Characteristics of Subscriber Interfaces

The following sections describe the characteristics of subscriber interfaces:

Relationship to Shared IP Interfaces

A subscriber interface is an extension of a shared IP interface. A shared IP interface is one of a group of IP interfaces that use the same layer 2 interface.

Shared IP interfaces are unidirectional—they can transmit but not receive traffic. In contrast, subscriber interfaces are bidirectional—they can both receive and transmit traffic.

For details about shared IP interfaces, see the Shared IP Interfaces section in the JunosE IP, IPv6, and IGP 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 3 illustrates the relationship between subscriber interfaces, an associated primary IP interface, and an associated Ethernet interface.

Figure 3: Subscriber Interfaces over Ethernet

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 the Using the Giaddr to Identify the Primary Interface for Dynamic Subscriber Interfaces section in DHCP Relay and BOOTP Relay Overview.

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 without VLANs. Using subscriber interfaces, the router can demultiplex or separate the traffic associated with different subscribers.

You can configure subscriber interfaces with VLANs. If you do so, the E Series router demultiplexes packets by using first the VLAN and then the subscriber interface.

Moving Interfaces

A shared IP interface that has associated subscriber demultiplexing attributes retains these attributes when it moves.

For details about moving shared IP interfaces, see the Moving IP Interfaces section in the JunosE IP, IPv6, and IGP Configuration Guide.

Preventing IP Spoofing

You can prevent IP spoofing on subscriber interfaces by using media access control (MAC) address validation.

For information about configuring MAC address validation, see the MAC Address Validation section in the JunosE IP, IPv6, and IGP Configuration Guide.

For information about the relationship between the MAC address validation state and dynamically created subscriber interfaces, see the Inheritance of MAC Address Validation State for Dynamic Subscriber Interfaces section in DHCP Relay and BOOTP Relay Overview.

Routing Protocols

You configure unicast routing protocols on subscriber interfaces in the same way that you configure routing protocols on primary IP interfaces, provided that you configure them to use unicast addressing when communicating with a peer. You can also enable multicast routing protocols such as IGMP on subscriber interfaces; however, we do not recommend this type of configuration.

Policies and QoS

You can configure policies, such as rate limiting and filtering, and quality of service (QoS) for subscriber interfaces in the same way that you do for primary IP interfaces. For more information, see the JunosE Policy Management Configuration Guide and the JunosE Quality of Service Configuration Guide.

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