BGP attributes

BGP attributes is a metric used to describe the

characteristics of a BGP paths. Attributes are

conatained 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.

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

Lets start from first... I m gonna explain each attributes

one by one  

Well Known: 

           Well known attributes are must be recognized by

each compliant of BGP implementations. Well known attributes

are propagated to other neighbors also.

Optional:

         Optional attributes are recognized by some

implementation of BGP & expected that not recognized by

everyone. Optional attributes are propagated to their

neighbors based on the meanings.

Mandatory:

          Mandatory is BGP well known attributes. Mandatory

attributes are must be present in all update message passed

between BGP peers. It is present in route description.

Discretionary:

              Discretionary is BGP well known attributes.

Discretionary attributes are may be present on update message.

AS Path: 

        AS path is BGP well known mandatory attributes. AS

path defines the list of Autonomous System that a route has

passed through. AS path attributes are used in path

selection process & lowest path attributes will be preferred

over highest one.

Next-Hop:

         Next hop is BGP well known mandatory attributes. In

BGP, next hop is the IP address which is used to reach the

advertising BGP router. 

For EBGP, next hop is the ip address of the connection

between the peers. 

For IBGP, EBGP next hop address is carried into local

autonomous system.

Origin:

       Origin is BGP well known mandatory attributes.

Origion attributes define how BGP router learned about the

particular route. Origin attribute have three possible

values i.e. internal (i), External (e) & Incomplete (?).

Internal(i):

            BGP learned about the route from Interior

routing protocol.

External(e):

             BGP learned about the router from External BGP.

Incomplete(?):

            The origin of the route is unknown or learned by

some other way. The origin of incomplete occurs when routes

are redistributed into BGP.