You notice traffic destined to 172.31.17.0/24 is traversing an OSPF path rather than BGP path. What is causing this behavior?

The behavior observed—that traffic destined for the 172.31.17.0/24 network is taking the OSPF path instead of the BGP path—can be attributed to the concept of routing protocol preference or administrative distance. In the case of OSPF and BGP, OSPF typically has a lower administrative distance than BGP when both protocols have routes to the same destination.

When a router has multiple routes to a destination from different protocols, it will prefer the route with the lower administrative distance. OSPF has an administrative distance of 110, while BGP's administrative distance is 20 for external routes (EBGP) and 200 for internal routes (IBGP). If the OSPF route to 172.31.17.0/24 is being chosen, it indicates that the router is applying the normal rules of preference for routing decisions, whereby it is selecting the path with a higher priority or reliability for the outgoing packets.

This scenario underscores the importance of understanding administrative distances assigned to different routing protocols when managing routing behavior in a network.