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Access-Line-Identifier-Based Dynamic VLANs Overview

Dynamic VLAN subscriber interfaces that are created based on the access-line identifier (ALI) are useful in configurations with a mix of DHCP and PPPoE subscriber sessions at the same household.

When you use service VLANs (S-VLANs) to carry one service to many subscribers (1:N), each subscriber or household can have different types of traffic on multiple VLANs. The access node embeds the ALI in DHCP and PPPoE control packets. To identify all subscriber sessions for an individual subscriber or a household, you can use the ALI. The ability to uniquely identify subscribers simplifies the application of services, such as CoS and filters, to individual subscribers or households.

Because an S-VLAN corresponds to a service rather than an individual subscriber, the router uses the ALI in DHCP and PPPoE control packets instead of VLAN encapsulation to uniquely identify subscribers and facilitate application of subscriber-based services. ALIs include the agent circuit identifier (ACI) and the agent remote identifier (ARI).


The ALI method for configuring the creation of dynamic VLANs is based on the receipt of a configured trusted option, which can be the ACI, the ARI, both the ACI and the ARI, or the absence of both of ACI and ARI. Another method, called the legacy ACI method, enables dynamic VLANs to be created based only on the ACI. When the legacy method is used and the ACI is not received, no VLAN is created. The ALI method provides greater flexibility than the legacy method; for example, it can be used when the access node embeds only the ARI instead of the ACI.

Although the agent circuit identifier is also an access-line identifier, we use specific terminology to distinguish between the two configuration methods:

  • The documentation continues to use the terms agent circuit identifier, ACI, and ACI-based to refer only to VLANs and interface sets configured with the legacy method, using the agent-circuit-identifier stanza for autoconfiguration.

  • The documentation uses the terms access-line identifier, ALI, and ALI-based to refer to VLANs and interface sets configured with the access-line-identifier method, using the line-identity stanza for autoconfiguration.

You must configure only one of these methods. A CLI check prevents you from configuring both of these methods. You can use the ALI method to achieve the same results as the legacy ACI method. Apart from the fact that the ALI method uses the line-identity stanza instead of the agent-circuit-identifier stanza for autoconfiguration, the configuration is the same for both methods. The legacy ACI method might be deprecated in the future in favor of the more generic ALI method. For information about ACI VLANs, see Agent Circuit Identifier-Based Dynamic VLANs Overview.

How ALI-Based Dynamic VLANs Work

The process for creating an ALI-based dynamic VLAN is as follows:

  1. The residential gateway at a household sends a connection request to the access node.

  2. The access node identifies the household and inserts an access-line-identifier value into the header of a DHCP or PPPoE control packet. The access-line identifier can be the ACI value, the ARI value, or both.Table 1 lists where the access node can insert the ALI value for DHCP, DHCPv6, and PPPoE control packets.

    Table 1: Location of the Access-Line Identifier in DHCP, DHCPv6, and PPPoE Control Packets

    DHCP Discover Packets

    DHCPv6 Solicit Packets

    PPPoE Active Discovery Initiation (PADI) and PPPoE Active Discovery Request (PADR) Control Packets


    Option 82, suboption 1

    Option 18

    DSL Forum Agent-Circuit-ID VSA [26-1]


    Option 82, suboption 2

    Option 37

    DSL Forum Agent-Remote-ID VSA [26-2]

    The access node inserts the same ALI value into the control packets for all subsequent sessions that originate from the same household.

    When neither the ACI nor the ARI is received and accept-no-ids is configured as the line identity trusted option, then the router creates the interface set using an internally generated default string as the identifier value. It creates one such interface set for each underlying logical interface.

  3. The access node forwards the control packets to the broadband network gateway (BNG).

  4. When the BNG receives the control packets, it extracts the ALI value in the header and uses this value to build a unique dynamic VLAN subscriber interface.

    Subsequent control traffic sent from the same household contains the same ALI value. The BNG groups subscriber interfaces that have the same ALI value into an ALI interface set, also called an ALI set.

The BNG can then apply CoS and policies to the ALI set to dynamically provision traffic for a household.

Interface Hierarchy When ALI Interface Sets Are Used

The following sections describe the components of an ALI-based dynamic VLAN configuration, from bottom to top of the interface stack.

Static Physical Interface

ALI-based dynamic VLAN configurations support the following physical interface types:

  • Gigabit Ethernet

  • Aggregated Ethernet

You can configure ALI-based dynamic VLAN subscriber interfaces on Modular Port Concentrators/Modular Interface Cards (MPCs/MICs) that face the access side of the network in an MX Series router.

Underlying VLAN Interface

After you define the ALI interface set, you must configure the underlying VLAN interface to enable creation of dynamic VLAN subscriber interfaces based on the ALI. You can configure the underlying VLAN interface either dynamically (with a dynamic profile) or statically.

ALI-based dynamic VLAN configurations support the following underlying VLAN interface types:

  • Gigabit Ethernet

  • VLAN demux (demux0)


If you configure an underlying VLAN interface to support creation of ALI-based dynamic VLANs, we recommend that you use this underlying interface only for subscriber interfaces that contain ALI information in their DHCP or PPPoE control packets. If the router receives DHCP or PPPoE control packets without this information on an underlying VLAN interface configured for ALI-based dynamic VLANs, the associated subscriber interfaces might not instantiate successfully. The exception to this behavior is when you have configured accept-no-ids as the trusted option.

Dynamic ALI Interface Set

The dynamic ALI interface set groups the DHCP and PPPoE subscriber sessions that belong to a particular household and share the same unique ALI value. The router creates one ALI interface set for each household.

You must create a dynamic profile that defines the ALI interface set. The interface set is represented in the profile by the predefined dynamic variable $junos-interface-set-name. When a DHCP or PPPoE subscriber accesses the router on a particular interface, the router obtains the ALI from the DHCP or PPPoE control packets transmitted on that interface. If the ALI matches the configured trusted option, the router dynamically creates the ALI interface set when the first subscriber from that household logs in.

ALI-Based Dynamic Subscriber Interface

You must create a dynamic profile to define either a dynamic PPPoE subscriber interface for PPPoE subscriber sessions, or a dynamic IP demultiplexer (IP demux) subscriber interface for DHCP subscriber sessions. The router creates the subscriber interface when a subscriber logs in on the associated underlying VLAN interface associated with the dynamic profile that defines the ALI interface set.