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redundancy. For more detail on high-availability network designs, refer to the white paper, Gigabit Campus Design, at:
http://www.cisco.com/en/US/partner/netsol/ns340/ns394/ns147/ns17/
networking_solutions_white_paper09186a00800a3e0b.shtml
Switch Fabric Module Redundancy
Since its introduction, the Cisco Catalyst 6500 Series has been built on a single 32-Gbps bus switching architecture that
provides the data path for all packets through the system. The Cisco Catalyst 6500 Series includes a 256-Gbps crossbar
switching fabric (the SFM for higher bandwidth capacities and 30+ Mpps of forwarding performance). The SFM is supported
in the Cisco Catalyst 6506 and the Cisco Catalyst 6509 chassis. The SFM2 is essentially the same fabric but designed to work
in all the Cisco Catalyst 6506, 6509, and 6513.
Switching Fabric Failover
The SFM also provides another level of hardware redundancy to the system. The single fabric channel versions of the
fabric-enabled line cards provide a connection to both the switching fabric and the existing system bus backplane. This allows
the Cisco Catalyst 6500 Series to use the SFM as the primary data path between fabric-enabled line cards. In the event that an
SFM fails, the system will fail over to the 32-Gbps bus to ensure that packet switching continues (albeit at the bus capacities
of 15 Mpps throughput and 32-Gbps bandwidth) and the network remains online. Additionally, a Cisco Catalyst 6500 Series
can be configured with dual SFMs (in slots 5 and 6 of a Catalyst 6506 or Catalyst 6509 or in slots 7 and 8 of a Catalyst 6513),
which provide a third level of fabric redundancy. In this configuration, a failure on the primary fabric module would result in
a switchover to the secondary fabric module for continued operation at 30 Mpps. Also, in the event of further fabric module
failures, the ability to switch over yet again to the system bus would still be available.
Switching Fabric Operation
Different combinations of SFMs, fabric-enabled line cards, and classic line cards in a chassis affect the internal switching
operation, which in turn affects the failover characteristics. This is an important point to understand as fabric-to-fabric or
fabric-to-bus failover scenarios are discussed. When an SFM is installed in a system of only fabric-enabled line cards, the
switching operation is called compact mode. This allows for 32-byte compacted headers (not the entire packet) to be sent
across the bus to the supervisor engine for each forwarding decision. The increase in efficiency for this operation allows for
inherent system performance capable of 30 Mpps. The data path for fabric-enabled cards is via the SFM.
If a classic line card is installed in a system with an SFM, the header format on the bus must be compatible with all the line
cards in the system. Because classic line cards do not support compact mode, the fabric-enabled line cards will change their
switching modes to truncated mode. Truncated mode allows the fabric-enabled line card to send packets in a 64-byte
header-only format that the classic line cards can understand. It is very important to note that the truncated mode still uses the
SFM as the data path between fabric-enabled line cards. Although the maximum centralized forwarding performance is 15
Mpps in a system of classic and fabric cards, the switch fabric is still used to provide higher bandwidth to the system. If
fabric-enabled line cards are installed in a system with no SFM, they will operate in flow-through mode even when classic
cards are present. This mode essentially programs the line card to operate in a classic mode whereby the entire packet is sent
across the system bus for a forwarding decision. A system in flow-through mode is capable of switching 15 Mpps and the data
path is via the 32-Gbps bus.
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