Cisco 3560G 48TS Software Configuration Guide - Page 616

Configuring IP Addressing Chapter 30 Configuring IP Unicast Routing Step 4 Step 5 Command show running-config copy running-config startup-config Purpose Verify your entry. (Optional) Save your entry in the configuration file. To restore the default and have the switch forward packets destined for a subnet of a network with no network default route to the best supernet route possible, use the ip classless global configuration command. Configuring Address Resolution Methods You can control interface-specific handling of IP by using address resolution. A device using IP can have both a local address or MAC address, which uniquely defines the device on its local segment or LAN, and a network address, which identifies the network to which the device belongs. The local address or MAC address is known as a data link address because it is contained in the data link layer (Layer 2) section of the packet header and is read by data link (Layer 2) devices. To communicate with a device on Ethernet, the software must determine the MAC address of the device. The process of determining the MAC address from an IP address is called address resolution. The process of determining the IP address from the MAC address is called reverse address resolution. The switch can use these forms of address resolution: • Address Resolution Protocol (ARP) is used to associate IP address with MAC addresses. Taking an IP address as input, ARP determines the associated MAC address and then stores the IP address/MAC address association in an ARP cache for rapid retrieval. Then the IP datagram is encapsulated in a link-layer frame and sent over the network. Encapsulation of IP datagrams and ARP requests or replies on IEEE 802 networks other than Ethernet is specified by the Subnetwork Access Protocol (SNAP). • Proxy ARP helps hosts with no routing tables determine the MAC addresses of hosts on other networks or subnets. If the switch (router) receives an ARP request for a host that is not on the same interface as the ARP request sender, and if the router has all of its routes to the host through other interfaces, it generates a proxy ARP packet giving its own local data link address. The host that sent the ARP request then sends its packets to the router, which forwards them to the intended host. Catalyst 3560 switches also use the Reverse Address Resolution Protocol (RARP), which functions the same as ARP does, except that the RARP packets request an IP address instead of a local MAC address. Using RARP requires a RARP server on the same network segment as the router interface. Use the ip rarp-server address interface configuration command to identify the server. For more information on RARP, refer to the Cisco IOS Configuration Fundamentals Configuration Guide for Release 12.1. You can perform these tasks to configure address resolution: • Define a Static ARP Cache, page 30-9 • Set ARP Encapsulation, page 30-10 • Enable Proxy ARP, page 30-10 30-8 Catalyst 3560 Switch Software Configuration Guide 78-16156-01