L2TP Dial-Out Process

The following is the dial-out process used in the network model illustrated in L2TP Dial-Out Overview:

  1. The router receives a trigger packet.
  2. The router builds a RADIUS Access-Request message and sends it to the RADIUS server that is associated with the virtual router on which the dial-out route is defined—typically, the RADIUS home server.
  3. The RADIUS server’s response to the Access-Request is similar to the response used for LAC incoming calls. Notable differences are that the IP addresses of the peer are interpreted as LAC addresses instead of LNS addresses. In addition, narrowband details, such as calling numbers, are returned.
  4. The LNS makes the outgoing call using a load-balancing or round-robin mechanism identical to the one that the E Series LAC uses for incoming calls. The LAC may also employ the LAC RADIUS in tunnel authentication.
  5. Once the LNS successfully completes a control connection and session with the LAC, the LAC performs the actual narrowband dial-out operation to the remote site using the information passed by the LNS during session setup.
  6. A PPP session is started on the remote customer premises equipment (CPE), and mutual PPP authentication is performed at the remote CPE and the LNS as follows:
    1. The LNS uses the LNS RADIUS server to validate the remote CPE’s PPP session, while the CPE can use its own RADIUS server to validate the LNS’s PPP session.
    2. The LNS uses the username and password that is returned in the first Access-Accept message.
  7. Once authentication is successful, an IP interface is built on top of the PPP interface at the LNS. Internet Protocol Control Protocol (IPCP) is negotiated, and the framed route that RADIUS returns as a result of the PPP authentication supersedes the dial-out route.

IP traffic can now flow freely between the home and remote sites.

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