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Commonly confused · Routing & Switching

iBGP vs eBGP: What's the Difference?

eBGP runs between routers in different autonomous systems and is how networks exchange routes across the Internet. iBGP runs between routers inside the same autonomous system to carry externally learned routes across it.

Internal BGP (iBGP)

Full definition
vs

External BGP (eBGP)

Full definition
Internal BGPExternal BGP
PeersSame AS numberDifferent AS numbers
Administrative distance (Cisco)20020
Default TTL255; peers need not be directly connected1; peers usually directly connected (ebgp-multihop to change)
AS_PATHNot modified when advertising to iBGP peersLocal AS prepended
Loop preventioniBGP split horizon; needs full mesh, route reflectors, or confederationsReject routes containing own AS in AS_PATH
Next hopUnchanged by default (next-hop-self often used)Set to the advertising router
Remember it

eBGP talks to the neighbors next door; iBGP passes the news around the house.

Exam tip

Know the AD values (20 vs 200), the iBGP full-mesh requirement and its fixes (route reflectors, confederations), and why next-hop-self is commonly configured.

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