BGP Best Path Selection: Origin


For the last few days I have been studying for my CCIE R&S lab and have been doing every practice lab I can get my hands on. The one thing they all seem to have in common is that controlling the routes that the traffic is taking is key. Each of them has requirements outlined as to which route traffic needs to take by default and you have to configure the network accordingly. One thing to keep in mind is the default way BGP routes are handled. When looking at BGP best path selection, origin is used in a way where the lowest is preferred:

IGP   ->   EGP   -> Incomplete

That is the order that routes are preferred assuming that everything else about the routes is the same. If you are in a situation where you have a case where you need to modify the origin, you can use a standard route-map to accomplish this:

 R1(config-route-map)#route-map INCOMING permit 10
 R1(config-route-map)#match ip address 100
 R1(config-route-map)#set origin ?
  igp         local IGP
  incomplete  unknown heritage

From there you can modify it as needed to change it to an either higher or lower priority by using either IGP or incomplete. Short, sweet, and to the point with this one. Just one more tool to have in your back pocket while designating the best path for traffic  through a network.


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