BGP Attributes Categories
BGP Attributes Categories are 1) WELL-KNOWN, MANDATORY , 2) WELL-KNOWN, DISCRETIONARY, 3) OPTIONAL, TRANSITIVE, 4) OPTIONAL, NON-TRANSITIVE. details are: [boxads]
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.[adsense]
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. [bodyads]
The attribute type codes used by Cisco are:
| Type Code byte value (as decimal) | Attribute Name | Category |
| 1 | ORIGIN | Well-known mandatory |
| 2 | AS_PATH | Well-known mandatory |
| 3 | NEXT_HOP | Well-known mandatory |
| 4 | MULTI_EXIT_DISC (MED) | Optional non-transitive |
| 5 | LOCAL_PREF | Well-known discretionary |
| 6 | ATOMIC_AGGREGATE | Well-known discretionary |
| 7 | AGGREGATOR | Optional transitive |
| 8 | COMMUNITY | Optional transitive |
| 9 | ORIGINATOR_ID | 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 |
| 16 | Extended communities | |
| 256 | Reserved for future development |
