BGP Attributes Categories

A quick copy-and-paste summary on BGP attribute categorization.

WELL-KNOWN, MANDATORY


AS-path: A list of the Autonomous Systems (AS) numbers that a route passes through to reach the destination. As the update passes through an AS the AS number is inserted at the beginning of the list. The AS-path attribute has a reverse-order list of AS passed through to get to the destination.

Next-hop: The next-hop address that is used to reach the destination.

Origin: Indicates how BGP learned a particular route. There are three possible types -- IGP (route is internal to the AS), EGP (learned via EBGP), or Incomplete (origin unknown or learned in a different way).


WELL-KNOWN, DISCRETIONARY

Local Preference: Defines the preferred exit point from the local AS for a specific route.

Atomic Aggregate: Set if a router advertises an aggregate causes path attribute information to be lost.


OPTIONAL, TRANSITIVE

Aggregator: Specifies the router ID and AS of the router that originated an aggregate prefix. Used in conjunction with the atomic aggregate attribute.

Community: Used to group routes that share common properties so that policies can be applied at the group level.


OPTIONAL, NON-TRANSITIVE

Multi-exit-discriminator (MED): Indicates the preferred path into an AS to external neighbors when multiple paths exist.


A list of path attributes is contained in BGP update messages. The attribute is variable length and consists of three fields: Attribute type consisting of a 1-byte attribute flags field and a 1-byte attribute code field, Attribute length field that is 1 or 2 bytes, and a variable length attribute value field. The attribute type codes used by Cisco are: 1-origin, 2-AS-path, 3-Next-hop, 4-MED, 5-Local preference, 6-Atomic aggregate, 7-aggregator, 8-community, 9-originator-ID, and 10-cluster list.

Comments

4 Responses to "BGP Attributes Categories"

Anonymous said... June 19, 2011 at 8:24 AM

Pete... there needs to be a correction regarding the ORIGIN code attribute... EGP is not the category of route learned via EBGP instead it refers to the successor of BGP... the original EGP protocol which is now a days obsolete, so you may never find a route with "e" as origin type... the rest is OK :D

Anonymous said... June 21, 2011 at 3:31 PM

Awesome Explnation!

Anonymous said... August 18, 2013 at 1:27 PM

nice

Tejas Jain said... December 28, 2013 at 1:20 AM

And as for the "incomplete" Origin attribute, it is when the routes learned via IGP are redistributed into BGP.

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