BGP PATH Attributes list

Path Attributes are the characteristics of an advertised BGP Route. BGP routing policy is set and communicated using the path attributes.

Path Attributes fall into one of the two categories

1. Well-known Path Attributes 2. Optional Path Attributes

Well-known: Meaning these attributes must be recognized by all the BGP implementations.

Well- Known BGP Path Attributes fall into two sub-categories known as

1. Mandatory (Called as Well-known Mandatory) 2. Discretionary (Called as Well-Known Discretionary)

Mandatory: This means the attribute must be always included and carried in all BGP update messages to peers. The BGP implementation has to recognize the attribute, accept it and also advertise it to its peers.

Discretionary: Meaning these are recognized by the BGP implementation but may or may not be sent in a specific Update message. Its up to the discretion of BGP Implementation to send or not to send these attributes in the update messages to the peers.

Optional: Meaning these attributes may or may not be supported by the BGP implementations.

Optional BGP Path attributes also fall into two sub-categories

1. Transitive (Called as Optional Transitive) 2. Non-transitive (Called as Optional Non-transitive)

Transitive: BGP process has to accept the path in which it is included and should pass it on to other peers even if these attributes are not supported. Meaning if any optional attribute is not recognized by a BGP implementation, then BGP looks to check if the transitive flag is set. If the transitive flag is set then BGP implementation should accept the attribute and advertise it to its other BGP Peers.

Non-transitive: If the BGP process does not recognize the attribute then it can ignore the update and not advertise the path to its peers. If the transitive flag is not set then BGP implementation can quietly ignore the attribute, it does not have to accept and advertise this attribute to its other peers.

The following chart lists all of those BGP Path Attributes.