computer science

Policy-based routing

Policy-based routing 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 Policy-based routing rather than just read about it. In short: In computer networking, policy-based routing (PBR) is a technique used to make routing decisions based on policies set by the network administrator. When a router receives a packet, it normally decides where to forward it based on the destination address in the packet, which is then used to look up an entry in a routing table.

Key takeaways

Policy-based routing 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 Policy-based routing to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Policy-based routing from memory before moving on to harder problems.

Reference excerpt

In computer networking, policy-based routing (PBR) is a technique used to make routing decisions based on policies set by the network administrator. When a router receives a packet, it normally decides where to forward it based on the destination address in the packet, which is then used to look up an entry in a routing table. However, in some cases, there may be a need to forward the packet based on other criteria. For example, a network administrator might want to forward a packet based on the source address, not the destination address. This permits routing of packets originating from different sources to different networks even when the destinations are the same and can be useful when interconnecting several private networks. Policy-based routing may also be based on the size of the packet, the protocol of the payload, or other information available in a packet header or payload. In the Cisco IOS, PBR is implemented using route maps. Linux supports multiple routing tables since version 2.2. FreeBSD supports PBR using either IPFW, IPFilter or OpenBSD's PF.

Examples PBR can be used to redirect traffic to a proxy server by using a PBR-aware L3-switch (router). In such a deployment, specific source traffic (e.g., HTTP, FTP) can be redirected to a cache engine. This is known as virtual inline deployment.

Notes

References

External links Policy routing Cisco Press article Policy based routing with Linux (ONLINE edition) Network overview by Rami Rosen

Worked examples

Example 1 — a first encounter with Policy-based routing

Start with the simplest possible case. Write down what Policy-based routing 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 Policy-based routing 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 Policy-based routing 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 Policy-based routing

In research: Policy-based routing 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 Policy-based routing 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: Policy-based routing is common in secondary-school and first-year university syllabi. It links to neighbouring topics Computer network stubs, Routing, so understanding it makes those chapters shorter.
In everyday life: Look for Policy-based routing 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 Policy-based routing in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Policy-based routing 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 Policy-based routing out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Policy-based routing in simple terms?

Why does Policy-based routing 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 Policy-based routing?

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 Policy-based routing.