CCIE RS – L2 WAN Circuit Technologies – PPPoE

Point to Point Protocol over Ethernet

Virtualizes Ethernet into multiple p2p sessions between clients and server

RFC 2516

  • • Adds 8 bytes of overhead
    • 2 bytes PPP
    • 6 bytes PPPoE
  • Change MTU on interface to 1492
  • MSS is changes to 1452 bytes
    • Allows for 40 bytes of TCP and IP header and 8 byte PPPoE header
  • Uses dialer interface
    • Pppoe-client dial-pool-number [#]

Configuration Example


Int [wan]
 Pppoe enable
 Pppoe-client dial-pool-number [#]
Int dialer [#]
 Mtu 1492
 Ip tcp adjust-mss 1452
 Encapsulation ppp
 Ip address negotiated (optional), can config with an address
 (optional) - ppp authentication - see above
 Dialer pool [#]
 Dialer-group 1


Bba-group pppoe global
Virtual-template [#]
Int [wan]
 Pppoe enable group global
Int virtual-template [#]
 Ip unnumbered [wan]
 Mtu 1492
 Peer default ip address pool PPPOEPOOL
 Ip local pool PPPOEPOOL [ip range]
  1. Client negotiates PPPoE using PADo, PADi and PADr with the server, both client and server move into PPP–>LCP phase
  2. Client/Server negotiate authentication, other parameters
  3. Server asks the client for a username/password (using pap, as pap is configured before chap)
  4. Client sends out a username/password configured in it’s dialer
  5. Server authenticates this username/password against its global username/password list (alternatively a AAA/Radius server)
  6. Client and Server both move on to IPCP phase
  7. Client sends an ip address of (asking for an ip address from the Server)
  8. Server hands out an IP address from its pool (in this case pppoepool)
  9. Client/Server finish IPCP phase and the link comes up


  • PADI – PPPoE Active Discovery Initiation
    • Multicast message from client to server to MAC ffff.ffff.ffff
  • PADO – PPPoE Active Discovery Offer
    • Reply to a PADI
    • Contains MAC of the server and it’s hostname
  • PADR – PPPoE Active Discovery Request
    • Response from client to server of a PADO
    • Confirms acceptance of the offer
  • PADS – PPPoE Active Discovery Session-confirmation
    • Response server to client of PADR
    • Session ID is given in this response
  • PADT – PPPoE Active Discovery Terminate
    • Termination of connection
    • Can be sent by either client or server

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