BGP Attributes Categories
BGP Attributes Categories are 1) WELL-KNOWN, MANDATORY , 2) WELL-KNOWN, DISCRETIONARY, 3) OPTIONAL, TRANSITIVE, 4) OPTIONAL, NON-TRANSITIVE. details are:
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).
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.[adsense]
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.
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:
|Type Code byte value (as decimal)||Attribute Name||Category|
|4||MULTI_EXIT_DISC (MED)||Optional non-transitive|
|10||Cluster List||Optional non-transitive|
|11||DPA||Designation Point Attribute|
|12||Advertiser||BGP/IDRP Route Server|
|13||RCID_PATH/CLUSTER_ID||BGP/IDRP Route Server|
|14||Multiprotocol Reachable NLRI||Optional non-transitive|
|15||Multiprotocol Unreachable NLRI||Optional non-transitive|
|256||Reserved for future development|