ZyXEL Communications Prestige 128Plus, User Manual

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Prestige 128+ ISDN Router
L2TP 10-3
In L2TP terminology, the NAS (network access server) at the ISP is the L2TP
client and is called a LAC (L2TP Access Concentrator). The Prestige at the
corporate LAN is the server and is called a LNS (L2TP Network Server). A
tunnel exists between the LAC and the LNS. The (mobile) user utilizes L2TP for
the Internet tunnel. In simple terms, a LAC forwards packets and must have a
GSTN connection; a LNS negotiates PPP with a user but does not necessarily
have a GSTN connection. Obviously, however, both the LAC and the LNS must
have access to the Internet. Both the LNS and LAC are called an L2TP endpoint.
After call connection, the user and the LNS negotiate PPP in exactly the same
fashion as a direct connection. However, instead of interfacing to the physical
device, e.g., ISDN, the LNS is talking to an L2TP tunnel, i.e., a logical device.
Please refer to the diagram above.
10.3.1 LNS (L2TP Network Server)
The LNS terminates a PPP connection - it handles the server side of the L2TP
protocol. Since L2TP runs on top of IP, the LNS may have only a single LAN or
WAN interface yet still be able to terminate calls arriving at any LAC's full range
of PPP interfaces (async, synchronous ISDN, V.120, etc.).
10.3.2 LAC (L2TP Access Concentrator)
The LAC relays the traffic between the LNS and the user. It may tunnel any
protocol carried within PPP.
For incoming calls, the LAC may negotiate LCP and authentication to discover
the apparent identity of the user; or it may use other mechanism, e.g., CLID. In
the case of PPP authentication, the LAC only performs partial negotiation, i.e.,
receiving PAP request or sending CHAP challenge and receiving response. Once
the user name (and hence the realm) is know, the LAC forwards all negotiation
data thus far gathered (LCP and authentication) to the LNS.
Note that a Prestige can be a LAC for one connection and a LNS for another at
the same time.