Advertise features before using them

BGP means Border Gateway Protocol. A BGP speaker is a router process exchanging routing information with a peer. OPEN is its opening BGP message after TCP connection setup. A capability is an optional protocol extension. A supporting speaker MAY include the Capabilities Optional Parameter in OPEN (§3; RFC4271 §4.2).

• BGP means Border Gateway Protocol. A BGP speaker is a router process exchanging routing information with a peer. OPEN is its opening BGP message after TCP connection setup. A capability is an optional protocol extension. A supporting speaker MAY include the Capabilities Optional Parameter in OPEN (§3; RFC4271 §4.2).
• A speaker learns its peer’s advertised capabilities from the received OPEN. RFC5492 describes using a capability on a peering after both peers have advertised it. Each extension’s own rules still matter; a shared code is not a substitute for checking its parameters (§3).
• Our principal example is Multiprotocol IPv6 unicast: both peers advertise the same Address Family Identifier (AFI) and Subsequent Address Family Identifier (SAFI). RFC4760 §8 requires each to advertise that particular tuple for bidirectional exchange. The chosen tuple is AFI 2 / SAFI 1; these mean IPv6 / unicast.
• An advertised feature is not an acknowledgement of the other peer’s list, a command to activate every extension, or proof of authenticated identity. Other OPEN checks, KEEPALIVEs and the BGP state machine still apply; capability eligibility alone is not an established session (RFC4271 §4.2; §7).
• Route Refresh is another concrete capability used in our wire examples. It advertises the ability to receive and handle ROUTE-REFRESH messages (RFC2918 §2). The helper compares shared advertisements; it does not implement that extension’s directional operation or actual routing updates.