Use IP and BGP to troubleshoot Internet connectivity
David Davis
Published: 24 Jun 2003 15:35 BST
If you have never seen BGP output info before, the data in Figure B may appear a bit daunting, but you just have to know what to look for. Take a look at the second line down. In this case, it says that there is one available path from this network (the BBC) to mine. That is important because there may often be multiple paths from one network to another. Then it shows the paths this router would take to get to your AS (which leads to your network). The fifth line down reads "6461 23215".
What do these numbers mean? BGP paths are listed starting from the source to the destination. So, the start (in this case, the BBC network, AS number not shown) first goes to AS 6461 and then to AS 23215, which is the AS where my IP address (the IP address I entered at the beginning) is located.
Who are the providers running these autonomous systems? Let's look it up at Arin. At the Arin Web site, enter 6461 in the Search box on the home page. You will find that the BBC network first goes through Abovenet Communications in White Plains, NY. From there it goes to AS 23215. If we, again, do a whois lookup on Arin, we see that AS 23215 is Megapath Networks, the ISP where my DSL line is located. So traffic from BBC goes to Abovenet and then to Megapath. This is a simple example, but you can use the same approach for Internet traffic from any network to any other network.
The second type of source, which you can check out if you want to go a little further and are interested in BGP, is Telnet-accessible BGP route servers on the Internet. These are real BGP routers, with full Internet routes, which you can log in to and do a limited set of BGP commands. Again, go to Traceroute.org and click on Route Servers. You'll get a list of about 20 route servers, each a Telnet link.
The best route server listed is the Oregon-ix.net Route Server, which is freely accessible to perform BGP commands. It has about 36 BGP peers, more than 800,000 BGP paths, and entries for more than 129,000 Internet networks. Using my example information on this route server, I can issue this command
show ip bgp 64.32.205.237
and get the output shown in Listing A.
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