Border Gateway Protocol Attributes

May 9, 2007 at 10:53 pm | Posted in BGP, Routing | Leave a comment

http://www.cramsession.com/articles/files/border-gateway-protocol-a-982003-1304.asp

Whether you are planning on taking the Building Scalable Cisco Networks (BSCN) or the Building Scalable Cisco Internetworks (BSCI) exam on the path to the CCNP certification, you are going to have to know BGP. For some reason, I have always had a hard time remembering the BGP attributes. This article is dedicated to the better understanding of BGP attributes.

A BGP attribute, or path attribute, is a metric used to describe the characteristics of a BGP path. Attributes are contained in update messages passed between BGP peers to advertise routes. There are four categories of BGP attributes. These are:

1) Well-known Mandatory
2) Well-known Discretionary
3) Optional Transitive
4) Optional Non-transitive

Optional transitive attributes may also be marked as partial.

Well-known attributes are attributes that all BGP implementations must recognize and are propagated to all BGP neighbors. Mandatory well-known attributes must appear in the route description. Discretionary attributes do not have to appear in the route description (they are not mandatory).

Optional attributes may not be supported by all BGP implementations. The transitive bit determines if an optional attribute is passed to BGP neighbors. If an optional attribute is transitive and not supported by a neighbor, it is passed and marked as partial. If it is non-transitive, a neighbor that does not implement the attribute deletes it.

The attributes that are commonly used in the Cisco Implementation of BGP are as follows:

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.

This article discussed the BGP attributes with the intent of preparing you for those confusing questions on the BSCN or BSCI exams. Hopefully, there is no more confusion on the various attributes that are used in the Cisco implementation of BGP.

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