Juniper Networks M5 - Manual (Page 31)

Packet Forwarding Engine
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 FEB provide the following functions:
•
Route lookups—The Internet Processor II ASIC on each FEB performs route lookups
using the forwarding table stored in SSRAM.
•
Creation and reassembly of data cells—The I/O Manager ASIC divides incoming packets
into 64-byte data cells for easier processing, and reassembles the cells for each packet
after the forwarding decision is made for it. There is one I/O Manager ASIC on the
M5 router and two on the M10 router.
•
Management of memory on the FEB—One Distributed Buffer Manager ASIC receives
the 64-byte data cells into which the I/O Manager ASIC divides incoming packets, and
uniformly allocates the cells throughout the memory buffers on the FEB.
•
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 FEB, which processes almost all of
them. The FEB sends any remaining exception packets to the Routing Engine for
further processing. When the FEB detects an error originating in the Packet Forwarding
Engine, it sends it to the Routing Engine using system logging (syslog) messages.
•
Monitoring of system components—The FEB monitors other system components for
failure and alarm conditions. It collects statistics from all sensors in the system and
relays them to the Routing Engine, which sets alarms as appropriate. For example, if
the temperature of a component exceeds the lower of two internally defined thresholds,
the Routing Engine issues a “high temperature” alarm. If the temperature exceeds the
higher threshold, the Routing Engine initiates a system shutdown.
•
Providing SONET/SDH clock source—The FEB generates a 19.44-MHz clock signal for
use by SONET/SDH interfaces.
FEB Components
An FEB has the following components (see Figure 5):
•
I/O Manager ASIC (one on the M5 router and two on the M10 router)—Divide incoming
packets into 64-byte data cells for easier processing, and reassemble the cells for each
packet after the forwarding decision is made for it.
•
Two Distributed Buffer Manager ASICs—Process incoming and outgoing packets: one
distributes data cells (which the I/O Manager ASIC derives from incoming packets) to
the memory buffers on the FEB, while the second forwards notification of routing
decisions to an I/O Manager ASIC.
•
One Internet Processor II ASIC—Performs route lookups and makes routing decisions.
•
Parity-protected synchronous SRAM (SSRAM)—Stores the forwarding table.
Hardware Component Overview 11