What is BGP attribute & How many types of attributes are used by BGP?

Question

It is generally said that BGP protocol belongs to Application Layer of OSI model but how is this possible ?

It is made use of by border routers which belongs to level-3 then will that mean border routers are special kind of routers which act at application layer?

How do they differ from general routers?

Answer 1

BGP attributes is a metric used to describe the characteristics of a BGP paths. Attributes are contained in
update messages passed between BGP peers for advertise routers.

BGP attributes are broadly divided into two types:

a) Well Known
b) Optional.

Well Known attributes are divided into two types -

a) Mandatory
b) Descretionary

Mandatory attributes are divided into three types -

a) AS Path
b) Next Hop
c) Origin

Origin attributes divided into three types -

a) Internal(i)
b) External(e)
c) Incomplete(?)

Descretionary attributes are divided into two types -

a) Local Preference
b) Atomic Aggregate

Optional attributes are divided into two types -

a) Transitive
b) Non Transitive

Transitive attributes are divided into two types -

a) Aggregator
b) Community

Community attributes are divided into for types -

a) No-export
b) No advertise
c) Internet
d) Local AS.

Non transitive attributes are divided into three types -

a) MED (Multi Exit Discriminator).
b) Originator
d) Cluster ID.