BGP – PATH ATTRIBUTES

A Path Attribute is a characteristics of an advertised BGP route.

Four Categories of PATH Attributes.

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

Well-Known Attributes – These are those BGP Path attributes which should be recognized by all BGP routers.

  • Well-Known Mandatory – Are those Well-Known attributes that must be included in all BGP Update messages.
  1. ORIGIN
  2. AS_PATH
  3. NEXT_HOP
  • Well-Known Discretionary – Are those Well-Known attribute, that may or may not include in a specific Update Messages.
  1.  LOCAL_PREF
  2. ATOMIC_AGGREGATE

Optional Attributes – There are those BGP attributes which may or not recognized or supported by a BGP router.

  • Optional Transitive – Are those optional attributes in which a BGP process should accept the PATH in which it is included and it should pass the Path in to its peers.
  1. AGGREGATOR
  2. COMMUNITY
  • Optional Non-Transitive – Are those optional attributes in which if a BGP process doesn’t recognize a attribute, it can just ignore the Update message and not need to pass to its peers.
  1. MULTI_EXIT_DISC(MED)
  2. ORGINATOR_ID
  3. CLUSTER_LIST

 

1. ORIGIN

Specifies the origin of the route. When BGP have multiple route , it uses ORIGIN as one factor to select the preferred route.

 * IGP     * EGP    * Incomplete

2. AS_PATH

It contains a list of ASs, through which the PATH will traverse to reach the destination. New AS number  prepend to the existing list.

3. NEXT_HOP

It contains the IP address of the Next-Hop router in the path specified.

Selecting Next-Hop :-

a. If the advertising peers are in different AS, NH = IP of the advertising router

b. If the advertising peers are in same AS and adv route is in same its AS, NH = IP address of the Neigbor that advertised.

c. If the advertising peers are in same AS, but adv route is in different AS, NH = IP address of the external Peer from which route is learned.

4. LOCAL_PREF

  • Used to communicate a BGP router’s degree of preference for an advertised route.
  • If an iBGP router receives multiple PATH to a same destionation, decision taking is done using this attribute.
  • Route with higest LOCAL_PREF is selected

5, MULTI_EXIT_DISC (MED)

  • When LOCAL_PREF can influence only traffic leaving the AS, MED can be used to influence the traffic that comes into AS.
  • One AS says to another AS, that which is its preferred ingress point.

 6. ATOMIC_AGGREGATE and AGGREGATOR

Atomic Aggregate attribute is used by the BGP speaking router when it advertise a aggregated route to its peer to mention that it is a less-specific route. The peers should keep this attribute when sending to its own peers.

Aggregator Attribute is used optionally to mention which the is origin router which aggregation happened and the AS which that router is residing.

 7. COMMUNITY

Simplifies policy enforcement. If a set of destinations have same properties and policies, we can make it as one Community and can be represented using COMMUNITY attribute.

COMMUNITY Attribute format :  AA:NN (Where AA – AS Number, NN – Community Identifier).

Well-Known Communities

INTERNET –  All routes advertised without any specific community by default belongs to INTERNET. It can be advertised freely without any restrictions.

NO_EXPORT – Routes received using this attributes cannot be advertised to eBGP peers.

NO_ADVERTISE – Cannot be advertised both eBGP and iBGP peers.

LOCAL_AS – Cannot advertise to eBGP peers including peers in other Autonomous System within a Confederation.

8. ORIGINATOR_ID

ORIGINATOR_ID attribute contains the RID of the originator of that specific route in the Local AS.

9. CLUSTER_LIST

Contains a sequence of Router Reflection cluster IDs through which the route have passed.

 

 

 

 

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