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).
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 System Basics Configuration Guide.
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:
- If there is 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.
- If there are two SFMs, the effect depends on which release
of the JUNOS Software is running on the router:
- With JUNOS Release 5.4 and later, the standby SFM assumes forwarding functions in less than one second.
- With JUNOS 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.
For SFM replacement instructions, see Replacing an SFM in an M40e Router.
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 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.
Figure 1 shows the Routing Engine location on the M40e and M160 router.
Figure 1: M40e and M160 Router Routing Engine Location
