3Com V6100, Руководство пользователя, Страница: 269 / 496

V6100 and V7122 User Guide
269
Each of the Ethernet frames carrying PPP session has a standard Ethernet header followed
by PPPoE header, and is sent with the remote host Ethernet MAC address (except for the
very first one, in the discovery phase, which is broadcasted to all hosts).
Further information on the transmission of PPPoE is available on the IETF website
(
http://www.ietf.org/rfc/rfc2516.txt ).
PPPoE in 3Com Gateways
The 3Com gateway contains a PPPoE client embedded in its software. When correctly
configured (see INI File parameters) the gateway can try to connect to a remote PPPoE
Access Concentrator.
When restarting the gateway after several BOOTP attempts, if PPPoE is enabled (see INI
File parameter “EnablePPPoE”), the gateway tries to initiate a PPP session.
The gateway initiates a PPPoE discovery phase to discover a PPPoE Access Concentrator.
It does this by broadcasting a discovery initialization packet (PADI). If an Access
Concentrator exists and replies, the gateway tries to connect to this Access Concentrator. If
this initial connection succeeds, then the PPP LCP phase will start - each side of the PPPoE
connection will send LCP configuration request to configure the PPP link.
The gateway PPPoE client supports both PAP and CHAP authentications. The type of
authentication protocol used is according to the request from the authentication server. In the
LCP configuration phase, the server requires a specific authentication (none, PAP or CHAP
are supported). The INI File parameters PPPoEUserName, PPPoEPassword and
PPPoEServerName are used to configure the authentication parameters. If the Access
Concentrator is configured to operate in PAP, the PPPoEUserName and PPPoEPassword
are used as Username and Password (in this case, the PPPoEServerName parameter is not
used). If the Access Concentrator is configured to operate in CHAP, the PPPoEUserName
parameter functions as Client Name (sent in the CHAP response packet), while the
PPPoEPassword functions as the shared secret (calculated along with the challenge to
produce the response). In this case, the PPPoEServerName is the name of the server (some
hosts can be configured to authenticate to multiple servers. In such hosts, the server name is
used to identify which secret should be used).
The 3Com gateway, being a PPPoE client, requests no authentication
After the gateway has been authenticated, it needs to configure a network layer protocol.
The gateway uses the IP protocol. Therefore, the used NCP will be IPCP (IP Configuration
Protocol). In this phase, if the INI File parameter PPPoEStaticIPAddress is defined, the
gateway requests the remote host to assign this address for its use.
When working in PPPoE environment, the gateway negotiates for its IP address (as
described above). However, if the user desires to disable the PPPoE client, the gateway can
be configured to use default values for IP address, subnet mask and default gateway. This
can be done using INI File parameters PPPoERecoverIPAddress,
PPPoERecoverSubnetMask and PPPoERecoverDfgwAddress. These parameters indicate to
the gateway that if the PPPoE is disabled and no BOOTP server is activated, as required in
the gateway to use a PPPoE environment, then the gateway should use these defaults for its
IP configuration.