Msc To Fabric Section And Queuing; Msc From Fabric Section; Msc Egress Packet Engine; Shaping And Queuing Function - Cisco CRS-1 System Description

Line Cards and PLIMs Overview
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MSC To Fabric Section and Queuing

The "to fabric" section of the board takes packets from the ingress packet engine, segments them into
fabric cells, and distributes (sprays) the cells into the four planes of the switch fabric. Because each MSC
has multiple connections per plane, the "to fabric" section distributes the cells over the links within a
fabric plane. The chassis midplane provides the path between the "to fabric" section and the switch
fabric (as shown by the dotted line in
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MSC From Fabric Section

The "from fabric" section of the board receives cells from the switch fabric and reassembles the cells
into IP packets. This section of the card then places the IP packets in one of its 8-K egress queues, which
helps the section adjust for the speed variations between the switch fabric and the egress packet engine.
Egress queues are serviced using a modified deficit round-robin (MDRR) algorithm. The dotted line in
Figure 5-2

MSC Egress Packet Engine

The transmit (egress) packet engine performs a lookup on the IP address or MPLS label of the egress
packet based on the information in the ingress MSC buffer header and on additional information in its
internal tables. The transmit (egress) packet engine performs transmit side features such as output
committed access rate (CAR), access lists, DiffServ policing, MAC layer encapsulation, and so on.

Shaping and Queuing Function

The transmit packet engine sends the egress packet to the shaping and queuing function (shape and
regulate queues function), which contains the output queues. Here the queues are mapped to ports and
classes of service (CoS) within a port. Random early-detection algorithms perform active queue
management to maintain low average queue occupancies and delays.

PLIM Physical Interface Section on Egress

On the transmit (egress) path, the physical interface controller provides the interface between the MSC
and the physical ports on the PLIM. For the egress path, the controller performs the following functions:
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Cisco CRS Carrier Routing System 4-Slot Line Card Chassis System Description
5-4
Chapter 5
Maintains per-interface and per-protocol byte-and-packet statistics
Maintains Netflow accounting
Implements a flexible dual-bucket policing mechanism
The first level performs ingress shaping and queuing, with a rate-shaping set of queues that are
normally used for input rate-shaping (that is, per input port or per subinterface within an input port),
but can also be used for other purposes, such as to shape high-priority traffic.
The second level consists of a set of destination queues where each destination queue maps to a
destination MSC, plus a multicast destination.
Note that the flexible queues are programmable through the Cisco IOS XR software.
indicates the path from the midplane to the "from fabric" section.
Support for the physical ports. Each physical interface controller can support up to four physical
ports and there can be up to four physical interface controllers on a PLIM.
Queuing for the ports
Line Cards, Physical Layer Interface Modules, and Shared Port Adapters Overview
Figure 5-2).
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