Thursday 7 May 2020

BGP CHAPTER 14 - Understanding BGP Attributes


In all our previous post we have seen Concept and Configuration of BGP neighborship , Advertisement of networks in BGP , BGP Synchronization rule , Auto Summary etc.

From this post onwards we will start focusing on BGP Attributes and understand what exactly role does BGP Attributes play in BGP .

As we are aware that each routing protocol uses some metric for path selection 
e.g. RIP uses Hop count , EIGPR uses a composite metric which includes Bandwidth and Delay (by default) , OSPF uses Cost i.e Bandwith to calculate the best route. 

Similarly BGP also uses something known as "Attributes" to the find the best route to reach a destination.

It is very important to have good understanding of BGP Attributes before understanding BGP Path Selection Process.



There are 4 categories of BGP Attributes :-

1) Well Known Mandatory -  
  • Supported (Known/Understand) by all BGP Routers
  • Must be present in BGP Update Message.
  • AS Path, Origin ,Next Hop.

2) Well Known Discretionary - 
  • Supported (Known/Understand) by all BGP Routers.
  • May or May not be present in BGP Update Message.
  • Local Preference

3) Optional Transitive -
  • May or May not be supported  a BGP Routers.
  • Passed on the other BGP Routers.
  • Aggregator , Community.
4) Optional Non-Transitive-
  • May or May not be supported by a BGP Routers.
  • If not supported may ignore the update and may not pass on to the other BGP Routers.
  • MED ,Router Cluster list , Router Originator List.

Hope you all like this post !!!

Thanks for Reading 

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