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An interior gateway protocol (IGP) is a routing protocol that distributes routing information among routers within one autonomous system (AS). It describes a protocol’s role and scope—not one particular protocol or routing algorithm.
What an IGP does
Routers use routing information to determine where to send network traffic. An IGP enables routers inside the same AS to exchange that information. 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 AS is the routing-domain boundary that gives “interior” its meaning here. An IGP operates within that boundary; the term does not specify a particular vendor, protocol implementation, or method for calculating routes.
How an IGP differs from BGP
IGP and BGP are not interchangeable names. They describe routing at different scopes: an IGP distributes routing information within an AS, while RFC 1812 identifies BGP-4 as an inter-AS routing protocol. In other words, the distinction is about where the routing information is exchanged, not simply which protocol is newer or more capable.
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Examples of interior gateway protocols
OSPF, IS-IS, and RIP are examples of IGPs. They share the within-an-AS role, but use different routing approaches.
| Protocol | Algorithm family | Metric or calculation |
|---|---|---|
| OSPF | Link-state | Not stated in the cited source notes |
| IS-IS | Link-state | Uses shortest-path-first calculations |
| RIP | Distance-vector | Uses hop count |
These categories describe how protocols approach route information; they do not change the basic definition of an IGP. The table’s classifications and details are supported by the cited protocol overviews; a specific metric for OSPF is not established in those source notes.
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OSPF and IS-IS
OSPF and IS-IS are link-state IGPs. IS-IS uses shortest-path-first calculations to determine routes. IS-IS also has an IP extension described in RFC 1195, as noted in Cisco’s IOS XE IS-IS documentation.
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RIP is a distance-vector IGP whose metric is hop count. That makes it a straightforward example of how an IGP can use a particular measure when evaluating routes.
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IGRP
IGRP is another example found in Cisco’s documentation, which describes it as an IGP. It is best understood here as a historical, vendor-developed example rather than as a claim about the default choice for contemporary networks.
Does “IGP” mean one specific protocol?
No. IGP is the category: it identifies protocols used to exchange routing information within an AS. OSPF, IS-IS, and RIP are individual examples in that category. Their algorithm families and route metrics differ, so the definition alone does not tell you which protocol a network uses.
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How to think about IGP selection
There is no universally best IGP established by these definitions and protocol overviews. A network’s requirements and implementation context determine which protocol is appropriate. The useful first distinction is scope: identify whether the routing exchange is within an AS or between ASes. For an interior-routing question, then compare the relevant protocols by their algorithm family and route-calculation approach.
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