The global R&E network consists of roughly 2,700 separate networks (autonomous systems). A significant percentage of these networks have connections to both an upstream R&E provider and one or more commodity ISPs. Each network independently makes routing decisions about which path to prefer when reaching various destinations.
Campuses typically connect to a regional R&E network (e.g., MERIT, LEARN, CENIC, NYSERNet), which in turn connects to Internet2. The R&E route between two campuses is often at least 4 AS hops. If both campuses use the same commodity ISP—which happens frequently—the commodity route might only be 2 hops. Without an explicit routing policy, BGP selects the shortest AS path, and the two campuses will route via the commodity ISP instead of the R&E network.
The recommended best practice is to use BGP Local Preference to prefer routes learned from R&E networks over routes learned from commodity ISPs. When this is done, campuses route via the R&E path regardless of AS-path length.
The Local Preference Probe (LPP) is a server connected to both the Internet2 R&E network and a commodity ISP. It sends probes from two source addresses to hosts across the 2,700 R&E networks and observes how each network routes its response back—does it arrive via the R&E path or the commodity path?
| Source | Address | Route design |
|---|---|---|
| Source A | 163.253.64.1 |
R&E route is prepended towards commodity |
| Source B | 163.253.63.63 |
R&E route is prepended towards R&E |
Both sources share the same commodity route. The difference is which direction the R&E route is prepended, so the two probes test whether the remote network chooses based on Local Preference or AS-path length.
| Color | Source A | Source B | Meaning |
|---|---|---|---|
| RE | R&E | R&E | The network is following the best practice of preferring R&E routes over commodity. BGP Local Preference favors R&E. |
| Path | R&E | Commodity | The network is using AS-path length to choose, so some destinations use R&E while others route via commodity. R&E and commodity have equal Local Preference. |
| Comm | Commodity | Commodity | The network appears to always prefer commodity, even when an R&E route exists. |
| Inconclusive | Missing / unexpected | Excluded from percentages. | |
Orange and red networks are connected to the global R&E network infrastructure, but are not effectively using it.
The stacked bar for each AS represents the ratio of probe response classifications across its entire customer cone—the AS itself plus every AS downstream of it in the R&E tree.
Multiple IP addresses are probed within each prefix, not just one per prefix. The percentages reflect the classification of every individual probe response across all prefixes in the cone. For example, if an AS and its downstream networks have 1,000 total probed IPs and 800 are classified as RE, 150 as Path, and 50 as Comm, the bar shows 80% / 15% / 5%. Inconclusive responses are excluded from both the count and the total.
A leaf AS shows only its own probe results. A transit AS near the root aggregates results across hundreds of downstream networks, giving a high-level view of routing policy in that part of the R&E topology. Expanding an AS’s prefixes shows the individual probed IP addresses within each prefix and their classifications.
An AS can appear as a child of multiple parents in the tree. This happens when a network is multi-homed to more than one R&E upstream—for example, a campus connected to both a regional network (like MERIT or CENIC) and directly to Internet2, or reachable through multiple regional peers. RouteViews sees AS paths through each upstream, creating multiple parent–child edges for the same AS.
Multi-parent ASes are identified by a badge next to the AS number (e.g., “3 parents”). Clicking the badge opens a per-parent detail panel showing:
The header displays a count of how many ASes in the tree have multiple parents.
The Highlight multi-homed button adds a blue left border to every row where the AS has more than one parent, making them easy to spot while scrolling.
The probe targets responsive IP addresses discovered by the USC/ISI Internet address survey. These addresses may include router interfaces, management planes, or other infrastructure devices whose return path may differ from the network’s general routing policy. For example, a router interface on a transit link may respond over that transit link regardless of the BGP Local Preference configured for the campus’s production traffic. As a result, individual probe results should be interpreted with caution—the aggregate percentages across many probed IPs within a prefix provide a more reliable picture than any single probe.
RouteViews (BGP paths & origins), RIPE Stat (AS names), CAIDA (fallback names), lpp-store (probe results). Report rebuilds daily at UTC midnight.
Part of the CICI-ROOTBEER project.
This material is based on research sponsored by the National Science Foundation (NSF) grant OAC-2530871. The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of NSF.