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CRS-1 Hardware Overview

After introducing the Cisco  CRS-1 router in a previous post, We are going to delve more into the hardware architecture of this router. I believe we have to start by defining the main hardware components of the CRS-1 router and briefly describe their functions. In later posts we are going to study each part in more details. Now lets continue our baby steps.

Note: I will be focusing on single chassis configurations.

CRS-1 Line Cards:

The router comes in 4,8 and 16 slots each  supports a capacity of 40 Gbps line card. Each line card has two separate physical components. The Modular service card (MSC)  which provides all the L3 forwarding functionality and  Physical interface module (PLIM). The PLIM provides all Layer1/Layer2 functions.  Each PLIM is connected to its associated MSC through the midplane.

The separation of the MSC and PLIM has its benefits. For example the MSC can work with multiple types of PLIMs and you can change the PLIM without changing the MSC or change the MSC without changing the PLIM.

Switch Fabric Cards:

The core of the CRS-1 platform is the switch fabric. The main function of the switch fabric is to connect line cards to each other. The CRS-1 switch fabric is designed as cell based, buffered three stage benes architecture for enhanced performance. The benes architecture provides high capacity non-blocking switch fabric.

Ingress packets are received at the PLIM and trasfered to the associated MSC for  forwarding. The MSC segments the packet into cells and distributes them to fabric planes which in turn deliver these cells to the appropriate egress MSC.

Route processor Cards:

The route processor is considered to be the brain of the CRS-1 platform. The RP maintains the system operation, responsible for monitoring functions, IOS-XR image and logs storage.

The route processor is responsible for all control plane functions then constructed information is distributed to the MSCs for distributed forwarding. No actual data forwarding is done on the RP unless the packets are destined to the router itself.

Each chassis comes with two redundant route processor cards. The two cards act in an active/standby relationship for redundancy and high availability.

This post concludes our second step. Please do share your experience and information with us.


  1. Nice summary.

    Thanks & B.regards
    Ahmed Elhoussiny,2x CCIE# 21988 (R&S-SP)
    Network Consultant & Cisco Academy Instructor

  2. The most simplest & clearest form of CRS Explanation i read so far.
    Thanks Wael, & if you have a good material to explain the structure in more details but in simple form, please send me.

  3. thanks a lot for great info in simple language :)

  4. Excellent explanation of CRS. Keep it up!!

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