The difference between PPPoE and DHCP

What is DHCP?

DHCP (Dynamic Host Configuration Protocol) is a standardized client/server network protocol that dynamically assigns IP addresses and other related configuration information to network devices. As you know, each device in a TCP/IP network should have a unique IP address to access the network. Therefore, if your network devices are not configured with DHCP, network administrators must manually configure IP addresses if they want to add new computers or move computers from one subnet to another. Generally, the DHCP architecture consists of a DHCP client, a DHCP server and a DHCP relay agent (usually a router or switch with DHCP), as shown in the following figure.

The DHCP protocol automatically assigns and updates IP addresses and other configuration information on the network. a DHCP server provides information to DHCP clients through a series of message exchanges. The DHCP protocol can be used to forward DHCP packets between the client and server when the client and server are not on the same physical subnet.

The DHCP protocol enables network users to configure anywhere on the network and automatically obtain an IP address when they reconnect. On the other hand, the DHCP protocol also provides a faster and more reliable way for network administrators to configure IP addresses, which helps reduce errors caused by manually configuring IP addresses. DHCP can also help save limited IP address space, since only hosts connected to the network are assigned IP addresses.

What is PPPoE?

PPPoE stands for Point-to-Point Protocol over Ethernet, which is a network tunneling protocol that encapsulates Point-to-Point Protocol (PPP) in the Ethernet framework. It enables an Ethernet host to connect to a remote access concentrator through a simple bridging device. Through the PPPoE protocol, the remote access device enables control and billing of each access user. Compared with traditional access methods, the PPPoE protocol has a high performance-to-price ratio, and it is widely used in a range of applications, including cell networking construction, and the current popular broadband access method, ADSL, uses the PPPoE protocol. Usually, PPPoE architecture consists of PPPoE client, PPPoE server, host and ADSL modem, etc.

With PPPoE, users can dial up from one router (PPPoE client) to another router (PPPoE server) via BRAS (Broadband Remote Access Server), and then establish a point-to-point connection and transmit packets over that connection. To use PPPoE, you need the username and password provided by your ISP to establish the connection. However, with today’s networks that integrate the modem into the connection, you only need to set up the username and password once and the modem can automatically connect to the network as soon as you turn it on.

Since the BRAS (Broadband Remote Access Server) has numerous users sharing the same physical connection, thus sending traffic to and from the broadband remote access device on the ISP network, the PPPoE protocol keeps track of the user’s traffic and which user should be billed.

What is the difference between PPPoE and DHCP?

The DHCP protocol is a method of assigning unique IP addresses to devices on the network (i.e. computers, smartphones, Gigabit Ethernet switches, etc.) to facilitate traffic. PPPoE is a method of encapsulating network traffic based on credential access (i.e. user names and passwords). PPPoE needs to be properly configured before users can actually connect to the Internet, but DHCP does not require configuration and is installed and ready to use. Therefore, using the DHCP protocol to connect to an ISP can eliminate problems caused by configuration. Just like a computer on a network, you do not need to pre-configure the computer. You simply leave everything on hold automatically and leave the configuration to the ISP server.

The specific processes of the two phases of the PPPoE protocol session discovery and session are as follows.

Discovery (Discovery) phase: In this phase, the user host broadcasts to find all the connected access concentrators (or switches) and obtain their Ethernet MAC addresses. It then selects the hosts to be connected and determines the identification number of the PPP session to be established. The discovery phase has four steps: the host broadcasts the originating packet (PADI), accesses the concentrator, the host selects an appropriate PADO packet and prepares to start the PPP session. When this phase is completed, both ends of the communication know the PPPoESESSION-ID and the Ethernet address of the other end and together they uniquely define the PPPoE session.

PPP session phase: The user host and the access concentrator conduct a PPP session based on the PPP session connection parameters negotiated during the discovery phase. Once the PPPoE session starts, PPP data can be sent in any other PPP encapsulated form. All Ethernet frames are unicast. the SESSION-ID of the PPPoE session must not change and must be the value assigned during the discovery phase.

The differences between PPPoE and DHCP can be summarized as follows.

The DHCP protocol does not require authentication and the IP address is not known to you when you turn on the network. All you have to do is wait for the DHCP server to assign you a random IP address from among all Internet IP addresses. However, the PPPoE protocol requires authentication first, and will only assign you a valid IP address if your account password is correct.

Since PPPoE protocol enables a large number of hosts to form a network unit and control the cost of each host for its billing, it can be widely used in enterprise networks, campus networks, etc. And the popular ADSL broadband access method has adopted PPPoE protocol. For DHCP protocol, it is usually used to dynamically assign IP addresses to a company’s LAN or Internet.

Conclusion

Both PPPoE and DHCP protocols are essential for network connectivity, which can be used to obtain IP addresses. The difference between PPPoE and DHCP protocols is the method of obtaining an IP. With PPPoE, you must first configure a username and password, which will allow you to authenticate the next time you turn on the network to obtain an IP address. With DHCP, an IP address will be automatically assigned to you without authentication.