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ARB
The Arbiter (ARB) is located on each line module. It uses a two−tiered method of arbitration to assign
queuing priorities and control data traffic through the switch. The arbiter controls the traffic coming to and
from the line modules. In addition, a Central Bus Arbiter located on the Supervisor Engine module obtains
permission to transmit frames to the switching engine.
The Central Bus Arbiter provides special handling of high−priority frames by using a round−robin approach.
Frames with other priority levels can be set to handle support of time−sensitive traffic, such as multimedia.
LTL
The Local Target Logic (LTL) works in conjunction with the EARL ASIC to determine if a frame is switched
to one individual port or sent to multiple ports. The LTL also helps identify the port or ports on the switch to
which the frame needs to be forwarded, and it can look at the frame to determine if the frame is a unicast or a
multicast frame for broadcast forwarding. This process is handled using index values provided by the EARL
ASIC table. The LTL then uses this information to select the port or ports to forward the frame to.
CBL
The Color Block Logic (CBL) blocks data frames from entering a port that does not belong to the same
VLAN as the port of arrival. This ASIC aids STP in deciding which ports to block and which ports to place in
the learning, listening, or forwarding modes.
Other Cisco Switch Processors, Buses, ASICs, and Logic Units
In addition to the items we just discussed, other ASICs and significant components are used in the Cisco 5000
architecture as well as that of other Cisco Catalyst and Gigabit Switch Routers (GSRs).
Note ASIC is not a Cisco term. ASICs are vendor specific, and differently named ASICs can be found on
other vendor networking products.
Let’s take a closer look at the functions of these switch components:
Content Addressable Memory (CAM)•
AXIS bus•
Cisco Express Forwarding (CEF) ASIC•
Phoenix ASIC•
Line Module Communication Processor (LCP)•
Synergy Advanced Gate−Array Engine (SAGE) ASIC•
Quad Token Ring Port (QTP) ASIC•
Quad Media Access Controller (QMAC)•
CAM
The CAM table is used by a bridge to make forwarding and filtering decisions. The CAM table contains MAC
addresses with port addresses leading to the physical interfaces. It uses a specialized interface that is faster
than RAM to make forwarding and filtering decisions. The CAM table updates information by examining
frames it receives from a segment and then updating the table with the source MAC address from the frame.
AXIS Bus
The architecture of the Catalyst 3900 centers around the AXIS bus, which uses a 520Mbps switching fabric
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