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RFC1661 - часть 39

Magic-Number Configuration Option, then it MUST NOT respond with a

Configure-Reject when it receives a Configure-Request with a

Magic-Number Configuration Option. That is, if an implementation

desires to use Magic Numbers, then it MUST also allow its peer to

do so. If an implementation does receive a Configure-Reject in

response to a Configure-Request, it can only mean that the link is

not looped-back, and that its peer will not be using Magic-

Numbers. In this case, an implementation SHOULD act as if the

negotiation had been successful (as if it had instead received a


The Magic-Number also may be used to detect looped-back links

during normal operation, as well as during Configuration Option

negotiation. All LCP Echo-Request, Echo-Reply, and Discard-

Request packets have a Magic-Number field. If Magic-Number has

been successfully negotiated, an implementation MUST transmit

these packets with the Magic-Number field set to its negotiated


The Magic-Number field of these packets SHOULD be inspected on

reception. All received Magic-Number fields MUST be equal to

either zero or the peer's unique Magic-Number, depending on

whether or not the peer negotiated a Magic-Number.

Simpson [Page 46]

RFC 1661 Point-to-Point Protocol July 1994

Reception of a Magic-Number field equal to the negotiated local

Magic-Number indicates a looped-back link. Reception of a Magic-

Number other than the negotiated local Magic-Number, the peer's

negotiated Magic-Number, or zero if the peer didn't negotiate one,

indicates a link which has been (mis)configured for communications

with a different peer.

Procedures for recovery from either case are unspecified, and may

vary from implementation to implementation. A somewhat

pessimistic procedure is to assume a LCP Down event. A further

Open event will begin the process of re-establishing the link,

which can't complete until the looped-back condition is

terminated, and Magic-Numbers are successfully negotiated.

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