Speed-Up Function; S2 And S3 Buffering; Failure Operation; Switch Fabric Card Description - Cisco CRS-1 System Description

Switch Fabric

Speed-up Function

Note The speed up function only applies to the Cisco CRS-1 and Cisco CRS-3. It is not applicable for the Cisco
CRS-X.
A line card chassis can contain up to 8 MSCs, each with up to 140 Gbps of bandwidth. To provide 140 Gbps
of switching capacity for each MSC, the switch fabric must actually provide additional bandwidth to
accommodate cell overhead, buffering, and congestion-avoidance mechanisms.
Congestion can occur in the switch fabric if multiple input data cells are being switched to the same destination
egress MSC. Typically, little congestion exists between the S1 and S2 stages because there is little or no
contention for individual links between the switch components. However, as multiple cells are switched from
the S2 and S3 stages to the same egress MSC, cells might contend for the same output link.
To reduce the possibility of data cells being delayed during periods of congestion, the switch fabric uses 2
times (2x) speedup to reduce contention for S2 and S3 output links. The switch fabric achieves 2x speedup
by providing two output links for every input link at the S2 and S3 stages.

S2 and S3 Buffering

Buffering is also used at the S2 and S3 stages of the switch fabric to alleviate any additional congestion that
the switch fabric speedup does not accommodate. To ensure that this buffering does not cause cells to arrive
out of sequence, the MSC resequences the cells before reassembling them into packets. To limit the amount
of buffering required, a back-pressure mechanism is used for flow control (which slows the transmission of
data cells to a congested destination). Back-pressure messages are carried in fabric cell headers.

Failure Operation

The routing system can withstand the loss of a single plane of the switch fabric with no impact on the system.
The loss of multiple planes results in linear and graceful degradation of performance, but does not cause the
routing system to fail.
Note the following:
• For the Cisco CRS-1 and Cisco CRS-3 routing systems, at least two planes of the switch fabric (an even
• For the Cisco CRS-X routing system, at least three planes of the switch fabric (an even plane, an odd

Switch Fabric Card Description

Depending on the card, the fabric card can be used in a CRS single-chassis system or a CRS back-to-back
system.
plane and an odd plane) must be active at all times. Otherwise, the switch fabric fails, causing a system
failure.
plane, and one plane in either slot 6 or 7) must be active at all times for the router to operate. Otherwise,
the switch fabric fails, causing a system failure.
Cisco CRS Carrier Routing System 8-Slot Line Card Chassis Enhanced Router System Description
Speed-up Function
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