Cisco CRS-1 System Description page 48

Chapter 5 Line Cards, Physical Layer Interface Modules, and Shared Port Adapters Overview
Line Cards and PLIMs Overview
Line cards support several forwarding protocols, including IPV4, IPV6, and MPLS. Note that the route
processor (RP) performs routing protocol functions and routing table distributions, while the line cards
actually forwards the data packets.
In the Cisco CRS 4-Slot line card chassis, both the line cards and PLIMs are installed in the front of the
chassis, and mate through the chassis midplane.
Note The following MSC functional description is also generally applicable to FP and LSP cards.
Figure 5-1 shows how the MSC takes ingress data through its associated PLIM and forwards the data to
the switch fabric where the data is switched to another MSC, FP, or LSP, which passes the egress data
out its associated PLIM.
Figure 5-1 Data Flow through the 4-Slot Line Card Chassis
Switch fabric
PLIM MSC S1 S2 S3 MSC PLIM
IP
IP
Ingress Egress
Data
Data
Data streams are received from the line side (ingress) through optic interfaces on the PLIM. The data
streams terminate on the PLIMs. Frames and packets are mapped based on the Layer 2 (L2) headers.
The MSC converts packets to and from cells and provides a common interface between the routing
system switch fabric and the assorted PLIMs. PLIMs provides the interface to user IP data. PLIMs
perform Layer 1 and Layer 2 functions, such as framing, clock recovery, serialization and deserialization,
channelization, and optical interfacing. Different PLIMs provide a range of optical interfaces, such as
very-short-reach (VSR), intermediate-reach (IR), or long-reach (LR).
A PLIM eight-byte header is built for packets entering the fabric. The PLIM header includes the port
number, the packet length, and some summarized layer-specific data. The L2 header is replaced with
PLIM headers and the packet is passed to the MSC for feature applications and forwarding.
The transmit path is essentially the opposite of the receive path. Packets are received from the drop side
(egress) from the MSC through the chassis midplane. The L2 header is based on the PLIM eight-byte
header received from the MSC. The packet is then forwarded to appropriate Layer 1 devices for framing
and transmission on the fiber.
A control interface on the PLIM is responsible for configuration; optic control and monitoring;
performance monitoring; packet count; error-packet count and low-level operations of the card; such as
PLIM card recognition; power up of the card; and voltage and temperature monitoring.
Cisco CRS Carrier Routing System 4-Slot Line Card Chassis System Description
5-2
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