What will happen if an update is received with an unrecognized optional non-transitive attribute?

According to the BGP-4 specification (RFC 4271), BGP path attributes are classified by their properties. An "optional non-transitive" attribute is one that a BGP implementation is not required to recognize. If a BGP speaker receives an update with an optional non-transitive attribute that it does not recognize, the protocol mandates that the router should accept the update but quietly ignore the unrecognized attribute. Because the attribute is "non-transitive," it must not be passed along to other BGP peers. No error notification is generated, as the attribute is "optional."

A. A notification is not sent for unrecognized optional attributes; this action is reserved for errors with well-known attributes.
B. The update is accepted. Rejecting the update and sending a notification is the behavior for an unrecognized well-known mandatory attribute.
D. The attribute is not propagated because its "non-transitive" property explicitly forbids this for unrecognized attributes.

1. IETF RFC 4271, A Border Gateway Protocol 4 (BGP-4), January 2006.

Section 5.1.2, Path Attributes: "If a BGP speaker receives an update with an unrecognized optional, non-transitive attribute, it SHOULD quietly ignore the attribute and not pass it on to other BGP speakers." This directly supports the correct answer.

2. Nokia 7750 SR OS R21.2.R1 BGP Configuration Guide, June 2021.

Section: BGP Path Attributes, Page 58: The document states, "Optional non-transitive attributes are discarded if not recognized." This confirms that the attribute is ignored/discarded while the rest of the update is processed.