BGP has many attributes in choosing the best path. However, not all attributes need to be present in every BGP update and need to be recognized or pass to other peers. There are some that can be silently discarded and ignored.

BGP attributes are classified into 4 types:

  1. Well-known Mandatory
  2. Well-known Discretionary
  3. Optional Transitive
  4. Optional Non-Transitive

Well-known Mandatory attributes, as the name suggests, it is a must in every BGP update to have it included. It should exist in every BGP update and must be recognized by all BGP speakers. Well-known mandatory attributes are AS Path, Next Hop address, and Origin.

Well-known Discretionary must be recognized by all BGP speakers but not necessarily included in every BGP update. It includes Local Preference and Atomic Aggregate.

Optional Transitive and Optional Non-Transitive are attributes that don’t need to be understood and supported by all BGP speakers. The main difference between these two is that if it is set as transitive, it means it has to advertise to all peers and if is set as non-transitive then it can be ignored and not pass to other peers. Optional Transitive attributes are Aggregator and Community. Optional Non-Transitive attributes are Originator ID and Cluster List.

Type Code ValueAttributeAttribute Type
1OriginWell-Known Mandatory
2AS_PathWell-Known Mandatory
3Next_HopWell-Known Mandatory
4Multi-Exit Discriminator (MED) Optional Non-Transitive
5Local_PreferenceWell-Known Discretionary
6Atomic_AggregateWell-Known Discretionary
7AggregatorOptional Transitive
8CommunityOptional Transitive
9Originator_IDOptional Non-Transitive
10Cluster_ListOptional Non-Transitive

BGP path attributes types doesn’t seem to be of importance for now. However, as we go further our BGP studies, we will realize the essence of it.

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