BGP path attributes

Attribute classification

Path attributes fall into four categories based on two flag bits:

Category	Recognition	Propagation	Example
Well-known mandatory	All implementations	Always forwarded	ORIGIN, AS_PATH, NEXT_HOP
Well-known discretionary	All implementations	Forwarded if present	LOCAL_PREF, ATOMIC_AGGREGATE
Optional transitive	May not be recognized	Forwarded with Partial bit set	AGGREGATOR, COMMUNITIES
Optional non-transitive	May not be recognized	Silently dropped	MULTI_EXIT_DISC

Attribute encoding

Each attribute is a TLV triplet: Attribute Flags (1 octet) + Type Code (1 octet) + Length (1 or 2 octets) + Value.

Flag bit	Position	Meaning when set
Optional	bit 0	Attribute is optional (not well-known)
Transitive	bit 1	Optional attribute is passed to peers
Partial	bit 2	Attribute was not recognized by some AS in the path
Extended Length	bit 3	Length field is 2 octets instead of 1

Attributes within an UPDATE are ordered by Type Code (ascending). A receiver handles out-of-order attributes. Duplicate Type Codes in the same UPDATE are an error.

ORIGIN (Type 1)

Well-known mandatory. Indicates how the route was injected into BGP.

Value	Meaning	Preference
0	IGP — originated from an interior protocol (network statement)	Highest
1	EGP — learned via the legacy Exterior Gateway Protocol	Middle
2	INCOMPLETE — redistributed from another source	Lowest

The Decision Process prefers lower ORIGIN values. A route redistributed from OSPF gets INCOMPLETE; a route originated via network or aggregate-address gets IGP.

AS_PATH (Type 2)

Well-known mandatory. Records the sequence of ASes the route has traversed. Provides loop detection — a router rejects routes containing its own ASN.

Segment types

Type	Name	Meaning
1	AS_SET	Unordered set (result of aggregation)
2	AS_SEQUENCE	Ordered list of ASes traversed

Each segment is encoded as: segment type (1 octet) + count (1 octet, number of AS numbers) + AS numbers (2 octets each, or 4 octets with RFC 6793).

Prepending rules

When originating a route to EBGP: single-element AS_SEQUENCE with own ASN. To IBGP: empty AS_PATH.

An AS_SET counts as path length 1 regardless of how many ASes it contains. RFC 9774 deprecates AS_SET and AS_CONFED_SET — new implementations reject routes containing them.

NEXT_HOP (Type 3)

Well-known mandatory. The IPv4 address of the router to use as next hop toward the advertised destinations.

NEXT_HOP rules

Scenario	NEXT_HOP value
EBGP, directly connected	Sender’s interface address facing the peer
EBGP, multihop	Sender’s session source address (default)
IBGP, externally learned	Preserved from EBGP peer (default)
IBGP, locally originated	Interface through which the network is reachable
next-hop-self configured	Sender’s own address regardless of source

The receiving router resolves NEXT_HOP recursively through the routing table to determine the actual forwarding interface. If unresolvable, the route is ineligible for selection.

MULTI_EXIT_DISC (Type 4)

Optional non-transitive. A 4-octet metric used to influence inbound traffic when multiple links connect to the same neighbor AS. Lower is preferred.

Key constraints:

• Comparable only between routes from the same neighbor AS
• Received over EBGP, may be propagated over IBGP within the same AS
• Never propagated to other external ASes
• Must be removed or evaluated before the Decision Process

LOCAL_PREF (Type 5)

Well-known discretionary. A 4-octet unsigned integer expressing the local AS’s degree of preference for a route. Higher is preferred.

Context	Behavior
Sent to IBGP peer	Required in all UPDATEs
Sent to EBGP peer	Never included (except confederations)
Received from EBGP	Ignored (except confederations)

LOCAL_PREF is the first discriminator in the Decision Process after policy filtering.

ATOMIC_AGGREGATE (Type 6)

Well-known discretionary. Zero-length attribute. Signals that the originator selected a less-specific route and dropped some AS_PATH information (AS_SET) during aggregation.

A receiver of ATOMIC_AGGREGATE must not deaggregate the prefix into more-specifics. The actual forwarding path may differ from what AS_PATH shows.

AGGREGATOR (Type 7)

Optional transitive. Carries the AS number (2 octets) and BGP Identifier (4 octets) of the router that formed the aggregate. Added during aggregation to preserve attribution.

Attribute summary table

Type	Name	Category	Length	Decision Process role
1	ORIGIN	WK mandatory	1 octet	Tie-break step 2
2	AS_PATH	WK mandatory	variable	Tie-break step 1 (shortest path)
3	NEXT_HOP	WK mandatory	4 octets	Resolvability check, IGP cost
4	MULTI_EXIT_DISC	Opt non-transitive	4 octets	Tie-break step 3 (same neighbor AS)
5	LOCAL_PREF	WK discretionary	4 octets	Primary selection (highest wins)
6	ATOMIC_AGGREGATE	WK discretionary	0	Informational
7	AGGREGATOR	Opt transitive	6 octets	Informational