Juniper Networks M5 - Manual (Page 50)

Packet Forwarding Engine Architecture
•
Midplane—Transports packets, notifications, and other signals between the PICs and
the Packet Forwarding Engine (as well as other system components).
•
Physical Interface Card (PIC)—Physically connects the router to fiber-optic or digital
network media. A controller ASIC in each PIC performs control functions specific to
the PIC media type.
•
Forwarding Engine Board (FEB)—Hosts the Internet Processor II ASIC, I/O Manager
ASIC (one on the M5 router, two on the M10 router), and two Distributed Buffer
Manager ASICs. An I/O Manager ASIC divides incoming packets into 64-byte data cells
and reassembles cells into data packets for transmission on the network after the
Internet Processor II ASIC decides how to forward the packets. One Distributed Buffer
Manager ASIC distributes the data cells among the memory buffers on the FEB and the
other notifies the PICs of the forwarding decision for outgoing packets.
Data Flow through the Packet Forwarding Engine
Use of ASICs promotes efficient movement of data packets through the system. Packets flow
through the Packet Forwarding Engine in the following sequence (see Figure 13):
1. Packets arrive at an incoming PIC interface.
2. The PIC passes the packets through the midplane to the FEB.
3. An I/O Manager ASIC on the FEB processes the packet headers and divides the packets
into 64-byte data cells.
4. A Distributed Buffer Manager ASIC on the FEB distributes the data cells throughout the
memory buffers on the FEB.
5. The Internet Processor II ASIC on the FEB performs a route lookup for each packet
and decides how to forward it.
6. The Internet Processor II ASIC notifies the second Distributed Buffer Manager ASIC
of the forwarding decision, and the Distributed Buffer Manager ASIC forwards the
notification to the appropriate outbound PIC.
7. The I/O Manager ASIC on the FEB reassembles data cells stored in shared memory into
data packets as they are ready for transmission and passes them to the outbound PIC.
8. The outbound PIC transmits the data packets.
30 M5 and M10 Internet Routers Hardware Guide