computer science

Route redistribution

Route redistribution is a computer science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Route redistribution rather than just read about it. In short: In a router, route redistribution allows a network that uses one routing protocol to route traffic dynamically based on information learned from another routing protocol. Routing On the Internet, routers forward data packets between computer networks.

Key takeaways

Route redistribution belongs to computer science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Route redistribution to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Route redistribution from memory before moving on to harder problems.

Reference excerpt

In a router, route redistribution allows a network that uses one routing protocol to route traffic dynamically based on information learned from another routing protocol.

Routing On the Internet, routers forward data packets between computer networks. In order for a router to determine where to forward each packet, it may use:

default or static routes that are manually configured, on-demand routing, dynamic routing protocols, or Policy-based routing. In a small network such as a home or home office, a default route is often used to send all traffic to the user's Internet service provider. Likewise, medium-sized networks such as branch offices or small Internet service providers may use default routes for traffic intended for the public Internet. But in medium-sized and large networks, routers use dynamic routing protocols to determine the best paths to various network destinations. Sometimes, a network may use more than one dynamic routing protocol, for example, if two different companies merge or if networking devices from multiple vendors are used. In such networks, it is often useful to redistribute information from one routing protocol into another.

Where used Route redistribution may be used to allow different networks belonging to the same company to communicate with each other. It may also be used when two companies merge and their networks use different routing protocols. And it may be used in conjunction with VRFs or MP-BGP.

Route redistribution pitfalls If not implemented carefully, route redistribution may cause routing loops. Each routing protocol contains loop-avoidance mechanisms, but the information needed for loops to be avoided is lost when routes are redistributed. For example, a distance-vector routing protocol may prevent loops by the "split-horizon" rule; that is to say, if a router learns a route from a particular interface it will not re-advertise the route out the same interface. In other words if A learns from B that the path to C is through B then it will not tell B to route packets destined for C through A. Likewise, a link-state routing protocol may keep a database containing the state of different links in the network, representing a "map" (so to speak) of the network. But the portion of the network whose routes are learned by redistribution are not "mapped" in the same way. The "where did I learn this route?" information is lost in the redistribution process. The chance of loops is increased if there are more than one router that performs redistribution in the same network, and if redistribution occurs in both directions (for example, both from EIGRP into OSPF and from OSPF into EIGRP).

References

External links David Davis, Cisco administration 101: Routing redistribution, http://www.techrepublic.com/article/cisco-administration-101-routing-redistribution (Dec. 8, 2005). Cisco Systems, Inc., Configuring a Gateway of Last Resort Using IP Commands, http://www.cisco.com/c/en/us/support/docs/ip/routing-information-protocol-rip/16448-default.html (last updated Aug. 10, 2005).

Worked examples

Example 1 — a first encounter with Route redistribution

Start with the simplest possible case. Write down what Route redistribution claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In computer science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Route redistribution before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Route redistribution ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Route redistribution

In research: Route redistribution appears in computer science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Route redistribution in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Route redistribution is common in secondary-school and first-year university syllabi. It links to neighbouring topics Routing algorithms, so understanding it makes those chapters shorter.
In everyday life: Look for Route redistribution outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Route redistribution in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Route redistribution means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Route redistribution out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Route redistribution in simple terms?

In a router, route redistribution allows a network that uses one routing protocol to route traffic dynamically based on information learned from another routing protocol. Routing On the Internet, routers forward data packets between computer networks.

Why does Route redistribution matter?

Because it connects several computer science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Route redistribution?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Route redistribution.