BGP Interview Questions and Answers
BGP separates senior candidates from juniors — it runs the internet and its logic is genuinely different from IGPs. These are the questions asked for network engineer and ISP roles, with the reasoning interviewers want to hear. Ground them in our BGP basics and path selection guide.
How to prepare
BGP interviews test whether you grasp policy-based path selection, not just connectivity. Know the attribute order (weight → local preference → AS-path…), the eBGP/iBGP distinction, and why iBGP needs a full mesh or route reflectors. Being able to explain how to influence inbound vs outbound traffic marks real experience.
Frequently asked questions
What is the difference between eBGP and iBGP?
eBGP runs between different autonomous systems (default AD 20, TTL 1); iBGP runs within one AS (AD 200) and does not re-advertise iBGP-learned routes, requiring a full mesh or route reflectors.
What is the first BGP path-selection attribute?
Weight — Cisco-proprietary and local to the router. Highest weight wins before any other attribute is considered.
What is the difference between weight and local preference?
Weight is local to one router and not advertised; local preference is shared across the whole AS and influences the AS's outbound path choice.
Why does iBGP require a full mesh?
Because a router won't re-advertise routes learned via iBGP to other iBGP peers (loop prevention) — so every iBGP router must peer with every other, unless route reflectors are used.
What is a route reflector?
A router that re-advertises iBGP routes to its clients, removing the need for a full iBGP mesh and greatly improving scalability.
How do you influence inbound traffic in BGP?
It's harder than outbound — commonly via AS-path prepending (making your path look longer on less-preferred links) or MED, since you're trying to influence a neighbouring AS's decision.
What are the main BGP neighbour states?
Idle, Connect, Active, OpenSent, OpenConfirm, Established. Established means the session is up and exchanging routes; stuck in Active usually means a connectivity or config problem.
What does AS-path prepending do?
Repeats your AS number in the path advertisement to make that route appear longer and therefore less preferred, steering traffic to another path.
Related articles
Want hands-on training?
Learn this on real Cisco lab devices with placement support at Attila Technologies, Ahmedabad.