Shared IP Interfaces

You can create multiple shared IP interfaces over the same layer 2 logical interface—for example, atm 5/3.101—enabling more than one IP interface to share the same logical resources. You can configure one or more shared IP interfaces. Data sent over shared interfaces uses the same layer 2 interface. You can configure shared interfaces as you would unshared IP interfaces. Each shared interface has its own statistics.

Some layer 2 interfaces require a primary IP interface to negotiate certain IP parameters—for example, IPCP for PPP, ARP for Ethernet, and Inverse ARP for Frame Relay. If you do not configure a primary IP interface in such cases, the layer 2 interface cannot become operationally up.

A primary IP interface is the default interface for receiving data that arrives on the layer 2 interface. If you configure shared IP interfaces for the same layer 2 interface as your primary IP interface, by default data received on the layer 2 interface is received on the virtual router corresponding to the primary IP interface. A primary IP interface and all of its shared IP interfaces have the same interface location. You can configure a shared IP interface to receive data on the same layer 2 interface as a primary IP interface. You can delete primary and shared IP interfaces independently of each other.

You can create a primary IP interface as you do any other IP interface, as shown in the following example:

You do not have to configure a primary IP interface if you do not need one as described above. In the absence of a primary interface, you can still configure shared IP interfaces; however, in this scenario, data received on the layer 2 interface is discarded.

You cannot create shared IP interfaces for the following kinds of interface:

• IP floating interfaces (IP interfaces that stack over MPLS stacked tunnels)
• Loopback interfaces
• Null interfaces

For information about configuring shared IP interfaces to receive data on the same layer 2 interface as a primary IP interface, see JunosE Broadband Access Configuration Guide.

Configuring Shared IP Interfaces

To share IP interfaces:

1. Create a layer 2 interface.
   host1(config)#interface atm 5/3 host1(config-if)#interface atm 5/3.101
2. (Optional) Create a primary IP interface.
   host1(config-if)#ip address 10.1.1.1 255.255.255.255 host1(config-if)#exit
3. Create the shared IP interface.
   host1(config)#interface ip si0
4. Associate the shared IP interface with the layer 2 interface by one of the following methods:
   • Statically
     host1(config-if)#ip share-interface atm 5/3.101
   • Dynamically
     host1:vr-a:vrf-1(config-if)#ip share-nexthop 10.0.0.1
5. To fully configure the shared interface, assign an address (or make the interface unnumbered).
   host1(config-if)#ip address 2.2.2.2 255.0.0.0

Moving IP Interfaces

You can move an IP shared interface from one layer 2 interface to another by issuing the ip share-interface command to specify a different layer 2 interface. Moving an IP interface does not affect interface statistics, packets forwarded through the interface, or policies attached to the IP interface.

IP Shared Interface Statistics

Each shared interface has its own statistics. Packets transmitted on a shared IP interface are always counted only in the shared IP interface.

Subscriber Interfaces

A subscriber interface is an extension of a shared IP 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 configuring and using subscriber interfaces, see JunosE Broadband Access Configuration Guide.