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Routing & Switching · Foundational

Interior Gateway Protocol (IGP)

A routing protocol used to exchange routes within a single autonomous system, such as OSPF, IS-IS, EIGRP or RIP.

Definition

IGPs are optimized for fast convergence and metric-based best-path selection inside one administrative domain. They usually carry infrastructure prefixes such as loopbacks and links, while BGP carries external or customer routes. In service provider networks the IGP provides reachability to BGP next hops and is the basis for MPLS LSPs.

Example

An enterprise runs OSPF as its IGP and eBGP to its ISP.

Exam tip

IGP vs EGP is about where the protocol runs, not its algorithm; BGP used inside an AS (iBGP) is still an EGP.

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