BGP Path Attributes

BGP attributes are divided into :

1) Well-known mandatory : it must be recognized by all BGP implementation. Everyone supports them.
                                           Mandatory attributes must be included in all BGP Update message

• ORIGIN
• AS-PATH
• NEXT_HOP

2) Well-known discretionay : may or may not be sent int a specific Update required to support the attribute

• LOCAL_PREF
• ATOMIC_AGGREGATE

3) Optional transitive : BGP process should accept the path in which transitive att is included, even if it
                                    doesn't support and it should pass the path on to its peers

• AGGREGATOR
• COMMUNITY

4) Optional nontransitive : BGP process that doesn't recognize the attribute can ignore Update and not
                                          advertise the path to its other peer

• MULTI_EXIT_DISC (MED)
• ORIGINATOR_ID
• CLUSTER_LIST

 

Routes are characterized by a set of attributes:

• Next Hop (NH) - external AS peer IP address. Changed only when advertised over EBGP. Passive IGP interface, next-hop-self or static routes recommended for IBGP operation.

• Local Preference - By default 100, higher is better. Used within AS.

• AS Path - List of ASs the route goes through. New ones are prepended. Confederations are in {brackets} and count as one segment. => LINK

• Origin - source of the route injected into BGP - IGP/EGP/Unknown. => LINK

• Multiple Exit Discriminator (MED) - suggests path into the AS. Taken into suggestions only for prefixes learned from multiple routers in the same AS. 0 by default, lower is better.

• Atomic Aggregate - indicator that a less specific route whas advertised. Never used by Junos.

• Aggregator - RID and AS of the router advertising the aggregate route into BGP.

• Community - administrative tag for bundling routes together <AS:number>. Well known communities are:-> LINK

• No-export - advertise to neighboring AS, but no further.

• No-advertise - send the route to neighboring router, but don't advertise inside AS.

• No-export-subconfed - advertise to neighboring subconf, but no further.

• Originator ID - RID of a router advertising the route to the first Route Reflector => LINK

• Cluster List - like AS path but for Route Reflector Clusters => LINK

Only 3 attributes have to be attached to all routes:
Next Hop
AS Path
Origin

They are all well-known (included in the original BGP specification) and mandatory (attached to all routes).

Other well-known (but not required to be attached) attributes are:
Local Preference
Atomic Aggregate

Attributes besides those 5 are optional and don't have to be understood by any BGP router. 

 source : “Internetworking Technology Overview”, “Routing TCP/IP Vol2”, cisco.com