Maximum Number of Paths Reached Notification for BGP

Introduction The BGP Cease NOTIFICATION message is used when a BGP speaker terminates its peering with a neighbor. defines subcodes for the Cease NOTIFICATION message to aid network operators in correlating network events and diagnosing BGP peering issues, including the "Maximum Number of Prefixes Reached" subcode. BGP ADD-PATH permits a BGP speaker to advertise multiple paths for the same address prefix. As a result, a BGP speaker may enforce a locally configured upper bound on the number of paths it accepts from a peer, distinct from any bound on the number of prefixes. This bound may be applied per prefix, for example using the PATHS-LIMIT Capability , or as an aggregate limit across an <AFI, SAFI>, for example through a local configuration knob that counts the additional paths toward a combined prefix-and-path limit. No existing Cease subcode precisely describes the termination of a peering because such a bound was reached. This document defines the "Maximum Number of Paths Reached" Cease NOTIFICATION subcode for this purpose.

Requirements Language The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

Maximum Number of Paths Reached Subcode IANA has allocated the value TBD (see ) for the "Maximum Number of Paths Reached" Cease NOTIFICATION message subcode. When a BGP speaker terminates a peering because the number of paths received from the neighbor for one or more <AFI, SAFI> exceeds a locally configured upper bound, the BGP speaker SHOULD send a NOTIFICATION message with the error code "Cease" and the error subcode "Maximum Number of Paths Reached". Following the format defined in , Section 4, the message MAY optionally include the Address Family information and the upper bound in the "Data" field, as shown in .

Maximum Number of Paths Reached Data Field The "Paths upper bound" field indicates the locally configured maximum number of paths, for the indicated <AFI, SAFI>, that triggered the termination. If a BGP speaker terminates a peering because the bound was exceeded for more than one <AFI, SAFI>, it MAY include the Data field for any one of them.

Operational Considerations This subcode is meaningful only when BGP ADD-PATH has been negotiated for at least one <AFI, SAFI> with the peer. A speaker that bounds only the number of prefixes continues to use the "Maximum Number of Prefixes Reached" subcode . A receiver may bound the number of received paths in two distinct ways. An aggregate bound limits the total number of paths a receiver will accept from the peer for an <AFI, SAFI>, primarily to protect against resource exhaustion; for example, an implementation might provide a configuration knob that counts the additional paths toward a combined prefix-and-path limit, such as "neighbor X maximum-prefix 10 include-additional-paths". A per-prefix bound limits the number of paths accepted for a single prefix. The PATHS-LIMIT Capability , when supported by both peers, lets a receiver negotiate the per-prefix bound with the sender, so that the sender can avoid exceeding it in the first place. This subcode is the complementary enforcement signal for either kind of bound: it indicates termination when the number of received paths exceeds the receiver's locally configured upper bound, regardless of whether the PATHS-LIMIT Capability was negotiated, for example because the sender ignored it, or sent more paths than indicated. Terminating the peering is the most disruptive response, because it discards all paths for all prefixes even when only a small number of prefixes exceed a per-prefix bound. Where feasible, a receiver SHOULD prefer graceful handling that preserves the session, as recommended in , over termination. For a per-prefix bound in particular, such handling includes treating the affected prefixes as withdrawn, or accepting only the permitted number of paths for those prefixes and discarding the excess, while logging the event for the operator. A receiver SHOULD send this subcode only when its local policy does terminate the peering, for example when an aggregate bound is exceeded and preserving the session would risk resource exhaustion. When BGP Graceful Restart with NOTIFICATION support has been negotiated, a speaker that sends or receives this subcode follows the procedures of . A speaker whose local policy requires the immediate discard of the routes received from the peer uses the Hard Reset mechanism of , carrying the "Cease" code and "Maximum Number of Paths Reached" subcode in the Data field of the Hard Reset message.

Security Considerations This document defines a subcode for the BGP Cease NOTIFICATION message that provides information to aid network operators in correlating network events and diagnosing BGP peering issues. This subcode is purely informational and has no impact on the BGP Finite State Machine beyond that already documented by , Sections 6.6 and 6.7.

IANA Considerations IANA is requested to assign a value from the "BGP Cease NOTIFICATION message subcodes" registry, with the name "Maximum Number of Paths Reached" and a reference to this document.

Acknowledgements TBD