V6100 and V7122 User Guide 269Each of the Ethernet frames carrying PPP session has a standard Ethernet header followedby PPPoE header, and is sent with the remote host Ethernet MAC address (except for thevery 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 GatewaysThe 3Com gateway contains a PPPoE client embedded in its software. When correctlyconfigured (see INI File parameters) the gateway can try to connect to a remote PPPoEAccess Concentrator.When restarting the gateway after several BOOTP attempts, if PPPoE is enabled (see INIFile 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 AccessConcentrator exists and replies, the gateway tries to connect to this Access Concentrator. Ifthis initial connection succeeds, then the PPP LCP phase will start - each side of the PPPoEconnection will send LCP configuration request to configure the PPP link.The gateway PPPoE client supports both PAP and CHAP authentications. The type ofauthentication protocol used is according to the request from the authentication server. In theLCP configuration phase, the server requires a specific authentication (none, PAP or CHAPare supported). The INI File parameters PPPoEUserName, PPPoEPassword andPPPoEServerName are used to configure the authentication parameters. If the AccessConcentrator is configured to operate in PAP, the PPPoEUserName and PPPoEPasswordare used as Username and Password (in this case, the PPPoEServerName parameter is notused). If the Access Concentrator is configured to operate in CHAP, the PPPoEUserNameparameter functions as Client Name (sent in the CHAP response packet), while thePPPoEPassword functions as the shared secret (calculated along with the challenge toproduce the response). In this case, the PPPoEServerName is the name of the server (somehosts can be configured to authenticate to multiple servers. In such hosts, the server name isused to identify which secret should be used).The 3Com gateway, being a PPPoE client, requests no authenticationAfter 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 ConfigurationProtocol). In this phase, if the INI File parameter PPPoEStaticIPAddress is defined, thegateway requests the remote host to assign this address for its use.When working in PPPoE environment, the gateway negotiates for its IP address (asdescribed above). However, if the user desires to disable the PPPoE client, the gateway canbe configured to use default values for IP address, subnet mask and default gateway. Thiscan be done using INI File parameters PPPoERecoverIPAddress,PPPoERecoverSubnetMask and PPPoERecoverDfgwAddress. These parameters indicate tothe gateway that if the PPPoE is disabled and no BOOTP server is activated, as required inthe gateway to use a PPPoE environment, then the gateway should use these defaults for itsIP configuration.
