What device has content-addressable memory (CAM) and what is
stored in it?

1 A: Switch, learned IP addresses
2 B: Router, learned IP addresses
3 C: Switch, learned MAC addresses
4 D: Router, learned MAC addresses

Answer Posted / pachicoo

Content-addressable memory is often used in computer
networking devices. For example, when a network switch
receives a data frame from one of its ports, it updates an
internal table with the frame's source MAC address and the
port it was received on. It then looks up the destination
MAC address in the table to determine what port the frame
needs to be forwarded to, and sends it out on that port.
The MAC address table is usually implemented with a "binary
CAM" so the destination port can be found very quickly,
reducing the switch's latency.

Ternary CAMs are often used in network routers, where each
address has two parts: the network address, which can vary
in size depending on the subnet configuration, and the host
address, which occupies the remaining bits. Each subnet has
a network mask that specifies which bits of the address are
the network address and which bits are the host address.
Routing is done by consulting a routing table maintained by
the router which contains each known destination network
address, the associated network mask, and the information
needed to route packets to that destination. Without CAM,
the router compares the destination address of the packet
to be routed with each entry in the routing table,
performing a logical AND with the network mask and
comparing it with the network address. If they are equal,
the corresponding routing information is used to forward
the packet. Using a "ternary CAM" for the routing table
makes the lookup process very efficient. The addresses are
stored using "don't care" for the host part of the address,
so looking up the destination address in the CAM
immediately retrieves the correct routing entry; both the
masking and comparison are done by the CAM hardware.

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