LNS offers a proxy LCP (with the proxy-lcp command) function where LCP-related information is cached temporarily in the LNS during the ICCN phase where L2TP control messages are exchanged between the LAC and LNS. The LNS can then use the cached information to bypass the LCP negotiation and immediately start the authentication state with the client. Furthermore, proxy authentication (using the proxy-authentication command) can also be enabled on the LNS to bypass authentication and the client can immediately start the IPCP negotiation phase. If the proxy LCP information conflicts from the LNS configuration, then the LNS forces the client to re-start LCP negotiation. LCP negotiation is not restarted in the proxy LCP mode when:
MRU is missing in the LastTxLcpConfReq AVP
The magic number is missing in LastTxLcpConfReq AVP
Async-Control-Character-Map (ACCM) with value = 0x00000000 is present in LastTxLcpConfReq and LastRxLcpConfReq AVP's
Address-and-Control-Field-Compression (ACFC) is present in LastTxLcpConfReq and LastRxLcpConfReq. Note that PPP frames with and without address and control field (0xFF03) in the PPP header are accepted. LCP frames without 0xFF03 are also accepted as valid frames.
Also, proxy-authentication that fails then forces the client to re-start the LCP negotiation again.