Figure 18: Subscriber Interfaces Over Ethernet; Relationship To Primary Ip Interfaces; Ethernet Interfaces And Vlans - Juniper JUNOSE 11.1.X - BROADBAND ACCESS CONFIGURATION GUIDE 6-4-2010 Configuration Manual

JUNOSe 11.1.x Broadband Access Configuration Guide

Relationship to Primary IP Interfaces

A subscriber interface operates only with a primary IP interface a normal IP interface
on a supported layer 2 interface, such as Ethernet. You create a primary interface
by assigning an IP address to the Ethernet interface. Although you can configure a
subscriber interface directly on an Ethernet interface, the subscriber interface does
not operate until you assign an IP address to the Ethernet interface.
To configure a subscriber interface you must associate either a source address or a
destination address with the interface. The router receives packets on a subscriber
interface after demultiplexing the packet according to the specified source address
or destination address. You can associate multiple source addresses or multiple
destination addresses with a subscriber interface. However, a single primary interface
and its associated subscriber interfaces can only demultiplex source addresses or
destination addresses at any given time.
For example, Figure 18 on page 606 illustrates the relationship between subscriber
interfaces, an associated primary IP interface, and an associated Ethernet interface.

Figure 18: Subscriber Interfaces over Ethernet

When the router receives traffic on a primary interface, the primary interface performs
a lookup in its demultiplexing table. If the result of the lookup is a subscriber interface,
the traffic is received on the associated subscriber interface.
NOTE: You can use the set dhcp relay giaddr-selects-interface command to specify
that the primary interface is identified by information in the giaddr field of DHCP
ACK messages. By default, the router identifies the primary interface based on the
interface used by the DHCP-destined packets. See "Using the Giaddr to Identify the
Primary Interface for Dynamic Subscriber Interfaces" on page 502 .

Ethernet Interfaces and VLANs

In the absence of VLANs, Ethernet does not have a demultiplexing layer. A subscriber
interface adds a demultiplexing layer for an Ethernet interface that is configured
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Subscriber Interfaces Overview