Subscriber Interfaces and PPPoE Overview

You can configure the router to dynamically create Point-to-Point Protocol over Ethernet (PPPoE) logical interfaces on statically created underlying Ethernet interfaces. The router automatically and transparently creates the dynamic interface in response to the receipt of a PPPoE Active Discovery Request (PADR) control packet on the underlying interface. Because the router creates a dynamic PPPoE logical interface on demand when a subscriber logs in to the network, dynamic PPPoE logical interfaces are also referred to as dynamic PPPoE subscriber interfaces.

To enable the router to create a dynamic PPPoE logical interface on an underlying Ethernet interface, you define the attributes of the PPPoE logical interface in a dynamic profile, and then attach the dynamic profile to an Ethernet interface configured with PPPoE encapsulation. When the router receives a PADR control packet from a PPPoE client on an underlying interface with a PPPoE dynamic profile attached, the router uses the attributes defined in the profile to instantiate a dynamic PPPoE subscriber interface to handle the PPPoE session.

This overview covers the following topics:

Benefits of Using Dynamic PPPoE Subscriber Interfaces

Configuring and using dynamic PPPoE subscriber interfaces offers the following benefits:

On-demand dynamic interface creation
Configuring dynamic PPPoE subscriber interfaces provides the flexibility of dynamically creating the PPPoE subscriber interface only when needed; that is, when a subscriber logs in on the associated underlying Ethernet interface. By contrast, statically created interfaces allocate and consume system resources when the interface is created. Configuring and using dynamically created interfaces helps you effectively and conveniently manage edge or access networks in which large numbers of subscribers are constantly logging in to and logging out from the network on a transient basis.
Dynamic removal of PPPoE subscriber interfaces without manual intervention
When the PPPoE subscriber logs out or the PPPoE session is terminated, the router dynamically deletes the associated PPPoE subscriber interface without your intervention, thereby restoring any consumed resources to the router.
Use of dynamic profiles to efficiently manage multiple subscriber interfaces
A dynamic profile is a set of characteristics that can be dynamically assigned to subscriber interfaces. By using a profile, you reduce the management of a large number of interfaces by applying a set of common characteristics to multiple interfaces. When you configure a dynamic profile for PPPoE, you use predefined dynamic variables in the profile to represent information that varies from subscriber to subscriber, such as the logical unit number and underlying interface name. These variables are dynamically replaced with the values supplied by the network when the subscriber logs in.
Denial of service (DoS) protection
You can optionally configure the underlying Ethernet interface with certain PPPoE-specific attributes that can reduce the potential for DoS attacks. Duplicate protection, which is disabled by default, prevents activation of another dynamic PPPoE logical interface on the underlying interface when a PPPoE logical interface for the same client is already active on the underlying interface. You can also specify the maximum number of PPPoE sessions that the router can activate on the underlying interface. By enabling duplicate protection and restricting the maximum number of PPPoE sessions on the underlying interface, you can ensure that a single toxic PPPoE client cannot monopolize allocation of the PPPoE session.
Support for dynamic PPPoE subscriber interface creation from PPPoE service name tables
You can assign a previously configured PPPoE dynamic profile to a named, empty, or any service entry in a PPPoE service name table, or to an agent circuit identifier/agent remote identifier (ACI/ARI) pair defined for these services. The router uses the attributes defined in the profile to instantiate a dynamic PPPoE subscriber interface based on the service name, ACI, and ARI information provided by the PPPoE client during PPPoE negotiation. To specify the routing instance in which to instantiate the dynamic PPPoE subscriber interface, you can assign a previously configured routing instance to a named, empty, or any service, or to an ACI/ARI pair defined for these services. The dynamic profile and routing instance configured for the PPPoE service name table overrides the dynamic profile and routing instance assigned to the PPPoE underlying interface on which the dynamic subscriber interface is created.

Supported Platforms for Dynamic PPPoE Subscriber Interfaces

Configuration of dynamic PPPoE subscriber interfaces over static underlying Ethernet interfaces is supported on the following routing platforms:

Intelligent Queuing 2 (IQ2) PICs on M120 Multiservice Edge Router and M320 Multiservice Edge Router
Trio MPC/MIC interfaces on MX Series Ethernet Service Routers

Sequence of Operations for PPPoE Subscriber Access

When a PPPoE subscriber logs in to the network, the PPPoE protocol defines the sequence of operations by which a connection is established and traffic flow is enabled on the dynamic PPPoE subscriber interface. Similarly, when the PPPoE subscriber logs out from the network, PPPoE defines the sequence that occurs to terminate the connection and remove the dynamic PPPoE subscriber interface from the router.

The router creates a dynamic PPPoE subscriber interface for each new PPPoE session, and removes the dynamic PPPoE subscriber interface when the session is terminated due to subscriber logout, PPP negotiation failure, or down status of the underlying Ethernet interface. Dynamic PPPoE subscriber interfaces are never reused for multiple PPPoE sessions.

Sequence When a PPPoE Subscriber Logs In

In a PPPoE subscriber network, the router acts as a remote access concentrator, also known as a PPPoE server. For a PPPoE client to initiate a PPPoE session with a PPPoE server, it must first perform PPPoE Discovery to identify the Ethernet MAC address of the remote access concentrator that can service its request. Based on the network topology, there may be more than one remote access concentrator with which the client can communicate. The Discovery process enables a PPPoE client to find all remote access concentrators and then select one to connect to.

The following sequence occurs when a PPPoE subscriber logs in to the network. Steps 1 through 5 in this sequence are part of the PPPoE Discovery process.

The PPPoE client broadcasts a PPPoE Active Discovery Initiation (PADI) packet to all remote access concentrators in the network.
One or more remote access concentrators respond to the PADI packet by sending a PPPoE Active Discovery Offer (PADO) packet, indicating that they can service the client request. The PADO packet includes the name of the access concentrator from which it was sent.
The client sends a unicast PPPoE Active Discovery Request (PADR) packet to the access concentrator it selects.
On receipt of the PADR packet on the underlying interface associated with a PPPoE dynamic profile, the router uses the attributes configured in the dynamic profile to create the dynamic PPPoE logical interface.
The router sends a PPPoE Active Discovery Session (PADS) packet to confirm establishment of the PPPoE connection.
The PPP Link Control Protocol (LCP) negotiates the PPP link between the client and the PPPoE server.
The subscriber is authenticated using the PPP authentication protocol (CHAP or PAP) configured in the PPPoE dynamic profile.
The PPP Network Control Protocol (NCP) negotiates the IP routing protocol and network family.
The PPP server issues an IP access address for the client, and the router adds the client access route to its routing table.
The router instantiates the dynamic profile and applies the attributes configured in the profile to the dynamic PPPoE subscriber interface.
PPP NCP negotiation completes, enabling traffic flow between the PPPoE client and the PPPoE server.

Sequence When a PPPoE Subscriber Logs Out

The following sequence occurs when a PPPoE subscriber logs out of the network:

The client terminates the PPP connection and the router receives an LCP termination request.
The router removes the client access router from its routing table.
The router sends or receives a PPPoE Active Discovery Termination (PADT) packet to end the PPPoE connection.
The router deactivates the subscriber, gathers final statistics for the PPPoE session, and sends the RADIUS server an Acct-Stop accounting message.
The router de-instantiates the PPPoE dynamic profile and removes the PPPoE logical interface. The router does not reuse the PPPoE logical interface for future dynamic PPPoE sessions.