AS 100 (a small ISP) has two routes to a destination: one through its provider AS 200 (2 hops, fast link) and one through a paying customer AS 300 (4 hops, slower link). Which route will AS 100 most likely prefer under standard BGP routing policy?
AThe route through AS 200, because BGP prefers paths with fewer AS hops
BThe route through AS 200, because BGP selects for lowest physical latency
CThe route through AS 300, because AS 100 earns revenue for carrying customer traffic and sets higher local preference for customer routes
DWhichever route has the lower BGP MED (multi-exit discriminator) value
BGP's highest-priority decision criterion is local preference, which is set by the operator to reflect business relationships — not path length or latency. Customer routes are preferred because the customer pays AS 100 for transit; provider routes are least preferred because AS 100 pays the provider. AS path length (option 0) and physical performance (option 1) are lower-priority criteria in BGP's decision algorithm. This is the fundamental distinction from interior protocols: BGP routing is a business decision, not a technical optimization.
Question 2 Multiple Choice
What is the primary technical purpose of BGP's path-vector mechanism — advertising the full list of AS numbers in each route announcement?
ATo allow receiving routers to calculate the total end-to-end latency of each path
BTo prevent routing loops by enabling routers to reject routes that already contain their own AS number
CTo compress route update messages so they consume less bandwidth than link-state flooding
DTo provide a path quality metric equivalent to OSPF's link-state cost calculation
The full AS path serves two purposes: loop prevention (if a router sees its own AS number already in the path, it rejects the route — it would be routing back to itself) and policy input (the path list is available for operators to make business-based decisions). BGP has no knowledge of link latency or performance (option 0), its updates are not compressed path representations (option 2), and it deliberately does not use a cost metric like OSPF (option 3). Loop prevention is the technical necessity; policy expressiveness is the operational value.
Question 3 True / False
BGP typically selects the route with the fewest AS hops to any given destination.
TTrue
FFalse
Answer: False
AS path length is only one criterion in BGP's multi-step decision algorithm, and it ranks below local preference. Because local preference reflects business relationships — customer routes are preferred over peer routes, which are preferred over provider routes — an operator will routinely select a longer AS path to a customer destination over a shorter path through a provider. 'Shortest path' is the OSPF paradigm; BGP's paradigm is 'policy-optimal path,' which may be quite long. Operators regularly add artificial AS path prepending to make their own routes appear longer and influence other ASes' routing decisions.
Question 4 True / False
A single misconfigured BGP route announcement by one autonomous system can redirect or blackhole traffic for millions of Internet users globally.
TTrue
FFalse
Answer: True
Because BGP glues together every AS on the Internet and route announcements propagate globally, a single AS incorrectly announcing ownership of a prefix (BGP hijacking) or advertising an overly specific route can redirect enormous volumes of traffic. A well-known example: in 2008, Pakistan Telecom accidentally advertised a more-specific route for YouTube's prefix, redirecting YouTube's global traffic through Pakistan and effectively taking YouTube offline worldwide for about two hours. BGP's trust model — routers accept advertised routes by default — makes this a persistent security vulnerability.
Question 5 Short Answer
Explain why BGP uses policy-based routing rather than shortest-path routing, and give a concrete example of how business relationships shape routing decisions.
Think about your answer, then reveal below.
Model answer: BGP connects independently operated autonomous systems with competing commercial interests — it is not a single cooperative network with a shared optimization goal. Shortest-path routing ignores whether carrying traffic is profitable or costly. The concrete mechanism is local preference: an ISP sets higher local preference for routes learned from paying customers than for routes through providers (where it pays for transit) or peers (free exchange). Even if the customer path is longer and slower, the ISP will route through it because it generates revenue. This means Internet traffic often takes economically optimal paths rather than technically optimal ones.
The three main relationship types — customer-provider, peer-peer, and transit — each generate different financial obligations that local preference encodes. 'Prefer customer > peer > provider' is the canonical BGP policy rule. The result is that the Internet's routing topology is shaped as much by business contracts as by physical infrastructure, which is why BGP misconfiguration or route hijacking can have such dramatic real-world consequences.