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: - To configure a
PPPoE client on an Ethernet interface, use the interface
ethernet command in global configuration mode to enter
interface configuration mode.
- Next, enable the PPPoE
on Ethernet interface.
- Finally, specify which dialer
interface to use. Use the pppoe-client dial-pool-number
command to bind the Ethernet interface to a dialer interface to
set the encapsulation to the PPPoE client.
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