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 16 illustrates the relationship between subscriber interfaces, an associated primary IP interface, and an associated Ethernet interface.
Figure 16: 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.
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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 . |
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