Broadband Access Aggregation and DSL Configuration Guide, Cisco IOS XE Release 3S

PPPoE on ATM

This feature module describes the PPP over Ethernet (PPPoE) on ATM feature. The PPPoE on ATM feature provides the ability to connect a network of hosts over a simple bridging-access device to a remote access concentrator.

Finding Feature Information

For the latest feature information and caveats, see the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the "Feature Information for PPPoE on ATM" section

Use Cisco Feature Navigator to find information about platform support and Cisco IOS XE software image support. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.

Contents

This document includes the following sections:

•Prerequisites for PPPoE on ATM

•Restrictions for PPPoE on ATM

•Information About PPPoE on ATM

•How to Configure PPPoE on ATM

•Configuration Examples for PPPoE on ATM

•Additional References

•Feature Information for PPPoE on ATM

•Glossary

Prerequisites for PPPoE on ATM

Before you can configure PPPoE on ATM, you need to specify a virtual template for the PPPoE sessions using the virtual-template command.

Restrictions for PPPoE on ATM

The following restrictions apply when PPPoE on ATM is used:

•PPPoE is not supported on Frame Relay.

•PPPoE over ATM AAl5Mux is not supported on ASR series 1000 routers.
For more information, refer to the PPPoEoA over ATM AAL5Mux feature: http://www.cisco.com/en/US/docs/ios/bbdsl/configuration/guide/bba_pppoeoa_aal5mux.html

•PPPoE is not supported on any other LAN interfaces such as FDDI and Token Ring.

•Fast switching is supported. PPPoE over RFC 1483 fibswitching is supported for IP. All other protocols are switched over process switching.

•Bridging is supported on the ATM permanent virtual connections (PVCs) running PPPoE.

•PPPoE is supported on ATM PVCs compliant with RFC 1483 only.

•Only dial-in mode is supported. Dial-out mode will not be supported.

Information About PPPoE on ATM

The PPPoE on ATM feature provides the ability to connect a network of hosts over a simple bridging-access device to a remote access concentrator. With this model, each host utilizes its own PPPoE stack and the user is presented with a familiar user interface. Access control, billing and type of service can be done on a per-user, rather than a per-site, basis. Before a point-to-point connection over Ethernet can be provided, each PPP session must learn the Ethernet address of the remote peer and establish a unique session identifier. A unique session identifier is provided by the PPPoE Discovery Stage protocol.

Figure 1 shows a sample network topology using PPPoE on ATM.

Figure 1 PPPoE on ATM Sample Network Topology

PPPoE Stage Protocols

PPPoE has two distinct stage protocols. The stage protocols are listed and summarized in Table 1.

There are four steps to the Discovery Stage:

1. Host broadcasts a PPPoE Active Discovery Initiation (PADI) packet.

2. When the access concentrator receives a PADI that it can serve, it replies by sending a PPPoE Active Discovery Offer (PADO) packet to the host.

3. Because the PADI was broadcast, the host may receive more than one PADO packet. The host looks through the PADO packets it receives and chooses one. The choice can be based on the AC name or the services offered. The host then sends a single PPPoE Active Discovery Request (PADR) packet to the access concentrator that it has chosen.

4. When the access concentrator receives a PADR packet, it prepares to begin a PPP session. It generates a unique SESSION_ID for the PPPoE session and replies to the host with a PPPoE Active Discovery Session-confirmation (PADS) packet.

When a host wishes to initiate a PPPoE session, it must first perform discovery to identify the Ethernet MAC address of the peer and establish a PPPOE SESSION_ID. Although PPP defines a peer-to-peer relationship, discovery is inherently a client/server relationship. In the discovery process, a host (the client) discovers an access concentrator (the server). Based on the network topology, there may be more than one access concentrator that the host can communicate with. The Discovery Stage allows the host to discover all access concentrators and then select one. When discovery is completed, both the host and the selected access concentrator have the information they will use to build their point-to-point connection over Ethernet.

Benefits of PPPoE on ATM

The PPPoE on ATM feature provides service-provider digital subscriber line (DSL) support. As service providers begin DSL deployments, two of their most significant goals are to ease and facilitate consumer end adoption and to preserve as much of the dialup model as possible. PPPoE serves to advance both of these goals by leveraging ethernet scale curves and embedded base (such as ATM NICs) and by preserving the point-to-point session used by internet service providers (ISPs) in today's dialup model.

Using a PPPoE client (available from RouterWare), a PPP session can be initiated on an Ethernet connected client through a standard ADSL modem. The session is transported over the ATM DSL link via RFC 1483 Ethernet bridged frames and can terminate either in the LAN emulation client (LEC) central office or the ISP point of presence (POP). The termination device can be an aggregation box such as the Cisco 6400 or a router such as the Cisco 7200 series platforms.

As customers deploy asymmetric DSL (ADSL), they will encounter the need to enable users to access remote-access concentrators via simple bridges connecting Ethernet and ATM networks.

How to Configure PPPoE on ATM

See the following sections for configuration tasks for the PPPoE on ATM feature. Each task in the list indicates if the task is optional or required.

