Among the reachability problems detected by Listen, two specific types
of routing problems (as detected by active probing) include: routing loops and forwarding errors due to unknown IP
addresses. Using traceroute, we were able to detect routing loops and
we inferred forwarding errors using the routing table entries at the
University exit router. A forwarding error arises when the
destination IP address in a packet is a genuine one but the router has
no next hop forwarding entry for the IP address. This can potentially
arise due to staleness of routes. Table 4 summarizes
the number of prefixes which are affected by each type of problem. In
particular, we observe routing loops to 
different prefixes and
forwarding errors to 
different prefixes. Additionally, Listen
detected 
other prefixes having other forms of reachability
problems.
To cite a few examples of reachability problems we observed: (a) A BGP daemon within our network attempted to connect to another such daemon within the destination prefix 193.148.15.0/24. The route to this prefix was perennially unreachable due to a routing loop. (b) The route to Yahoo-NET prefix 207.126.224.0/20 was fluctuating. During many periods, the route was detected as unavailable.