convergence

One of the most fascinating aspects of routing is a concept known as convergence. Quite simply, whenever a change occurs in a network’s topology, or shape, all the routers in that network must develop a new understanding of what the network’s topology is.This process is both  collaborative and independent; the routers share information with each other, but must independently calculate the impacts of the topology change on their own routes. Because they must mutually develop an agreement of the new topology independently from different perspectives, they are said to converge on this consensus.

Convergence is necessary because routers are intelligent devices that can make their own routing decisions. This is simultaneously a source of strength and vulnerability. Under normal operating conditions, this independent and distributed intelligence is a source of tremendous advantage. During changes in the network’s topology, the process of converging on a new consensus of the network’s shape may actually introduce instability and routing problem.

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