BSCI: BGP Attributes I
Posted by Aragoen Celtdra on 17th December 2008
A BGP attribute or path attribute is a characteristic of an advertised BGP route to define routing policies and maintain a stable routing environment
- Attributes can be:
- Well-known or Optional
- Mandatory or Discretionary
- The path attributes described above fall in four categories:
- Well-known mandatory
- Well-known discretionary
- Optional transitive
- Optional nontransitive
Well-Known Attributes
- A well-known attribute is one that all BGP implementations must recognize and propagate to BGP neighbors.
- Well-known mandatory – must appear in all BGP updates.
- Well-known discretionary – does not have to be present in all BGP updates.
Optional Attributes
- Attributes that are not well-known.
- Transitive – a BGP process should accept the path in which it is included, even if it doesn’t support the attribute, and it should pass the path on to its peers.
- Non-transitive – a BGP process that does not recognize the attribute can ignore the Update in which it is included and not advertise the path to its other peers.
- BGP routers that implement an optional attribute might propagate it to other BGP neighbors, based on its meaning.
- BGP routers that do not implement an optional transitive attribute should pass it to other BGP routers untouched and mark the attribute as partial.
- BGP routers that do not implement an optional non-transitive attribute must delete the attributes and must pass it to other BGP routers.
Defined BGP attributes:
- Well-known mandatory
- AS-Path
- Next Hop
- Origin
- Well-known discretionary
- Local Preference
- Atomic Aggregate
- Optional Transitive
- Aggregator
- Community
- Optional Non-transitive
- Multiexit-discriminator (MED)
- Cisco also has its own defined weight attribute for BGP.
- It is configured locally on a router and is not propagated to any other BGP routers.
BGP Attribute Type Codes
- Type code 1 – Origin
- Type code 2 – AS-path
- Type code 3 – Next-hop
- Type code 4 – MED
- Type code 5 – Local preference
- Type code 6 – Atomic aggregate
- Type code 7 – Aggregator
- Type code 8 (Cisco-defined) – Community
- Type code 9 (Cisco-defined) – Originator-ID
- Type code 10 (Cisco-defined) – Cluster list
Resources:
This entry is not an authoritative guide. These are merely notes and rehash of the primary text materials and resources that I use. For a thorough guide of the BSCI course, consider purchasing Building Scalable Cisco Internetworks (BSCI) (Authorized Self-Study Guide) (3rd Edition) by Diane Teare and Catherine Paquet; Routing TCP/IP, Volume 1 (2nd Edition) (CCIE Professional Development) by Jeff Doyle and Jennifer Carroll; as well as following the links on the resources section of this entry.
Posted in BGP, BSCI Exam Prep, CCNP, Routing Protocols | No Comments » | 