Router RI and Router R2 run BGP. Both routers are in the AS65234 The route for router R2 exists in the BGP routing table of the router RI, but not in the IP routing table of the router RI . So what causes the problem to occur? (Single Choice Questions).

For a BGP route to be installed into the IP routing table (global RIB), it must first be present in the BGP routing table (Loc-RIB) and then be selected as the best path. A critical condition for a route to be considered valid and eligible for best path selection is that its NEXTHOP attribute must be reachable (i.e., there must be a route to the next-hop address in the IP routing table). If R1 has the route for R2 in its BGP table but not its IP routing table, the most common reason is that the next-hop address associated with that BGP route is not resolvable by R1. This makes the route invalid and thus not optimal; it cannot be selected as the best path and will not be installed in the IP routing table. Another possibility is that a route to the same destination exists from another routing protocol with a better (lower) administrative distance. In both cases, the BGP route is not considered the optimal route to install.

A. Synchronization is turned off: Turning synchronization off (the modern default) allows a router to advertise an iBGP route without it being in the IGP. This setting does not prevent a valid route from being installed in the local IP routing table.
B. The BGP neighbor relationship is Down: If the neighbor relationship were down, R1 would not have received the route from R2, and it would not be in the BGP routing table at all.
C. Router R1 does not enable BGP Multi-hop: This option is for eBGP neighbors that are not directly connected. The question specifies that both routers are in the same AS (AS65234), making them iBGP peers. iBGP peers do not need to be directly connected by default, so this setting is irrelevant.

1. Huawei. (2022). CloudEngine S12700E V200R021C00 Configuration Guide - IP Routing. BGP Configuration > BGP Fundamentals > BGP Route Selection. This guide explains that for a BGP route to be considered optimal, its next hop must be reachable. If the next hop is unreachable, the route is considered unavailable and will not be installed in the IP routing table.

2. Huawei. (2021). NE40E V800R012C10 Configuration Guide - IP Routing. BGP Configuration > BGP Principles > BGP Route Advertisement Rules. This document details the conditions under which BGP routes are selected and advertised, highlighting the prerequisite of a reachable next hop for a route to be considered valid.