Address Resolution - Cisco Catalyst 9500 series Configuration Manual

Configuring IP Unicast Routing

Address Resolution

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 learn the MAC address of the device. The process of learning the
MAC address from an IP address is called address resolution. The process of learning the IP address from
the MAC address is called reverse address resolution.
The Device can use these forms of address resolution:
• Address Resolution Protocol (ARP) is used to associate IP address with MAC addresses. Taking an IP
• Proxy ARP helps hosts with no routing tables learn the MAC addresses of hosts on other networks or
The Device also uses 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, see the Cisco IOS Configuration Fundamentals Configuration Guide
Proxy ARP
Proxy ARP, the most common method for learning about other routes, enables an Ethernet host with no routing
information to communicate with hosts on other networks or subnets. The host assumes that all hosts are on
the same local Ethernet and that they can use ARP to learn their MAC addresses. If a Device receives an ARP
request for a host that is not on the same network as the sender, the Device evaluates whether it has the best
route to that host. If it does, it sends an ARP reply packet with its own Ethernet MAC address, and the host
that sent the request sends the packet to the Device, which forwards it to the intended host. Proxy ARP treats
all networks as if they are local and performs ARP requests for every IP address.
ICMP Router Discovery Protocol
Router discovery allows the Device to dynamically learn about routes to other networks using ICMP router
discovery protocol (IRDP). IRDP allows hosts to locate routers. When operating as a client, the Device
generates router discovery packets. When operating as a host, the Device receives router discovery packets.
The Device can also listen to Routing Information Protocol (RIP) routing updates and use this information to
infer locations of routers. The Device does not actually store the routing tables sent by routing devices; it
address as input, ARP learns 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).
subnets. If the Device (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.
Routing Configuration Guide, Cisco IOS XE Everest 16.6.x (Catalyst 9500 Switches)
Address Resolution
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