
Dial VPN Layer 2 Tunneling
303509-A Rev 00 2-11
Remote Router Configuration
If the host at the remote site is a Bay Networks router, you may need to configure
a dial-on-demand circuit for the remote router’s dial-up interface to the LAC at the
ISP network.
Enable RIP on both the dial-on-demand circuit and the attached LAN interface of
the remote router, so that the LNS can learn routing information from the remote
router. To avoid unnecessarily activating the circuit because of RIP packets,
enable dial-optimized routing for the dial-on-demand circuit.
In addition, configure a default or static route for the remote router, which uses the
next-hop address that corresponds to the L2TP IP interface address of the LNS.
This default or static route enables the remote router to deliver L2TP packets to
the LNS.
Starting an L2TP Session
The connection process for Layer 2 tunnels is similar to that for Layer 3, but the
end points of the tunnels are different. In L2TP tunneling, the end point of the PPP
connection from a LAC or a remote access server (RAS) extends to an L2TP
network server (LNS). Multiple users can communicate through a single tunnel
between the same LAC and LNS pair. Each user transmits and receives data in an
individual L2TP session.
Packets flow across an L2TP tunnel during an L2TP session. An L2TP session is
created when an end-to-end WAN connection is established between the remote
host and the LNS.
The L2TP portion of the packets sent through the tunnel contains a header with a
call ID field (also called a session ID) and a tunnel ID field. The call ID field,
which indicates the session that the WAN packet belongs to, is negotiated between
the LAC and the LNS when the L2TP call is set up. The tunnel ID specifies the
tunnel that the L2TP session is using.
In addition to the fields in the header, the L2TP packet contains a call serial
number, which is a unique number for each L2TP call. This number matches the
call to the L2TP session.
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