Tracing BGP AS path to your IP

We can trace an external ip address using trace route command. But that will only indicate the path taken from our PC to the other host which will ultimately affect your upload bandwidth.

To get a fair idea of our download bandwidth, the reverse path i.e. the path taken by packets to travel from the external host to our local host, must be considered. For this purpose we need a facility to perform reverse trace route.

This facility is provided by websites popularly known as network looking glass. The looking glass of an ISP  contains scripts which fire network commands from the ISP's routers. These commands may be executed by any external looking glass user.

Some useful looking glass URLs:

http://lg.level3.net
https://lg.airtel.in/              (Airtel)
http://lg.llnw.net/index.cgi
http://lg.softlayer.com/
http://lg.as6453.net/lg/      (TATA)
http://ubs.flagtel.com/lg     (Reliance)
http://www.as3257.net/lg/
http://lg.choopa.net/

BGP path trace and ip trace route are two of the popular facilities provided by a looking glass.

Explaining the BGP Path trace output from http://lg.level3.net,

We can select the router location of the specific ISP --- Seattle,WA.
Then we input our IP address with an optional prefix --- 117.207.216.204.

The output of this BGP table query tells that there are two possible paths from that router.
It further displays the sequence of ASNs(Autonomous System Numbers) which lie on the BGP AS-translation path.
Here, both the paths go via AS3561(Savvis) and then finally to the destination AS9829(BSNL).
The green lines signify the currently selected best active path selected on the basis of BGP algorithms.

IP trace route is same as the normal tracert command in Windows but it traces the reverse path.