BGP Path Attributes

ORIGIN:
specifies the origin of the routing update.

The Origin attribute can contain one of these three values
1. IGP
2. EGP
3. Incomplete

If BGP has multiple routes then ORIGIN is one of the factor in determining the preferred route. IGP is the highest preferred ORIGIN value followed by EGP and Incomplete ORIGIN Attribute is the lowest preferred ORIGIN value of the three.

NLRI: Network Layer Reachability Information (NLRI) is exchanged between BGProuters using UPDATE messages. An NLRI is composed of a LENGTH and a PREFIX.

Incomplete: Usually misunderstood, this value means that the NLRI was learnt from some other means, it does not mean that the route is faulty, it only specifies that the information to determine the ORIGIN is not complete. All redistributed routes into BGP have an Incomplete ORIGIN attribute, since the origin of these routes cannot be determined.  

 

AS_PATH ATTRIBUTE:

AS_Path describes the inter-AS path taken to reach a destination. It gives a list of AS Numbers traversed when reaching to a destination. 

 Every BGP speaker when advertising a route to a peer will include its own AS number in the NLRI. The subsequent BGP speakers who advertise this route will add their own AS number to the AS_Path, the subsequent AS numbers get prepended to the list. The end result is the AS_Path attribute is able to describe all the autonomous systems it has traversed, beginning with the most recent AS and ending with the originating AS.

 NEXT_HOP ATTRIBUTE:

NEXT_HOP Attribute specifies the next hop IP address to reach the destination advertised in the NLRI. If the BGP Peers are in different Autonomous Systems then the NEXT_HOP IP address that will be sent in the update message will be the IP address of the advertising router.

 Local Preference (LOCAL_PREF):

LOCAL_PREF specifies the BGP Speaker’s degree of preference for an advertised route.
The higher the value of Local Preference attribute the more preferred the route is.

ATOMIC_AGGREGATE and AGGREGATOR:

 

ATOMIC_AGGREGATOR path attribute does  route aggregation on the routes that are non identical but point to the same destination. In effect if summarizes the routes when advertising them to the BGP peer.

MULTI_EXIT_DISC ATTRIBUTE: (MED)

MED is an optional non-transitive attribute and it is used to influence how the incoming traffic comes into an AS.

 

In this example, AS-100 advertises the route from two different entry points with the MED value of 50 0n one router and a value of 100 on another. The Preferred entry point will be the router which is advertising the routes with the MED value of 50, since this is the lower value. Also notice that AS-200 will not advertise the AS-100 MED values to AS-300 in its outgoing route advertisements, since MED is an optional non-transitive attribute.