PPPoE: Point to Point Protocol over Ethernet

Point to Point Protocol over Ethernet (PPPoE) visualizes Ethernet multiple point to point sessions between client hosts and an access concentrator….essentially turning a broadcast Ethernet domain into a point to multi-point environment.

The PPPoE client feature in IOS allows the router as opposed as an end user host workstation to serve as the client in the network.  This allows entire LANs to connect to the internet over a single PPPoE connection terminated to a router.

PPP interface IP addresses are assigned using an upstream DHCP server and the IP Configuration Protocol (IPCP), which is a sub protocol of PPP.  IP address negotiation must be enabled on the dialer interface in the router for it to obtain an IP address.

PPPoE also introduces an additional 8byte transport overhead, (2 bytes for the PPP header and 6 bytes for PPPoE)  in order to adjust for this in the 1500 byte MTU, you have to decrease the MTU to 1492 bytes so the entire encapsulated packet fits within the 1500 byte Ethernet frame.  For TCP sessions the Max segment size is reduced to 1452 this allows for 40 bytes in TCP and IP headers and 8 bytes in the PPPoE totaling 1500 bytes that fit into an ordinary Ethernet frame.

MTU mismatches can prevent a PPPoE connection from establishing or carrying large datagrams, so this is a good place to check when troubleshooting connections.



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