28-8
Catalyst 3750 Metro Switch Software Configuration Guide
78-15870-01
Chapter 28 Configuring IP Unicast Routing
Configuring IP Addressing
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 3750 Metro 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 28-8
• Set ARP Encapsulation, page 28-9
• Enable Proxy ARP, page 28-10
Define a Static ARP Cache
ARP and other address resolution protocols provide dynamic mapping between IP addresses and MAC
addresses. Because most hosts support dynamic address resolution, you usually do not need to specify
static ARP cache entries. If you must define a static ARP cache entry, you can do so globally, which
installs a permanent entry in the ARP cache that the switch uses to translate IP addresses into MAC
addresses. Optionally, you can also specify that the switch respond to ARP requests as if it were the
owner of the specified IP address. If you do not want the ARP entry to be permanent, you can specify a
timeout period for the ARP entry.
Beginning in privileged EXEC mode, follow these steps to provide static mapping between IP addresses
and MAC addresses:
Command Purpose
Step 1
configure terminal Enter global configuration mode.
Step 2
arp ip-address hardware-address type Globally associate an IP address with a MAC (hardware) address
in the ARP cache, and specify encapsulation type as one of
these:
• arpa—ARP encapsulation for Ethernet interfaces
• snap—Subnetwork Address Protocol encapsulation for
Token Ring and FDDI interfaces
• sap—HP’s ARP type
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