Shared Interfaces Overview

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A shared interface is a physical interface that is owned by the Root System Domain (RSD), with logical interfaces under it assigned to different Protected System Domains (PSDs).

Shared Interfaces Basics

Any Flexible PIC Concentrator (FPC) that has not been assigned to a specific PSD can be used to host shared interfaces. With JUNOS Release 9.3, only SONET Physical Interface Cards (PICs) can be configured as shared interfaces. On the RSD, multiple logical interfaces are configured on the physical SONET interface and each individual logical interface is assigned to a different PSD. On the PSD, each assigned logical SONET interface is configured and peered with an uplink tunnel interface (ut-fpc/pic/slot), which transports packets between the PSD and the shared interface on the RSD. See Figure 3.

Figure 3: Shared Interfaces

Image g016952.gif

When applied to shared interfaces:

JUNOS features that are configured under logical interfaces, such as class-of-service (CoS) classifiers and rewrites, firewall filters, and policers, are configured on the PSD.
JUNOS features that are configured under physical interfaces, such as drop profiles and schedule maps, are configured on the RSD.

Traffic Flow on Shared Interfaces

The packets belonging to a shared interface pass between the Packet Forwarding Engine on the SONET PIC in the RSD and the Packet Forwarding Engine on the tunnel PIC in the PSD through a cross-connect in the forwarding fabric.

The physical shared interface must be configured on both the RSD and the PSD with Frame Relay encapsulation. Frame Relay encapsulation enables a change in state to be communicated between the RSD and the PSD. Status is communicated through Local Management Interface (LMI) packets exchanged on data-link connection identifier (DLCI) 0. LMI packet exchanges are managed by the RSD.

Note: Point-to-multipoint Frame Relay is not supported.

Traffic flow from the PSD to the RSD over a shared interface is as follows:

A packet destined for the shared PIC at the RSD is received on an interface at the PSD and sent to the Packet Forwarding Engine on the PSD’s tunnel PIC. (The tunnel PIC is configured to peer with the shared PIC at the RSD.)
The packet is sent out of the tunnel interface.
The tunnel PIC loops the packet back to the input side of its Packet Forwarding Engine and the packet is sent over the switch fabric to the Packet Forwarding Engine on the shared PIC at the RSD.
The packet is then sent out the shared interface.

Traffic flow from the RSD to the PSD is as follows:

The Packet Forwarding Engine on the shared PIC at the RSD determines on which logical interface the packet arrived.
Based on the RSD configuration, the PSD that is associated with this logical interface is known and the packet is sent over the switch fabric to the tunnel PIC at that PSD.
The packet is sent out the tunnel interface.
The tunnel PIC loops the packet back to the input side of its Packet Forwarding Engine and the packet is then handled as if it had arrived on a directly-connected PIC.

Supported Platforms and PICs

Shared interfaces are owned by the RSD. With JUNOS Release 9.3, shared interfaces can only be configured on SONET PICs installed on Enhanced Services (ES) FPCs. Because ES FPCs are not supported on the T320 router, only a T640 or T1600 routing node can be configured as an RSD with shared interfaces.

With JUNOS Release 9.3, the following SONET PICs support shared interfaces:

4-port OC48 SONET, SFP
1-port OC192 SONET, XFP
4-port OC192 SONET, XFP
1-port OC768 SONET, SR

To configure shared interfaces, each PSD requires a single tunnel PIC. With JUNOS Release 9.3, interfaces that are configured to peer with shared interfaces are supported only on tunnel PICs that are mounted on Type 3 Enhanced Scaling FPCs (FPC3-ES) and on any Type 4 FPC (FPC4).