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    M40e Switching and Forwarding Module (SFM) Description

    The Switching and Forwarding Module (SFM) performs route lookup, filtering, and switching on incoming data packets, then directs outbound packets to the appropriate FPC for transmission to the network. It can process 40 million packets per second (Mpps).

    Figure 1: M40e Switching and Forwarding Module

    Image g001184.gif

    SFM Slots

    One or two SFMs can be installed into the midplane from the rear of the chassis, as shown in M40e Chassis Description. Only one SFM is active at a time, with the optional second SFM in standby mode. By default, the SFM in slot SFM 0 is active. To modify the default, include the appropriate sfm statement at the [edit chassis redundancy] hierarchy level of the configuration, as described in the section about SFM redundancy in the Junos OS System Basics Configuration Guide.

    SFM Redundancy

    SFMs are hot-pluggable, as described in M40e Field-Replaceable Units (FRUs). Removing the standby SFM has no effect on router function. If the active SFM fails or is removed from the chassis, the effect depends on how many SFMs are installed:

    • One SFM—Forwarding halts until the SFM is replaced and functioning again. It takes approximately one minute for the replaced SFM to boot and become active; reading in router configuration information can take additional time, depending on the complexity of the configuration.
    • Two redundant SFMs—The effect depends on which release of the Junos OS is running on the router:
      • With Junos OS Release 5.4 and later, the standby SFM assumes forwarding functions in less than one second.
      • With Junos OS Release 5.3 and earlier, forwarding halts while the standby SFM boots and becomes active, which takes approximately one minute; synchronizing router configuration information can take additional time, depending on the complexity of the configuration.

    SFM Function

    The SFM communicates with the Routing Engine using a dedicated 100-Mbps Fast Ethernet link. The link transfers:

    • Routing table data from the Routing Engine to the forwarding table in the Internet Processor II ASIC.
    • Routing link-state updates and other packets destined for the router that have been received through the router interfaces from the SFM to the Routing Engine.

    The ASICs and other components on the SFM provide the functions:

    • Route lookups—The Internet Processor II ASIC on each SFM performs route lookups using the forwarding table stored in SSRAM.
    • Management of shared memory on the FPCs—One Distributed Buffer Manager ASIC receives the 64-byte data cells into which the active I/O Manager ASIC on each FPC divides incoming packets, and uniformly allocates them throughout the shared memory buffers located on the FPCs.
    • Transfer of outgoing data packets—The second Distributed Buffer Manager ASIC passes notification of the forwarding decision for each packet to an I/O Manager ASIC so that data cells for the outgoing packet can be reassembled for transmission to the network.
    • Transfer of exception and control packets—The Internet Processor II ASIC passes exception packets to the microprocessor on the SFM, which processes almost all of them. The SFM sends any remaining exception packets to the Routing Engine for further processing. When the SFM detects an error originating in the Packet Forwarding Engine, it sends it to the Routing Engine using system logging (syslog) messages.

    SFM Components

    Each SFM is a two-board system, as shown in Figure 1. It has the components:

    • Two Distributed Buffer Manager ASICs—Process incoming and outgoing packets: one distributes data cells (which the I/O Manager ASIC on each FPC derives from incoming packets) to the shared memory buffers on the FPCs, while the second forwards notification of routing decisions to the I/O Manager ASICs.
    • One Internet Processor II ASIC—Performs route lookups and makes routing decisions.
    • Parity-protected SSRAM—Stores the forwarding table.
    • Processor subsystem—Manages SFM functions and handles exception packets. The processor has the components:
      • One PowerPC 603e processor
      • Parity-protected Level 2 cache
      • Parity-protected DRAM
    • EEPROM—Stores the serial number and revision level.
    • Offline button—Prepares the SFM for removal from the router when pressed.
    • Two LEDs—Indicate SFM status. There is a green one labeled OK and an yellow one labeled FAIL. M40e SFM LEDs describes the LED states.
    • Ejector handles and locking tabs—Control the locking system that secures the SFM in the chassis.

    Note: For specific information about SFM components (for example, the amount of SSRAM and DRAM), issue the show chassis sfm detail command.


    Published: 2010-10-28