Switch Fabric Overview
The switch fabric is the core of the Cisco CRS routing system. The switch fabric is implemented through switch fabric cards installed in the chassis. The switch fabric uses a cell-switched, buffered three-stage Benes switch fabric architecture. The switch fabric receives user data from a modular services card (MSC) and performs the switching necessary to route the data to the appropriate egress MSC.
The switch fabric is divided into eight planes (plane 0 to plane 7) that are used to evenly distribute traffic across the switch fabric. Each switch fabric plane is independent and not synchronized with one another. Each cell traverses the switch fabric using a single switch fabric plane. (Cells are not bit sliced across the switch fabric.) And unlike the CRS 16-slot, the CRS 8-slot has only 4 physical switch fabric cards.
There are four types of switch fabric cards used in the line card chassis: CRS-8-FC/S, CRS-8-FC140/S, CRS-8-FC400/S (200G mode) and CRS-8-FC400/M (200G mode). Each fabric card implements all three stages of the switch fabric.
This figure shows the basic path of IP data packets through the Cisco CRS routing system switch fabric. Note that the figure shows a single-shelf system, in which all three stages of the switch fabric are provided by switch fabric cards in the line card chassis. In a multishelf system, Stage 2 of the switch fabric is provided by S2 fabric cards in the fabric card chassis.
Ingress data packets are received at a physical interface on a PLIM and transferred to the associated MSC, where the packets are segmented into cells for efficient switching by the switch fabric hardware. Each MSC has multiple connections to each switch fabric plane, which it uses to distribute cells to each fabric plane. On egress, cells are reassembled into data packets before being transmitted by the egress MSC.
Note |
The cell structure used in the Cisco CRS switch fabric is a Cisco-specific cell structure and is not related to Asynchronous Transfer Mode (ATM) cells. |