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Validation of LCP Peer Magic Number

If the peer has not negotiated an LCP magic number, you can configure the router to ignore a mismatch of the LCP peer magic number and retain the PPP connection.

Previously, the router terminated a PPP connection with a non-conforming peer when it received LCP echo request packets or LCP echo reply packets from the peer with a magic number that did not match the LCP peer magic number on the router. This is still the current default behavior if you do not explicitly configure the router to ignore the LCP peer magic number mismatch if the peer has not negotiated the magic number and retain the PPP connection.

Configuring the router to ignore the peer magic number mismatch and retain the PPP connection is useful if your network includes peers that send a non-null or invalid magic number in the LCP echo request and reply packets despite having not negotiated the magic number. In this situation, the router expects to receive a null magic number from the peer, and terminates the PPP connection unless you configure it to ignore the peer magic number mismatch and retain the connection.

To configure the router to ignore the LCP peer magic number mismatch and retain the PPP connection, use the ppp magic-number ignore-mismatch command from Interface Configuration mode or Subinterface Configuration mode. For more information, see ppp magic-number ignore-mismatch.

To verify configuration of LCP peer magic number validation on the router, you can use the show ppp interface command. For more information, see show ppp interface.

Keep the following points in mind when configuring the router to ignore the peer magic number mismatch and retain the PPP connection:

If the peer negotiates the magic number but sends the router an LCP echo request or reply packet that contains a null or invalid magic number, the router strictly terminates the PPP connection. The router can ignore a mismatch of the LCP peer magic number only when the peer has not negotiated the magic number.
Using the ppp magic-number disable command to disable negotiation of the magic number on the router does not affect validation of the peer magic number. When you issue the ppp magic-number disable command, the router sets only the local magic number to null, but does not change or validate the peer magic number. (For more information, see ppp magic-number disable.)

You can also configure validation of the LCP peer magic number for static MLPPP interfaces, dynamic PPP interfaces, and dynamic MLPPP interfaces. For more information about configuring static MLPPP interfaces, see Configuring Multilink PPP. For more information about using profiles to configure dynamic PPP and dynamic MLPPP interfaces, see Configuring a Dynamic Interface from a Profile in Configuring Dynamic Interfaces.

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