•Enabling PPP over ATM (Required)

•Creating and Configuring a Virtual Template (Optional)

•Specifying an ATM Subinterface (Optional)

•Creating an ATM PVC (Required)

•Enabling PPPoE on an ATM PVC (Required)

Enabling PPP over ATM

After you configure the Cisco router or access server for Ethernet encapsulation, you must configure the physical interface with the PVC and apply a virtual template with PPP encapsulation to the PVC that it applies to. To configure the physical interface that will carry the PPPoE session and link it to the appropriate virtual template interface, use the following commands:

SUMMARY STEPS

1. enable

2. configure terminal

3. bba-group pppoe {group-name | global}

4. virtual-template template-number

5. sessions per-vc limit per-vc-limit [threshold threshold-value]

6. sessions per-mac limit per-mac-limit

7. exit

8. interface atm slot/subslot/port[.subinterface] [multipoint | point-to-point]

9. ip address ip-address mask [secondary]

10. range [range-name] pvc start-vpi/start-vci end-vpi/end-vci

11. dbs enable [aggregated | maximum]

12. protocol pppoe group {group-name | global}
or
encapsulation aal5snap

13. create-on-demand

14. end

Note You can use the virtual-template, sessions per-vc, and sessions per-mac commands in any order.

DETAILED STEPS

bba-group pppoe {group-name | global} 

virtual-template template-number

sessions per-vc limit per-vc-limit [threshold 
threshold-value]

range [range-name] pvc start-vpi/start-vci 
end-vpi/end-vci 

dbs enable [aggregated | maximum]

protocol pppoe group {group-name | global}

Router(config-if-atm-range-pvc)# encapsulation 
aal5snap

Creating and Configuring a Virtual Template

Prior to configuring the ATM PVC for PPPoE on ATM, you typically create and configure a virtual template.

Other optional configuration commands can be added to the virtual template configuration. All PPP parameters are managed within the virtual template configuration. Configuration changes made to the virtual template are automatically propagated to the individual virtual access interfaces. Multiple virtual access interfaces can spawn from a single virtual template; hence, multiple PVCs can use a single virtual template.

Cisco IOS software supports up to 25 virtual template configurations. If greater numbers of tailored configurations are required, an authentication, authorization, and accounting (AAA) server may be employed.

If the parameters of the virtual template are not explicitly defined before the ATM PVC is configured, the PPP interface is brought up using default values from the virtual template identified. Some parameters (such as an IP address) take effect only if specified before the PPP interface comes up. Therefore, Cisco recommends that you explicitly create and configure the virtual template before configuring the ATM PVC to ensure such parameters take effect. Alternatively, if parameters are specified after the ATM PVC has already been configured, you should issue a shutdown command followed by a no shutdown command on the ATM subinterface to restart the interface; this restart will cause the newly configured parameters (such as an IP address) to take effect.

Network addresses for the PPP-over-ATM connections are not configured on the main ATM interface or subinterface. Instead, these connections are configured on the appropriate virtual template or obtained via AAA.

The virtual templates support all standard PPP configuration commands; however, not all configurations are supported by the PPP-over-ATM virtual access interfaces. These restrictions are enforced at the time the virtual template configuration is applied (cloned) to the virtual access interface. These restrictions are described in the following paragraphs.

Only standard first-in, first-out (FIFO) queueing is supported when applied to PPP-over-ATM virtual access interfaces. Other types of queueing that are typically configured on the main interface are not (for example, fair queueing). If configured, these configuration lines are ignored when applied to a PPP-over-ATM interface.

Although Cisco Express Forwarding (CEF) switching is supported, fast switching, flow, and optimum switching are not; these configurations are ignored on the PPP-over-ATM virtual access interface. CEF is enabled by default for IP. All other protocol traffic will be processed switched.

Note The PPP reliable link that uses Link Access Procedure, Balanced (LAPB) is not supported.

Because an ATM PVC is configured for this feature, the following standard PPP features are not applicable and should not be configured:

•Asynchronous interfaces

•Dialup connections

•Callback on PPP

To create and configure a virtual template, use the following commands:

SUMMARY STEPS

1. enable

2. configure terminal

3. interface virtual-template number

4. encapsulation ppp

5. ip unnumbered gigabitethernet slot/subslot/port[.subinterface]

6. end

DETAILED STEPS

interface virtual-template number

encapsulation ppp

ip unnumbered gigabitethernet 
slot/subslot/port[.subinterface]

Specifying an ATM Subinterface

After you create a virtual template for PPPoE on ATM, specify a multipoint or point-to-point subinterface per PVC connection. To specify an ATM multipoint subinterface, use the following commands:

SUMMARY STEPS

1. enable

2. configure terminal

3. interface atm slot/subslot/port[.subinterface] [multipoint | point-to-point]

4. end

DETAILED STEPS

interface atm slot/subslot/port[.subinterface] 
[multipoint | point-to-point]

Creating an ATM PVC

After you create a virtual template and specify an ATM subinterface, you must create an ATM PVC.

