BGP Path Attributes Types

Sassenach Kenzie — Wed, 01 Nov 2017 18:25:00 +0000

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 passed to other peers. There are some that can be silently discarded and ignored.

BGP attributes are classified into 4 types:

Well-known Mandatory
Well-known Discretionary
Optional Transitive
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 passed to other peers. Optional Transitive attributes are Aggregator and Community. Optional Non-Transitive attributes are Originator ID and Cluster List.

Type Code Value	Attribute	Attribute Type
1	Origin	Well-Known Mandatory
2	AS_Path	Well-Known Mandatory
3	Next_Hop	Well-Known Mandatory
4	Multi-Exit Discriminator (MED)	Optional Non-Transitive
5	Local_Preference	Well-Known Discretionary
6	Atomic_Aggregate	Well-Known Discretionary
7	Aggregator	Optional Transitive
8	Community	Optional Transitive
9	Originator_ID	Optional Non-Transitive
10	Cluster_List	Optional Non-Transitive

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

Next Hop – Sass Learns

BGP Path Attributes Types