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
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.
Figure 1. Basic Cisco CRS
Carrier Routing System Switch Fabric
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.
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.