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I am trying to understand BGP and looking glass server concepts. The IP address of stackexchange is 198.252.206.140. Now, I use the looking glass server from here.

  • I selected BGP network and clicked on submit button.
  • I give the starting location of the looking glass server as Australia and try to trace the traffic till stackexchange. So, I give the IP address as 198.252.206.140.

Now, this is the output I get.

BGP routing table entry for 198.252.206.0/24, version 315807603
Paths: (4 available, best #3, table default)
  Advertised to update-groups:
     15     19    
  4637 174 25791, (aggregated by 25791 198.252.206.3)
    134.159.98.29 (metric 693) from 195.206.69.80 (195.206.69.80)
      Origin IGP, metric 0, localpref 90, valid, confed-internal, atomic-aggregate
      Community: 3300:4 3300:1804 3300:1999 3300:3070 3300:3071
  4637 174 25791, (aggregated by 25791 198.252.206.3)
    134.159.208.153 (metric 693) from 195.206.69.81 (195.206.69.81)
      Origin IGP, metric 0, localpref 90, valid, confed-internal, atomic-aggregate
      Community: 3300:4 3300:1804 3300:1999 3300:3070 3300:3071
  4637 174 25791, (aggregated by 25791 198.252.206.3)
    134.159.188.77 from 134.159.188.77 (202.84.219.195)
      Origin IGP, localpref 90, valid, external, atomic-aggregate, best
      Community: 3300:4 3300:1804 3300:1999 3300:3070 3300:3071
  4637 174 25791, (aggregated by 25791 198.252.206.3), (received-only)
    134.159.188.77 from 134.159.188.77 (202.84.219.195)
      Origin IGP, localpref 100, valid, external, atomic-aggregate  

As I understand currently, the numbers 4637, 174 and 25791 are the as numbers of the route. Also, as per the output, I get 4 paths and the best path is #3. So what is that I have to look in this output to see why #3 is the best path or route? Or in other words, how to decipher this output from the looking glass server?

I went through this question which helped me understand the concepts. But am still looking for a way to decipher the output from the looking glass server.

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The 4637, 174 and 25791 are numbers of Autonomous Systems that packet must pass in order to achieve a target 198.252.206.140.

BGP uses several attributes to decide the best path. In fact there are 10 main attributes which are browses sequentially. That's called path selection.

BGP Path selection

When all attributes for different paths are the same. As in your case. The path with lowest Router ID (tenth attribute) will be selected as best. That's the reason why path #3 is the best. [I overlooked the 'IGP cost' the eighth attribute, please see it in below comments.]

In case that some path to destination network will have just one more preferred attribute and other attributes will be same (for example highest 'local preference' than other path has). The router will choose that path as the best. But be aware other attributes of the above 'local preference' will have to be same for each path. In our example each router will has to have the same weight.

By the way... You may ask why the the fourth path form your routing table entry is not the best because of his highest local preference (100 others has 90 localpref). But the Fourth path has in parentheses 'received-only', that means that the route has only been received, but not entered in the routing table (inbound soft reconfiguration).

I hope that's what you wanted to explain.

Daniel

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  • Thanks for the answer. How do I know that the router ID is used for path selection in this example?
    – Ramesh
    Dec 15 '14 at 17:59
  • Ouch. Forget to 'Router ID'. I overlooked something. The metric value 693 in parentheses for first two paths. This metric is the reason why was chosen the third path in you example. It' s the eighth attribute in path selection and that value means IGP metric. Be aware, all attributes of the above 'IGP Cost' are same. That's the reason why he came to attribute 8 (IGP COST). It selects sequentially. Dec 15 '14 at 18:24

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