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Outbyte Driver Updater FREEScan for outdated or missing drivers - takes under a minuteDriver Scan →Outbyte PC Repair FREEClear out junk files and repair common Windows errorsFree Scan →An interior gateway protocol (IGP) distributes routing information among routers within a single autonomous system (AS). It describes a protocol’s role and scope—not one specific protocol or routing algorithm. OSPF, IS-IS, and RIP are examples.
What does interior gateway protocol mean?
RFC 1812, section 7.2.1, defines an IGP as a protocol “used to distribute routing information between the various routers in a particular AS.” An autonomous system is the routing-domain boundary that matters here: an IGP exchanges routing information inside that domain.
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So, “interior” refers to routing within an AS. It does not mean that IGP is a single protocol, a particular vendor’s product, or a synonym for one routing algorithm.
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The distinction is primarily about where routing information is exchanged. RFC 1812 identifies BGP-4 as an inter-AS protocol, while an IGP distributes routing information among routers within an AS. They serve different scopes and are not interchangeable terms.
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Which protocols are IGPs?
OSPF, IS-IS, and RIP are examples of IGPs. They share the within-an-AS role, but their algorithm families and routing metrics differ.
| Protocol | Algorithm family | Metric or calculation |
|---|---|---|
| OSPF | Link-state | Uses shortest-path calculations; the cited overview does not establish a specific metric value here. |
| IS-IS | Link-state | Uses shortest-path-first calculations. |
| RIP | Distance-vector | Uses hop count as its metric. |
IGRP is another example described in Cisco documentation, but it is a historical, vendor-developed protocol; its inclusion does not make it the default contemporary choice.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Does the IGP definition identify the best protocol?
No. The definition establishes the protocol’s scope, not which IGP a network should use. Choosing among protocols depends on network requirements and implementation context. The cited material does not establish a universally best IGP.
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