(ISP). It is important that the ATM interface
status is up, the line protocol is up, and that the IP address
provided by the ISP is being pinged. If there is no output for
60 seconds, debugging the VPI/VCI values is probably incorrect
and you should contact ISP support. Finally, verify the VPI/VCI
values and make the necessary changes to the configuration.
When you are finished with this troubleshooting task, turn off
debugging by using the undebug all command.
Content 2.7 Troubleshooting Broadband ADSL
Configurations 2.7.6 Layer 2: Is Data Being
Received from the ISP? If the VPI/VCI values are correct
and the PVC is active, then the next step is to verify that
data is being sent and received on the ATM interface. The
show interfaces atm command shows the interface status
and counters for incoming and outgoing packets. If the incoming
and outgoing packet counters are incrementing, the router is
receiving and sending packets from the ISP; therefore, data is
received from the ISP and is sent to the ISP.
Content
2.7 Troubleshooting Broadband ADSL
Configurations 2.7.7 Proper PPP
Negotiation With Layer 1 set up properly, correct VPI/VCI
in use, PVC active, and data being received and sent, the next
step is to ensure that a PPP session is established properly
between the Cisco CPE router and the aggregation router of the
service provider. You can observe the PPP negotiation process
by issuing the debug ppp negotiation and debug ppp
authentication commands. PPP session setup occurs in three
stages: - Link control protocol (LCP): A
mandatory phase in which parameters to establish, configure,
and test the data-link connection are negotiated.
- Authentication: In this optional phase,
authentication is performed with the authentication protocol
(CHAP or PAP) agreed upon in LCP negotiation.
- Network Control Protocol (NCP): This mandatory phase
establishes and configures different network-layer protocols.
The most common Layer 3 protocol negotiated is IP. The routers
exchange IPCP messages to negotiate options specific to the IP
protocol.
The debug output in Figure shows the
successful PPP session establishment. In case the PPP session
does not come up successfully, there are four main points of
failure in a PPP negotiation: - There is no response
from the remote device (aggregation router of the service
provider).
- LCP is not opened.
- PAP or CHAP
authentication failure occurs.
- IPCP failures.
Content 2.8 PPPoE Simulation
Practice 2.8.1 PPPoE Simulation Practice
e-Lab Activity e-Lab Activity: PPPoE Simulation This
task will provide practice configuring a Cisco ADSL router for
connectivity to an ISP using PPPoE. In addition, it will be
necessary to configure Dynamic Host Configuration Protocol
(DHCP) services for IP addressing of local PCs at the
teleworker’s location; also, basic Port Address Translation
(PAT) will need to be configured.