PPPoA protocol stack. With PPPoA, the CPE routes the packets from the end-user PC over ATM to an aggregation router. The PPP session is established between the CPE and the aggregation router. Unlike PPPoE, PPPoA does not require host-based (PPPoE client) software. Figure shows how a PPP session is established with PPPoA between the CPE and the aggregation router. The CPE device must have a PPP username and password configured for authentication to the aggregation router that terminates the PPP session from the CPE. The aggregation router that authenticates the users can either use a local database on the aggregation router or a AAA server. The PPPoA session authentication can be based on PAP or CHAP. After the PPP username and password are authenticated, IPCP negotiation takes place and the IP address is assigned to the CPE. After the IP address has been assigned, a host route is established on both the CPE and the aggregation router. The aggregation router must assign only one IP address to the CPE, and the CPE can be configured as a DHCP server and use NAT and PAT to support multiple hosts connected via Ethernet behind the CPE.
Content 2.6 Configuring the CPE as the PPPoE or PPPoA Client 2.6.1 Configuring the CPE as the PPPoE Client There are two interfaces to configure, depending on the deployment requirements: the Ethernet interface or the ATM interface. Use the following PPPoE DSL configuration steps in addition to dial-on-demand routing (DDR)-derived commands: Step 1 Configure Ethernet 0/1 interface of the Cisco router with a PPPoE client configuration. Step 2 Create and configure the dialer interface of the Cisco router for PPPoE with a negotiated IP address and an MTU size of 1492. Step 3 Configure Ethernet 0/0 interface of the Cisco router with PAT to allow the sharing of the dynamic public IP address of the dialer interface. Use the ip tcp mss-adjust command on this interface to limit TCP maximum segment size because of PPPoE overhead. Step 4 Configure the Cisco router to be the DHCP server for the end-user PCs that are behind the router. Step 5 Configure a static default route on the Cisco router. Note
Prior to Cisco IOS software Release 12.2(13)T, you had to first configure a PPPoE virtual private dialup network (VPDN) group before the steps described above. You only had to perform this preliminary step for PPPoE, not for PPPoA. As an example, Figure shows the first two steps for configuring a CPE router acting as the PPPoE client: Step 1 Configure Ethernet interface. Step 2 Configure dialer interface. Figure shows the remaining configuration steps for a CPE router acting as the PPPoE client. Note that the figure shows abbreviated configuration scripts to reduce clutter. The full script includes all the lines from the previous steps: Step 3 Configure the Ethernet0/0 interface with PAT and an adjusted MSS size of 1452. Step 4 Enable a DHCP server on the router for clients in the customer network. Step 5 Configure an IP default route.
Content 2.6 Configuring the CPE as the PPPoE or PPPoA Client 2.6.2 Configuring the CPE as the PPPoE Client over the ATM Interface Configuring the CPE as the PPPoE client over an ATM interface follows the same steps as configuring the CPE over the Ethernet interface except that you configure the ATM interface rather than an Ethernet interface in the first step. Use the PPPoE DSL configuration steps listed here in addition to DDR-derived commands: Step 1 Configure the ATM interface (ADSL interface) of the Cisco router with an ATM permanent virtual circuit (PVC), encapsulation. Note
The pvc command establishes settings for the virtual circuit – virtual path identifier/virtual channel identifier (VPI/VCI) – on the Cisco router and must match the configuration by the service provider. ATM uses the VPI/VCI to identify an ATM VC. Step 2 Create and configure the dialer interface of the Cisco router for PPPoE with a negotiated IP address and an MTU size of 1492. Step 3 Configure Ethernet 0/0 interface of the Cisco router with PAT to allow the sharing of the dynamic public IP address of the dialer interface. Also configure the TCP maximum segment size. Step 4 Configure the Cisco router to allow it to be the DHCP server for the end-user PCs behind it. Step 5 Configure a static default route on the Cisco router. Figure shows a sample configuration for a CPE router acting as the PPPoE client over an ATM interface: Step 1 Configure ATM interface. Step 2 Configure the dialer interface. Figure shows the remaining configuration steps for a CPE router acting as the PPPoE client. Note that the figure shows abbreviated configuration scripts to reduce clutter. The full script includes all the lines from the previous steps: Step 3 Configure the Ethernet0/0 interface with PAT and an adjusted MSS size of 1452. Step 4 Enable a DHCP server on the router for clients in the customer network. Step 5 Configure an IP default route.
Content 2.6 Configuring the CPE as the PPPoE or PPPoA Client 2.6.3 Configuring a PPPoE Client Figure lists the commands used to configure the Ethernet interface (ADSL interface) of the Cisco router with an ATM PVC and encapsulation: Figure is an example of how to configure a PPPoE client on a router.
Content 2.6 Configuring the CPE as the PPPoE or PPPoA Client 2.6.4 Configuring the PPPoE DSL Dialer Interface Use the commands in Figure and in the following table to configure the PPPoE DSL dialer configuration. Dialer Commands for PPPoE DSL Configuration
Command Description interface dialer number Configure the dialer interface. ip address negotiated Enables a dynamic address from the service provider using IPCP. With IPCP, DSL routers automatically negotiate a globally unique (registered or public) IP address for the dialer interface from the service provider aggregation router. encapsulation ppp Specifies PPP encapsulation for the dialer interface. no cdp enable (Optional) Stops Cisco Discovery Protocol (CDP) advertisements from going out through the dialer interface. Figure shows the additional commands used for PPPoE DSL dialer configuration: Additional Dialer Commands for DSL Configuration
Command Description dialer pool pool_number Specifies which pool the dialer interface is assigned to. ip mtu mtu_size Sets the maximum Ethernet payload size. Reduces the MTU size from 1500 to 1492 because the PPPoE header and PPP protocol ID together require 8 bytes. ppp authentication chap [callin] (Optional) Configures the CHAP. With the keyword callin, the access server only authenticates the remote device if the remote device initiates the call. ppp chap password password (Optional) This command defines an interface-specific CHAP password. Note
Unlike an ISDN DDR configuration, DSL is always on. Therefore, a dialer list is not required to identify interesting traffic. Figure shows an example of configuring a PPPoE dialer interface on the router.
Content 2.6 Configuring the CPE as the PPPoE or PPPoA Client 2.6.5 Adjusting MSS and MTU Size In earlier topics that explained how to configure the CPE as the PPPoE Client over the Ethernet and ATM Interfaces, the configuration used the ip tcp adjust-mss and ip