Foundation Summary

The Foundation Summary is a collection of tables, lists, and other information that provides a convenient review of many key concepts in this chapter. If you are already comfortable with the topics in this chapter, this summary might help you recall a few details. If you just read this chapter, this review should help solidify some key facts. If you are doing your final prep before the exam, the following information is a convenient way to review the day before the exam:

• Layer 2 switches learn incoming MAC addresses and record their locations based on the inbound switch ports.

• Layer 2 switching information is stored in the Content Addressable Memory (CAM) table. The CAM is consulted to find the outbound switch port when forwarding frames.

• Multilayer switching looks at the Layer 2 addresses, along with Layers 3 and 4 address and port information, to forward packets.

• Multilayer switching is performed in hardware using the Cisco Express Forwarding (CEF) method.

• CEF builds Layer 3 destination information from routing tables and Layer 2 data. This information is stored in hardware as a Forwarding Information Base (FIB) table.

• Multilayer switches can make many policy decisions in parallel, using the Ternary Content Addressable Memory (TCAM) contents.

• TCAM combines a 134-bit value, or pattern (made up of addresses, port numbers, or other appropriate fields), with a 134-bit mask to yield a result value. The result instructs the switch hardware on how to finish forwarding the packet.

• Access lists for security (traditional router ACLs and VLAN ACLs) and QoS ACLs are compiled or merged into TCAM entries. These access lists then can be processed on each packet that passes through the switch, as a single table lookup.

• As a packet exits a multilayer switch, it must be rewritten so that its header and checksum values are valid. The fields in the original packet that the switch updates are as follows:

  The source MAC address becomes the Layer 3 switch MAC address.

  The destination MAC address becomes the next-hop MAC address.

  The IP TTL value is decremented by one.

  The IP checksum is recomputed.

  The Ethernet frame checksum is recomputed.

Table 3-3. Switching Table Commands

Task	Command Syntax
Set the CAM table aging time.	mac address-table aging-time seconds
Configure a static CAM entry.	mac address-table static mac-address vlan vlan-id interface type mod/num
Clear a CAM table entry.	clear mac address-table dynamic [address mac-address | interface type mod/num | vlan vlan-id]
View the CAM table.	show mac address-table dynamic [address mac-address | interface type mod/num | vlan vlan-id]
View the CAM table size.	show mac address-table count