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CCNA Certification All-In-One For Dummies by Silviu Angelescu

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Chapter 3: Controlling Network Traffic with Cisco Switches

Exam Objectives

Describing how MAC addresses are handled in local and remote transmissions

Describing Layer 2 switch modes

Setting the duplex mode of a Layer 2 switch

Reviewing MAC address table thrashing and broadcast storms and seeing how STP fixes these two issues

Read this chapter to find out how Cisco Layer 2 switches control and optimize traffic in a local-area network (LAN).

Sending to MAC Addresses in Remote Networks

In the following sections, you discover how a sending host device interacts with a Layer 2 switch and with a gateway (a router) to send frames to a target device located in a remote network. Most data transfers involve sending and receiving devices that are not located in the same LAN. Data frames need to be sent to the LAN gateway first. Next, the LAN gateway routes the data frames wrapped in packets through a wide-area network (WAN) to the destination LAN.

Sending frames within the LAN

A host determines the MAC address of a target device in the local network by first looking into its Address Resolution Protocol (ARP) table, searching for the MAC address that corresponds to the IP address of the target device:

If the sending host finds an entry in its ARP table corresponding to the IP address of the target device, it simply writes that MAC address in the destination MAC address field in the Ethernet frame.

If the sending host does not find an entry in its ARP table corresponding to the IP address ...

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