Data Flow Through an M-series Router

Figure 3 illustrates the flow of data packets through an M-series router, using the M40e router architecture as an example. In this example, data flows in the following sequence:

1. A packet enters through the incoming PIC, which parses and de-encapsulates the packet, then passes it to the FPC.
2. On the FPC, the Packet Director ASIC distributes packets to the active I/O Manager ASICs, where each is divided into cells and sent across the midplane to the Switching and Forwarding Modules (SFMs). (On the M40e router, only one SFM is online at a time.) In addition, the behavior aggregate (BA) classifier determines the forwarding treatment for each packet.

Figure 3: Data Flow Through an M40e Router

3. When cells arrive at an SFM, the Distributed Buffer Manager ASIC writes them into packet buffer memory, which is distributed evenly across the router’s FPCs. The Distributed Buffer Manager ASIC also extracts information needed for route lookups and passes the information to the Internet Processor II ASIC.
4. The Internet Processor II ASIC performs the lookup in the full forwarding table, and finds the outgoing interface and specific next hop for each packet. In addition, the Internet Processor II ASIC performs filtering, policing, sampling and mulitfield classification, if configured.
5. The forwarding table forwards all unicast packets that do not have options and any multicast packets that have been previously cached. Packets with options are sent to the Routing Engine for resolution.
6. After the Internet Processor II has determined the next hop, it notifies a second Distributed Buffer Manager ASIC, which forwards the notification to the outgoing FPC. Queueing policy and rewrites occur at this time on the egress router. A pointer to the packet is queued at the outgoing port.
7. When the packet pointer reaches the front of the queue and is ready for transmission, the cells are read from packet buffer memory and are reassembled into the packet, which is passed to the outgoing PIC interface.
8. The PIC performs media-specific processing and sends the packet into the network.