Recommended Free Tools
An interior routing protocol, formally called an Interior Gateway Protocol (IGP), is a routing protocol that routers use to exchange routing information with each other inside a single Autonomous System (AS). RFC 4271 (January 2006) defines it in nearly those words: “a routing protocol used to exchange routing information among routers within a single Autonomous System.”
What “interior” means
“Interior” describes the scope of the protocol. It works inside one routing domain, the Autonomous System, and not between domains. The classic AS model is one technical administration that runs an IGP and common metrics for its internal routes. A separate inter-AS protocol handles reachability to other ASes. Inside the AS, the IGP lets routers coordinate which destinations are reachable and by which paths.
![Computer Networking Bible: [3 in 1] The Complete Crash Course to Effectively Design, Implement and Manage Networks. Including Sections on Security, Performance and Scalability](https://m.media-amazon.com/images/I/41H4YJnxKgL._SL160_.jpg)
As an Amazon Associate I earn from qualifying purchases.
What an IGP does
An IGP distributes reachability information so that each router can pick routes within the domain. The definition does not prescribe one particular route-selection algorithm. The mechanics differ from protocol to protocol.
What’s actually slowing this PC down?
Pick the symptom - the matching free tool is one click away.
Cisco’s introduction to IGRP shows one example. In that distance-vector protocol, gateways learn routes by exchanging updates with adjacent gateways. They then compare path metrics to choose between routes. That describes IGRP and protocols like it. It is not a description of every IGP.
#1 Best Overall
Interior versus exterior routing
| Aspect | Interior (IGP) | Exterior / inter-AS |
|---|---|---|
| Scope | Routers within a single AS | Routing between different ASes |
| Typical management | One administration, or closely coordinated ones (Cisco’s description) | Connects separate sets of networks |
| Metrics | Classically common across the AS | Not a single shared internal metric |
Common misconceptions
- It is not defined by ownership. The standards wording is about exchanging routes within an AS. Single ownership is a typical case, not the definition.
- One AS does not always mean one IGP. The classic model uses a single IGP and common metrics. RFC 4271 notes that an AS may use multiple IGPs and metric sets and still present a consistent interior routing plan to other systems.
- Not every IGP works like IGRP. Update exchange and metric comparison are shown by one example. Protocols differ in routing method (for example link-state versus distance-vector), metrics, convergence behavior and operational constraints. Those are the dimensions to use when comparing OSPF, IS-IS, RIP or EIGRP. The definition alone does not say which is better.
You do not need any router hardware to understand the term. It is a concept defined by the standards.
Quick Recap
Best Value
Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon at the time of purchase will apply.
