Switching and Forwarding Module (SFM)

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).

Up to four SFMs can be installed in the router, processing a total of 160 Mpps. The SFMs are hot-pluggable, as described in Field-Replaceable Units (FRUs). Removing or inserting an SFM causes a brief interruption in forwarding performance (about 500 ms) as the Packet Forwarding Engine reconfigures the distribution of packets across the remaining SFMs.

For SFM replacement instructions, see Replacing an SFM.

The SFM communicates with the Routing Engine using a dedicated 100-Mbps Fast Ethernet link that transfers routing table data from the Routing Engine to the forwarding table in the Internet Processor II ASIC. The link is also used to transfer from the SFM to the Routing Engine routing link-state updates and other packets destined for the router that have been received through the router interfaces.

The ASICs and other components on the SFM provide the following 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 I/O Manager ASICs on each FPC divide 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 8. It has the following 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 following 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 amber one labeled FAIL. Table 7 describes the LED states.
• Ejector handles and locking tabs—Control the locking system that secures the SFM in the chassis.

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

Table 7: States for SFM LEDs