11 Path Attributes Overview
BGP does not use a single metric. Instead, each route carries a set of Path Attributes (PAs) that describe it and drive best-path selection. This note covers how attributes are classified; each individual attribute has its own note.
The classification system
Path attributes fall into four categories, based on two properties: whether an attribute is well-known or optional, and whether it is transitive or non-transitive.
- Well-known: must be recognized by every BGP implementation, from any vendor.
- Optional: may or may not be recognized; support is not guaranteed.
- Transitive: if not recognized, it is still passed on to other peers.
- Non-transitive: if not recognized, it is not passed on.
Because BGP is an open standard, it can differ slightly between vendors, which is exactly why the well-known versus optional distinction matters.
Well-known mandatory
Must be recognized by all implementations and must be present in every Update message.
- Origin
- AS-PATH
- Next-Hop
Well-known discretionary
Must be recognized by all implementations, but may or may not be present in a given Update (not mandatory).
- Local Preference
- Atomic Aggregate
Optional transitive
May not be recognized. If a router does not recognize such an attribute, it must still accept it and pass it on to its peers, even without understanding it.
- Aggregator
- Community
Optional non-transitive
May not be recognized. If a router does not recognize such an attribute, it may drop it and not forward it to its peers.
- MED
Summary table
| Category | Recognized by all? | Passed on if unrecognized? | Examples |
|---|---|---|---|
| Well-known mandatory | Yes | n/a (always present) | Origin, AS-PATH, Next-Hop |
| Well-known discretionary | Yes | n/a | Local Preference, Atomic Aggregate |
| Optional transitive | No | Yes | Aggregator, Community |
| Optional non-transitive | No | No | MED |
A note on Weight
Weight is a Cisco-proprietary attribute and does not fit this standard classification, because it is never included in Update messages between routers. It is purely local to a single router and is covered in note 12.
Atomic Aggregate classification
Atomic Aggregate is well-known discretionary. The related Aggregator attribute is optional transitive. These two are easy to confuse because both relate to summarization; they are covered in note 20.
Self-check
Q1 — What do the two classification axes describe?
A) Inbound versus outbound, and permit versus deny
B) Well-known versus optional, and transitive versus non-transitive
C) eBGP versus iBGP, and local versus global
D) Mandatory versus discretionary, and IPv4 versus IPv6
Respuesta
B is correct. Attributes are classified by whether they are well-known or optional, and whether they are transitive or non-transitive.
- A) False — those describe filtering direction and ACL actions, not attributes.
- C) False — peer type is not an attribute classification axis.
- D) False — mandatory/discretionary is a sub-split of well-known, and address family is unrelated here.
Q2 — Which attributes are well-known mandatory?
A) Local Preference, Atomic Aggregate, MED
B) Origin, AS-PATH, Next-Hop
C) Weight, Local Preference, Community
D) Aggregator, Community, MED
Respuesta
B is correct. Origin, AS-PATH, and Next-Hop are well-known mandatory: recognized by all and present in every Update.
- A) False — Local Preference and Atomic Aggregate are discretionary; MED is optional non-transitive.
- C) False — Weight is Cisco-proprietary and local; the others are not mandatory.
- D) False — those are optional attributes, not well-known mandatory.
Q3 — What must a router do with an unrecognized optional transitive attribute?
A) Drop it and not forward it
B) Accept it and pass it on to its peers
C) Reject the entire Update
D) Convert it to a well-known attribute
Respuesta
B is correct. Optional transitive means that even if unrecognized, the attribute is accepted and forwarded to peers unchanged.
- A) False — dropping when unrecognized is the non-transitive behavior.
- C) False — the Update is not rejected over an unrecognized optional attribute.
- D) False — attributes are not reclassified in transit.
Q4 — Why does Weight not fit the standard classification?
A) Because it is well-known mandatory
B) Because it is Cisco-proprietary and never included in Updates between routers (purely local)
C) Because it is optional transitive
D) Because it only applies to IPv6
Respuesta
B is correct. Weight is Cisco-proprietary and local to one router; it is never sent in Updates, so it falls outside the standard well-known/optional model.
- A) False — it is not a standard well-known attribute at all.
- C) False — it is not transmitted, so transitivity does not apply.
- D) False — Weight is not address-family specific.