1.

Explain the Path selection criteria used in BGP.

Answer»

BGP seeks to reduce the number of paths available to only one best path; it does not load balance by DEFAULT. To do so, it looks at the following path properties of any loop-free, synchronised (if synchronisation is enabled) ROUTES with an accessible next-hop:

  • PICK the route that has the most weight.
  • Choose the route with the highest local preference if weight is not specified.
  • Choose routes that began with this ROUTER.
  • Choose the Autonomous System path that is the shortest.
  • Select the path with the lowest origin code (lowest is I next is e, and last is?).
  • If the same Autonomous System advertises the available routes, choose the path with the lowest MED.
  • Opt for an EBGP route rather than an IBGP route.
  • Choose the route that passes through the IGP neighbour with the lowest IGP metric.
  • Pick the oldest route.
  • Choose the path that passes through the neighbour with the smallest router ID.
  • Select the path that passes through the neighbour with the smallest IP ADDRESS.


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