Understanding reachability gaps due to default routes
Some Research & Education (R&E) networks take the full R&E routing table from a peer such as Internet2, but point a default route at their commodity provider instead of carrying the full commodity table. When a destination prefix is a more‑specific of a covering route that already exists in the R&E table, those “default‑route” networks follow the covering route and never learn the more‑specific.
If the network announcing the covering less‑specific does not itself carry traffic onward to the more‑specific, the result is a reachability gap: networks with the full commodity table reach the prefix; networks following a default route to R&E do not.
What each Sankey shows
For each gap prefix, CAIDA Ark vantage points (VPs) inside R&E networks traceroute toward a responsive address in that prefix. Each VP is labelled full_table (carries the full table) or default_route (defaults to commodity). Nodes are ASes; each ribbon is a single traceroute path, its width the number of traces sharing that exact path.
How to read it
- VP source ASes on the left, ordered by responsiveness — the most responsive on top, default‑via‑R&E on the bottom. A ▶ cap marks where each traceroute is born; ribbons without a cap are passing through.
- One ◎ target on the right. Every trace aims at the same probed prefix. Traces that arrive plug into the green target bar; traces that fail fade out short of it, ending in a red ✗ with a dotted line marking the leg that never happened.
- Node colours: green = the prefix origin and the ◎ target; amber = covering‑route ASes that create the gap; slate = VP / transit.
A reachability gap happens when a more‑specific prefix lives only in the commodity table and the network announcing the covering R&E less‑specific does not itself carry traffic onward to it. R&E networks that follow a default route to commodity then can’t reach the more‑specific, while networks carrying the full table can.
Each Sankey overlays R&E vantage‑point traceroutes: full_table VPs carry the full table and reach the prefix; default_route VPs follow the default route and largely die short of the target, marked ✗.
Click a ribbon to highlight that traceroute end-to-end; click it again to list the matching hop-by-hop traces. Click empty space to clear.