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===AS PATH inference===
This technique relies on various [[Border Gateway Protocol|BGP]] collectors who collect routing updates and tables and provide this information publicly. Each BGP entry contains a [[Path Vector]] attribute called the AS Path. This path represents an [[autonomous system (Internet)|autonomous system]] forwarding path from a given origin for a given set of [[prefixes]]. These paths can be used to infer AS-level connectivity and in turn be used to build AS topology graphs. However, these paths do not necessarily reflect how data is actually forwarded and adjacencies between AS nodes only represent a policy relationship between them. A single AS link can in reality be several router links. It is also much harder to infer peerings between two AS nodes as these peering relationships are only propagated to an ISP's customer networks. Nevertheless, support for this type of mapping is increasing as more and more ISP's offer to peer with public route collectors such as [[Route-Views]] and [[Réseaux IP Européens|RIPE]]. New toolsets are emerging such as Cyclops and [[NetViews]] that take advantage of a new experimental BGP collector [[BGPMon]]. NetViews can not only build topology maps in seconds but visualize topology changes moments after occurring at the actual router. Hence, routing dynamics can be visualized in real time.
In comparison to what the tools using BGPMon does there is another tool [[netTransformer]] able to discover and generate BGP peering maps either through SNMP polling or by converting MRT dumps<ref>https://tools.ietf.org/html/rfc6396</ref> to a [[graphml]] file format. netTransformer allows us also to perform network diffs between any two dumps and thus to reason how does the BGP peering has evolved through the years.<ref>{{cite web|url=http://www.slideshare.net/itransformersCrew/tracking-network-evolution-process-with-nettransformer-bulgarian-internet-bgp-peering-evolution-from-2001-till-now-network-evolution|title=Tracking network evolution process with netTransformer & Bulgarian In…|date=2 November 2014|publisher=|accessdate=30 August 2016}}</ref> [[WhatsUp Gold]],
{{Image frame|width=800|content=[[Image:NetTransformer Internet BGP map.jpg|800px]]
|caption=Internet BGP peering map (red - multi homed AS, green stubs) |link=BGP peering|align=center}}
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