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cisco 100-105 dumps free.16.5.1 Local ARP TablesAs stated in Chapters 5 and 9, ARP is used to resolve a Layer 3 IP address to a Layer 2 MAC address. When a LAN device in a subnet needs to access resources outside the subnet, it must forward frames to the MAC address of the default gateway (router), which is obtained using ARP resolution. The router creates a local ARP table to keep track of the IP address to MAC address mapping when it receives traffic on the interface. To view the contents of this table, use the command show arp or showiparp.IOS# show ip arp Protocol Address Age (min) Hardware Addr Type Interface Internet 10.0.6.2 -0007.0e46.4070 ARPA FastEthernet0/0 Internet 172.30.6.2 - 0007.0e46.4071 ARPA FastEthernet0/1 Internet 172.30.6.7 0 0050.5480.7e01 ARPA FastEthernet0/1Hyphen (.) in the (min) column indicates that the address is local to the router and the time value indicates that the IP/MAC address mapping is dynamically learned by the router. In the above example, the last entry is dynamically learned at the last minute. If a specific MAC address is not seen within the specified time, it is removed from the ARP table. In addition, whenever a frame matches an existing entry in the table, the expiration time for that entry is reset to zero. The Type column indicates the type of Ethernet encapsulation (ARPA, SNAP, or SAP) used by the frame: the type of Ethernet encapsulation used by TCP/IP is ARPA.The command showiparp is important because as long as the mappings for the router interfaces are seen in the ARP table and these entries are updated, there are no problems with Layer 2 connectivity to these interfaces.Video 16.08.A multimedia demonstration of viewing ARP tables on a router is included in the companion digital resource to this book.16.5.2 Cisco Discovery Protocol (CDP)CDP is a Cisco-specific data link layer protocol that was originally introduced in IOS version 10.3. CDP is supported on many, but not all, Cisco devices, including Cisco routers and Catalyst switches. On devices that support CDP, CDP is enabled by default. By default, incoming CDP messages are not forwarded to other devices. In other words, on the current device, you can only see CDP messages from other Cisco devices that are directly connected to it. Given that CDP uses multicast to propagate information, most people do not properly recognize this; they assume that the Cisco switch floods this traffic, but CDP is an exception in networks consisting of Cisco devices.Exam Essentials You must know that whenever a CDP frame is received from a directly connected Cisco neighbor, it means that at least the data link layer is operating properly. CDP messages are not propagated to other Cisco devices to which the directly connected Cisco device is connected.