The CAM Table or MAC address Table

Ethernet switches store the MAC addresses of all connected devices in a table known as CAM or MAC address table, and use it to make forwarding decisions. The CAM table is also known as the MAC forward table, MAC filter table, MAC address table, the switching table, or the bridging table.

A CAM table stores information using entries. There are two ways to add entries to the CAM table: static and dynamic. In the static method, you manually add entries to the CAM table. In the dynamic method, the switch automatically adds entries to the CAM table.

The static method is complex and tedious. Administrators use this method to add only the MAC addresses of critical resources, such as servers and firewalls, for security reasons. The dynamic method is easy. It is also the default method on all Cisco switches. In this method, switches automatically learn the MAC addresses of the connected devices from incoming frames.

The CAM table saves each MAC address in a separate entry. With each MAC address, it also saves port and aging information. The switch uses the port information to make forwarding decisions. It uses the aging information to remove old entries.

A switch uses two configuration types: the running and startup configurations. It uses running configuration to save all the dynamically learned information and configuration changes you make on the switch.

When you restart a switch, the running configuration is lost. To preserve configuration across reboots, switches use the startup configuration. They save the startup configuration in NVRAM. When a switch boots, it loads the startup configuration from NVRAM to RAM.

Switches save the CAM table entries in the running configuration. To preserve entries across reboots, you need to save them in the startup configuration. The following privileged exec mode command saves the running configuration in the startup configuration.

Switch#copy running-config startup-config

CAM tables are not shareable

CAM tables are not shareable. A switch cannot share its CAM table with another switch. Similarly, it cannot use another switch's CAM table. Each switch has to build its own CAM table.

Viewing CAM table entries

To view the entries of the CAM table, use the "show mac-address-table" command in privileged exec mode. This command displays the real-time entries of the CAM table. If you run this command immediately after starting a switch, it displays an empty CAM table. The switch adds a device MAC address to the CAM table only when it receives a frame from that device on one of its ports.

The following image shows the CAM table just after the switch is on.

The following image shows the CAM table after the switch has learned the MAC addresses of its connected devices.

Key points: -

• Switches use a CAM table to store the MAC addresses of connected devices.
• The CAM table stores each MAC address in a separate entry, along with the port and aging information.
• The switch uses port information to make forwarding decisions.
• The switch uses aging information to age out old entries.
• There are two ways to update the CAM table: manual and dynamic.
• In the manual method, you manually add MAC addresses to the CAM table.
• In the dynamic method, the switch automatically adds the MAC addresses of connected devices to its database from incoming frames.
• Switches save CAM table entries in the running configuration.
• The running configuration doesn't persist across a reboot.
• To preserve the running configuration after the reboot, you need to copy it into the startup configuration.

Conclusion

The CAM table is a vital component of Ethernet switches. Each switch maintains its own CAM table. It enables efficient network traffic forwarding by mapping MAC addresses to specific ports. Understanding how CAM tables operate, including the differences between static and dynamic entry methods, as well as the importance of saving configurations, ensures robust switch performance and network reliability.