Cabletron Systems CSX400 - Manual (Page 79)

Configuring the PPP Protocol 4-15
CSX WAN Configuration
Initial MRU
The initial Maximum Receive Unit (MRU) that the local PPP entity will advertise
to the remote entity. If the value of this variable is 0, then the local PPP entity will
not advertise any MRU to the remote entity and the default MRU will be
assumed. Changes to this object take effect when the link is next restarted.
The default MRU for the CSX is 8191.
Magic Num. Neg.
The Magic Number Negotiation Option enables the detection of looped-back
links and other Data Link Layer anomalies.
If Magic Number Negotiation is set to True, the local node will attempt to
perform Magic Number Negotiation with the remote node; if set to False, the
negotiation is not performed by the local node. In any event, the local node will
comply with any Magic Number negotiations attempted by the remote node, per
the PPP speciÞcation. By default, the Magic Number is not negotiated, and zero is
inserted where a Magic Number might otherwise be used. Changes to this object
take effect when the link is next restarted.
The Magic Number is a number that should be selected randomly by the device
implementing PPP (e.g., based on unique seed, such as a machine serial number, a
network hardware address, a time-of-day clock, or the precise measurement of
the inter-arrival time of physical events such as packet reception on other
connected networks).
When a PPP peer has the Magic Number ConÞguration option enabled, and
receives a ConÞgure-Request with a randomly generated Magic Number, it
compares that number with that of the last ConÞgure-Request it received.
¥ If the two Magic Numbers are different, then the link is not looped-back, and
the Magic Number SHOULD be acknowledged.
¥ If the two Magic Numbers are equal, then it is possible (but not certain) that
the link is looped-back and that the ConÞgure-Request just received was
actually the last one issued by the receiving peer. Further checks are carried
out to determine whether a loopback condition exists.
The Magic Number also may be used to detect looped-back links during normal
operation, as well as during ConÞguration Option negotiation. All LCP Echo-
Request, Echo-Reply, and Discard-Request packets have a Magic Number Þeld. If
a Magic Number has been successfully negotiated, an implementation MUST
transmit these packets with the Magic Number Þeld set to its negotiated Magic
Number. The Magic Number Þeld of these packets SHOULD be inspected on
reception.
All received Magic Number Þelds MUST be equal to either zero or the peerÕs
unique Magic Number, depending on whether or not the peer negotiated a Magic
Number.