Switch Fabric Operation - Cisco CRS-1 System Description

Switch Fabric Operation

Figure 22: 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.
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.
Switch Fabric Operation
Several switch element components on each switch fabric card perform the functions to implement each of
the three stages (S1, S2, and S3) of the switch fabric. Each stage performs a different function:
• Stage 1 (S1)—Distributes traffic to Stage 2 of the fabric plane. Stage 1 elements receive cells from the
• Stage 2 (S2)—Performs switching, provides 2 times (2x) speedup of cells, and performs the first stage
• Stage 3 (S3)—Performs switching, provides 2 times (2x) speedup of cells, and performs a second level
Cisco CRS Carrier Routing System 8-Slot Line Card Chassis Enhanced Router System Description
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ingress MSC and PLIM (or RP) and distribute the cells to Stage 2 (S2) of the fabric plane. Cells are
distributed to S2 elements in round-robin fashion; one cell goes to the first S2 element, the next cell goes
to the next S2 element, the next cell goes to the third S2 element, and so on, and then back to the first
S2 in sequence.
of the multicast function. Stage 2 elements receive cells from Stage 1 and route them toward the
appropriate:
• Egress MSC and PLIM (single-shelf system)
• Egress Cisco CRS 8-Slot Line Card Chassis Enhanced router (multishelf system)
of the multicast function. Stage 3 elements receive cells from Stage 2 and perform the switching necessary
to route each cell to the appropriate egress MSC and PLIM.
Switch Fabric