The peak rate value is typically identical to the average rate or some suitable multiple thereof.

The average rate value should be set to the line rate available at the remote site, because the remote line rate will typically have the lowest speed of the connection.

For example, if the remote site has a T1 link, set the line rate to 1.536 Mbps. Because the average rate calculation on the ATM PVC includes the cell headers, a line rate value plus 10 or 15 percent may result in better remote line utilization.

The burst size depends on the number of cells that can be buffered by receiving ATM switches and is coordinated with the ATM network connection provider. If this value is not specified, the default, which is the equivalent to one maximum length frame on the interface, is used.

Operations, Administration and Maintenance (OAM) F5 cell loopback is provided by the remote AXIS shelf so OAM may be enabled. However, PPPoE on ATM is not typically an end-to-end ATM connection, and therefore enabling OAM is not recommended.

Once you configure the router for PPPoE on ATM, the PPP subsystem starts and the router attempts to send a PPP configure request to the remote peer. If the peer does not respond, the router periodically goes into a "listen" state and waits for a configuration request from the peer. After a timeout (typically 45 seconds), the router again attempts to reach the remote router by sending configuration requests.

To create an ATM PVC, use the following commands:

SUMMARY STEPS

1. enable

2. configure terminal

3. interface atm slot/subslot/port[.subinterface] multipoint | point-to-point

4. pvc [name] vpi/vci

5. encapsulation aal5snap

6. end

DETAILED STEPS

pvc [name] vpi/vci

encapsulation aal5snap

Enabling PPPoE on an ATM PVC

To enable PPPoE on an ATM PVC, use the following commands:

SUMMARY STEPS

1. enable

2. configure terminal

3. interface atm slot/subslot/port[.subinterface] [multipoint | point-to-point]

4. pvc [name] vpi/vci

5. pppoe max-sessions number-of-sessions [threshold-sessions number of sessions]

6. protocol pppoe

7. end

DETAILED STEPS

Configuration Examples for PPPoE on ATM

This section provides the following configuration example:

•PPPoE on ATM : Example

PPPoE on ATM : Example

The following example configures PPPoE on ATM to accept dial-in PPPoE sessions. The virtual access interface for the PPP session is cloned form virtual template interface 1. On subinterface ATM 2/0.1, ATM PVC with VPI 0 and VCI 60 is configured with Logical Link Control (LLC)/Subnetwork Access Protocol (SNAP) encapsulation and is configured to run PPPoE.

bba-group pppoe pppoe-group 

 virtual-template 1

 sessions per-vc limit 1 
 sessions per-mac limit 4000

interface atm 2/0.1 multipoint

 ip address 192.0.10.2 255.255.255.0 secondary

 range pvc 1/100 1/202

 pvc 0/60

  dbs enable

  encapsulation aal5snap

  protocol pppoe group two

  create on-demand

interface virtual-template 1

  ip addr 10.0.1.2 255.255.255.0

  mtu 1492

Where to Go Next

•If you want to enable PPP authentication on the virtual template using the ppp authentication chap command, refer to the "Configuring Virtual Template Interfaces" chapter in the Cisco IOS Dial Solutions Configuration Guide.

•If you want to configure an authentication, authorization, and accounting (AAA) server, refer to the "Configuring per-User Configuration" chapter in the Cisco IOS Dial Solutions Configuration Guide.

Additional References

The following sections provide references related to the PPPoE on ATM feature.

Related Documents

Standards

MIBs

RFCs

Technical Assistance

Feature Information for PPPoE on ATM

Table 2 lists the features in this module and provides links to specific configuration information.

Use Cisco Feature Navigator to find information about platform support and software image support. Cisco Feature Navigator enables you to determine which Cisco IOS XE software images support a specific software release, feature set, or platform. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.

Note Table 2 lists only the Cisco IOS XE software release that introduced support for a given feature in a given Cisco IOS XE software release train. Unless noted otherwise, subsequent releases of that Cisco IOS XE software release train also support that feature.

Glossary

AAL5—ATM Adaptation Layer 5

ADSL—Asymmetric Digital Subscriber Line

ATM—Asynchronous Transfer Mode

CPCS—Common Part of Convergence Sublayer

CPI—Common Part Indicator

CRC—Cyclic Redundancy Check

DSLAM—Digital Subscriber Line Access Multiplexer

FCS—Frame Check Sequence

IETF—Internet Engineering Task Force

ID-Identifier

IP—Internet Protocol

L2TP—Layer two Tunneling Protocol

LAN—Local Area Network

LLC—Logical Link Control

MAC—Media Access Control

PDU—Protocol Data Unit

PPP—Point to Point Protocol

PPPoE—Point to Point Protocol over Ethernet

PVC—Permanent Virtual Connection

VPDN—Virtual Private Dialup Network

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Any Internet Protocol (IP) addresses and phone numbers used in this document are not intended to be actual addresses and phone numbers. Any examples, command display output, network topology diagrams, and other figures included in the document are shown for illustrative purposes only. Any use of actual IP addresses or phone numbers in illustrative content is unintentional and coincidental.

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