An edge router R1 learns 10.10.10.0/24 from an ISP via eBGP and advertises it to an internal iBGP neighbor R2. R2 installs the route but cannot reach the route's next-hop address. What explains the problem?

iBGP by default preserves the next-hop attribute learned from an eBGP peer. This means R1 advertised the route 10.10.10.0/24 to R2 with the original next-hop (the ISP router's IP address) instead of its own IP address. Since R2 is an internal router, it likely doesn't have a direct route to the ISP router's IP, making the next-hop unreachable. R1 should be configured to set next-hop-self for its iBGP advertisements to R2. R2 enabling next-hop-self on its session to R1 would only affect routes R2 advertises, not routes it receives. BGP does not automatically rewrite the next-hop for iBGP advertisements; this behavior is specific to eBGP. The eBGP TTL expiring would prevent R1 from learning the route in the first place, not cause an unreachable next-hop for an already learned route.