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How to configure PPPoE?PPPoE provides an emulated (and optionally authenticated) point-to-point link across a shared medium, typically a broadband aggregation network such as those found in DSL service providers. In fact, a very common scenario is to run a PPPoE client on the customer side (commonly on a SOHO Linksys or similar brand router), which connects to and obtains its configuration from the PPPoE server (head-end router) at the ISP side. Note that ATM is typically run between the customer's modem and the DSLAM, though it will be transparent in this lab since our PPPoE client exists on a separate device. Server ConfigurationThe first task at the ISP end is to configure a Broadband Aggregation (BBA) group which will handle incoming PPPoE connection attempts. We'll name this MyGroup, and bind it to a virtual template to be created shortly. Here we can also apply PPPoE session limits. For example, we can limit the number of sessions established per client MAC address (setting this limit to 2 allows a new session to be established immediately if the prior session was orphaned and is waiting to expire). This is an optional step. Next we'll create the virtual template for the customer-facing interface. When a PPPoE client initiates a session with this router, the router automatically spawns a virtual interface to represent that point-to-point connection. At a minimum, we'll need to configure two items on our virtual template: an IP address, and a pool of IP addresses from which clients are assigned a negotiated address (similar in operation to DHCP). You may be wondering where the IP pool is defined. Well, it isn't; that's what we have to do next. Back in global configuration mode, we define a local IP pool named MyPool with the starting and ending addresses of an IP range. If you've configured DHCP on IOS before, you should find this task very familiar. Last, we need to enable our PPPoE group on the interface facing the customer network. Note that this interface should not have an IP address; the addressing is provided by our virtual template. Client ConfigurationClient configuration is relatively simple. We create a dialer interface to handle the PPPoE connection, and tie it to a physical interface which provides the transport. Creating our PPPoE dialer interface: The line ip address negotiated instructs the client to use an IP address provided by the PPPoE server.The PPP header adds 8 bytes of overhead to each frame. Assuming the default Ethernet MTU of 1500 bytes, we'll want to lower our MTU on the dialer interface to 1492 to avoid unnecessary fragmentation. Lastly we assign our ISP-facing interface to our newly created PPPoE dial group: If all is well, you should see a notification indicating the PPPoE session has successfully formed: We can verify that interface Dialer1 has negotiated an IP address from the ISP router: show pppoe session shows our PPPoE session with the ISP router terminated on Dialer0, via FastEthernet0/0:AuthenticationOf course, at this point anyone can connect via PPPoE. Generally we only want to provide service to trusted (e.g. paying) customers, so adding some low-layer authentication would be a prudent step. PPP can use PAP or CHAP to authenticate clients, with the later heavily preferred. On our ISP router, we'll create a local user account name CPE and the password MyPassword. (In real practice, account creation is typically performed on a back-end server and referenced via RADIUS or TACACS+ rather than being stored locally.) Next we enforce CHAP authentication on our virtual template: This will terminate our client session, as we can see from the logs on CPE: To reestablish the connection from CPE, we'll need to enter the proper credentials: We should see the PPPoE session come back up a few seconds later after successfully authenticating. debug ppp authentication can be used on the ISP router to monitor the CHAP exchange: |
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Post date: 2009-06-07 00:08:09 Post date GMT: 2009-06-06 16:08:09 Post modified date: 2010-07-22 23:35:24 Post modified date GMT: 2010-07-22 15:35:24 |
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