Illustration 1: PGCIL user wants to access cricbuzz website to see live cricket match score. How packet traverse from source to destination to access internet Via AS By AS is shown as below
How can user access internet ??
First of all which users who wants to access internet, need to advertise particular user prefix IP block in all upstream of ISP end.
Same way this IP block need to be accepted at customer end for return traffic.
Once IP block acceptance done need to check in looking glass in trace route log end to end IP reach ability is there or not.
Illustration- 2 Kindly check reach-ability of IP block :102.220.198.0/24 towards internet. During trace-route log we observe request time out after 4th hop. How to investigate this case??
If IP block: 102.220.198.0/24 is advertised properly towards all peering ISP then it’s reflected in Looking glass how packet traverse end to end. Also, it gives assurance that IP block advertisement done properly.
Fig:2 trace route log of user IP block
If request time out occurs after 4th hop in trace route need to login in router and check route-table of 182.79.149.102 to investigate at which particular section issue occurs to reach ability towards internet for this IP block users.
Generally, BGP takes shortest-AS path to traverse packet from source to destination. Nowadays users prefix block advertisement done at multiple peering location towards upstream. Hence, if any OFC cut happens, traffic automatically switches over 2nd shortest AS-path route.
Note:
1.BGP table always reflects best possible routes entry.
- By default local preference value is 100. Local preference is used for path-manipulation of BGP and it generally used for exit path of an Autonomous system.
