BGP Attributes

bgp1When BGP sends an UPDATE packet to a peer it sends path attributes associated with the prefix. These path attributes are then used by BGP to pick the best route to a destination. BGP attributes are similar to metrics in OSPF and EIGRP in the way it uses them to decided on the best route.

There are four categories of path attribute

  • Well-known mandatory – This attribute MUST exist in the BGP UPDATE. If this attribute is missing a NOTIFICATION error is generated and the session is closed.
  • Well-known discretionary – This attribute must be recognized by all BGP implementations but does not have to be included in every BGP UPDATE message.

The following two categories are optional (as the name suggests) and are not required to be supported by all BGP implementations

  • Optional transitive – If the attribute is not recognized by the BGP implementation but the transitive flag is set the attribute should be accepted and passed along to other peers.
  • Optional non-transitive – If the attribute is not recognized by the BGP implementation but the transitive flag is not set the attribute should be ignored and not passed on to other peers.
Wartość bajtowa atrybutu (dziesiętnie i szesnastkowo) Nazwa atrybutu
1   0x1 ORIGIN Well-known mandatory
2  0x2 AS_PATH Well-known mandatory
3  0x3 NEXT_HOP Well-known mandatory
4  0x4 MULTI_EXIT_DISC (MED) Optional non-transitive
5  0x5 LOCAL_PREF Well-known discretionary
6  0x6 ATOMIC_AGGREGATE Well-known discretionary
7  0x7 AGGREGATOR Optional transitive
8  0x8 COMMUNITY Optional transitive
9 0x9 ORIGINATOR_ID Optional non-transitive
10 0xa Cluster List Optional non-transitive
11 0xb DPA Designation Point Attribute
12  0xc Advertiser BGP/IDRP Route Server
14  0xe Multiprotocol Reachable NLRI Optional non-transitive
15  0xf Multiprotocol Unreachable NLRI Optional non-transitive
16 0x10 Extended communities  
256  0x100 Reserved for future development  

BGP works through these attributes in this specific order when choosing a path. There is an order used to path selection.

(W) L L A  O M N I

  1. Highest WEIGHT (This is a cisco proprietary value)
  2. Highest LOCAL_PREF
  3. Prefer a route that is LOCALLY SOURCED
  4. Shortest AS_PATH
  5. ORIGIN of the prefix (external, internal, unknown)
  6. Lowest MED
  7. External BGP routes are preferred over internal BGP routes (NEIGHBOR TYPE)
  8. If no external route select path with the lowest IGP cost to the next hop router for iBGP. (IGP COST)
  9. The most recent route
  10. The lowest BGP router ID.

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