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Agent Circuit Identifier-Based Dynamic VLANs Overview

Dynamic VLAN subscriber interfaces that are created based on the agent circuit identifier (ACI) value 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. To identify all subscriber sessions for an individual subscriber or a household, you can use the value of the ACI string. 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 ACI information in DHCP and PPPoE control packets instead of VLAN encapsulation to uniquely identify subscribers and facilitate application of subscriber-based services.


The legacy ACI method for configuring the creation of dynamic VLANs is based on the receipt of only the ACI. When the ACI is not received, no VLAN is created. An alternative method provides greater flexibility than the legacy method. The access-line-identifier (ALI) method enables dynamic VLANs to be created based on receipt of the ACI, the agent remote identifier (ARI), both the ACI and the ARI, or the absence of both of ACI and ARI.

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 ALI VLANs, see Access-Line-Identifier-Based Dynamic VLANs Overview.

How ACI-Based Dynamic VLANs Work

The process for creating an ACI-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 ACI value into the header of a DHCP or PPPoE control packet. The access node can insert the ACI value into one of the following DHCP options or PPPoE control packets:

    • Option 82 of DHCP packets

    • Option 18 of DHPv6 packets

    • The DSL Forum Agent-Circuit-ID VSA [26-1] (option 0x105) of PPPoE Active Discovery Initiation (PADI) and PPPoE Active Discovery Request (PADR) control packets

    The access node inserts the same ACI value to all subsequent sessions that originate from the same household.

  3. The access node forwards the control packets to the BNG.

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

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

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

Interface Hierarchy When ACI Interface Sets Are Used

The following describes the components of an ACI-based dynamic VLAN configuration, from bottom to top of the interface stack:

Static Physical Interface

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

  • Gigabit Ethernet

  • Aggregated Ethernet

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

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

  • Gigabit Ethernet

  • VLAN demux (demux0)


When you configure an underlying VLAN interface to support creation of ACI-based dynamic VLANs, we recommend that you use this underlying interface only for subscriber interfaces that contain agent-circuit-identifier information in their DHCP or PPPoE control packets. If the router receives DHCP or PPPoE control packets without agent-circuit-identifier information on an underlying VLAN interface configured for ACI-based dynamic VLANs, the associated subscriber interfaces might not instantiate successfully.

Dynamic ACI Interface Set

The dynamic ACI interface set groups the DHCP and PPPoE subscriber sessions that belong to a particular household and share a common unique ACI value. The router creates one ACI interface set per household.

You must create a dynamic profile to define the ACI interface set, which is represented in the profile by the Junos OS predefined dynamic variable $junos-interface-set-name. When a DHCP or PPPoE subscriber accesses the router on a particular interface, the router obtains the agent-circuit-identifier information from the DHCP or PPPoE control packets transmitted on that interface and dynamically creates the ACI interface set when the first subscriber from that household logs in.

ACI-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 ACI interface set.