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Document No. 650-100-700, Issue 1 13-19
Configuring Access Lists
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F-chip: Forwarding-chip (F-chip) is an application specific
integrated circuit (ASIC) that forwards recognized packets via Fast
Path or unrecognized packets via Slow Path. It learns or flushes L3
forwarding entries (L3FE) as directed by the CPU. In earlier
versions of the hardware, the F-Chip was referred to as the Packet
Routing Engine (PRE).
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Fast Path: When an F-chip is able to recognize and forward a
packet to the destination port without CPU intervention. Both FIRE
(media modules) and FORE (supervisor module) are Fast Path
forwarding mechanisms.
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FIRE (Fast In-Band Routing Engine): When an 80-series
Ethernet media module has been licensed for routing, the F-Chips
on that module provide fast, direct forwarding of L3 packets to their
destination ports. The destination ports can be on the same module
or a different module.
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Flow: A set of routed packets that get the same Layer 3 Forwarding
Entry (L3FE) due to equivalent address characteristics. The
complexity of this equivalence will affect the frequency that packets
match a single Flow. In the simplest case, such as no ACL present, a
Flow is identified only by the destination address of the packet, for a
high rate of L3FE re-use. In the most complex case, such as an ACL
that specifies the entire 5-tuple, the granularity of identification will
cause a very low rate of re-use.
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FORE (Fast Out-of-Band Routing Engine): When an 80-series
Ethernet media module is not licensed for routing, the media
modules forward all routed packets to the supervisor. The F-Chip on
the supervisor module provides fast, direct forwarding of L3 packets
to their destination ports.
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Hash Mode: The F-chip uses a 12-bit key to initially locate
available memory for Flows. In DA-only mode (default) it builds the
key from a sample of the Flow’s destination address (DA). In SA-
DA mode, it uses both the source and destination addresses. The F-
chip automatically selects which 12 bits to use, and when to re-hash
with a different key.
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Ingress F-chip: The F-chip that receives a packet from an external
source.
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Layer 3 Forwarding Entry (L3FE): When a packet arrives on a
media module, the F-Chip needs to know where to forward that
packet. The L3 forwarding cache is a list that identifies the path
taken by a data packet through the switch. Each entry in this list is a
L3FE. Each entry is identified by a combination of the Flow’s 5-
tuple and the corresponding ACL rule. If the F-chip does not find a
match in the L3 forwarding cache, the packet is sent Slow Path to
the CPU for processing and forwarding. Once the CPU has