building a `core' AS graph from two data types deriving topological combinatorial core recursively strip outdeg.0 and 2-loops bidirectional connectivity remains should be 100% if sufficient coverage compare using (1) BGP and (2) forward IP topology data (1) BGP routing data AS graph (RouteViews tables) 2.77% in combinatorial core (327 out of 11823) accumulation does not change much adding links from less to more specifics: adds 4400 links to 25K increases combin.core from 300 to 900 AS (2) probed forward topology AS graph, 1 Aug 2001: 9858 nodes out of 11.5K 2556 (26%) in comb.core 2102 (21%) in g.c. which reaches 9835 caveat: traceroute links with third party addresses multipath noise (may be 1% cases, exact number unknown)