Understanding the Switching Table
Content Addressable Memory (CAM) is used for MAC tables for layer two
switching.
■ Used for Catalyst 4500 layer 2 forwarding tables
■ Used for Catalyst 6500 layer 2 and Netflow forwarding tables
■ Contains binary values (0 or 1)
■ Match must be exact
In comparison, MLS uses Ternary Content Addressable Memory (TCAM).
■ Used for Catalyst 3500/3700, 4500, and 6500 layer 3 switching
■ Ternary (3) values (0, 1, or wildcard)
■ Entries are in VMR form
—Value—Pattern to be matched.
—Mask—Masking bits associated with pattern.
—Result—Consequences of a match (permit/deny or more complex
information).
Understanding Switch Forwarding Architectures
In a Centralized Forwarding model, the CPU controls forwarding decisions:
■ Decision made by single table
■ Used by 4500 and 6500
With Distributed Forwarding, the forwarding decisions are spread throughout
the interface ASICs:
■ Decision made at port or module
■ Used by 3500/3700 and 6500 with distributed forwarding card
■ NetFlow switching
■ Decision made cooperatively by Route Processor and MLS
■ First packet switched in software, result cached
■ Subsequent packets switched in hardware