In Border Gateway Protocol BGP path selection, when a router receives an advertised network prefix and must decide whether that route is actually usable, what is the very first condition that BGP checks to determine whether the prefix is reachable?

Introduction / Context:
This question examines your understanding of the initial reachability check performed by Border Gateway Protocol when it considers routes learned from BGP peers. Before a BGP router evaluates attributes such as AS path length, local preference, or origin type, it must decide whether the advertised path is even usable from the perspective of the local routing table. That first reachability check focuses on the next hop address for the BGP route.

Given Data / Assumptions:
- We are dealing with BGP routes learned from one or more neighbors.
- Each BGP path has attributes including next hop, AS path, local preference, and possible MED values.
- The question specifically asks for the first check BGP performs to determine if the prefix is accessible.
- We assume that the BGP session itself is already established and exchanging updates successfully.

Concept / Approach:
BGP is a path vector protocol and relies on the underlying Interior Gateway Protocol or static routes to reach the next hop of each learned route. The next hop attribute identifies the IP address to which packets are forwarded in order to reach the advertised prefix. If the local router has no route to that next hop address in its own routing table, the BGP route is considered unusable and is not installed, regardless of how attractive other attributes might be. Only after confirming reachability of the next hop does BGP begin the normal best path selection process.

Step-by-Step Solution:
Step 1: Recall that every BGP route includes a next hop attribute that points to a gateway router along the path.Step 2: Understand that BGP does not perform data plane forwarding by itself and instead forwards packets using routes already present in the local routing table from IGPs or static configuration.Step 3: Therefore, BGP must verify that the next hop address for a candidate route is itself reachable via some non BGP entry in the local table.Step 4: If there is no valid route to the next hop, the BGP route is marked as unusable and is not considered when computing the best path.Step 5: Only when next hop reachability is confirmed does BGP compare attributes such as local preference, AS path length, origin code, and MED to select the best route.

Verification / Alternative check:
A practical way to see this behavior is to think about a common lab where you forget to redistribute a static route to a BGP next hop. The BGP table may show several prefixes learned from a neighbor, but the router code will not install those prefixes into the main routing table until it has a way to reach the next hop. Running a show ip route command and noticing that the next hop address is missing explains why the BGP route is inactive.

Why Other Options Are Wrong:
The shortest AS path length is important but is evaluated only after reachability is confirmed. The highest local preference value is also a later selection step, not the first check. Whether the route is internal BGP or external BGP again affects tie breaking at later stages, not basic reachability. The lowest MED value is used when comparing multiple routes from the same neighboring AS, but MED is irrelevant if the next hop itself cannot be reached.

Common Pitfalls:
Many learners jump directly to metrics such as AS path or local preference and forget that a route which cannot reach its next hop is useless no matter how attractive the attributes look. Another pitfall is confusing the BGP next hop attribute with the neighbor IP address, which may be different especially in multihop or route reflector designs. Always remember that BGP relies on the underlying routing table to reach the next hop and that this check is the gateway to any further best path processing.

Final Answer:
BGP first checks whether the next hop address for the advertised prefix is reachable in the local IP routing table.