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156 Summit 300-48 Switch Software User Guide
IP Unicast Routing
Proxy ARP
Proxy Address Resolution Protocol (ARP) was first invented so that ARP-capable devices could respond
to ARP Request packets on behalf of ARP-incapable devices. Proxy ARP can also be used to achieve
router redundancy and simplify IP client configuration. The switch supports proxy ARP for this type of
network configuration. The section describes some example of how to use proxy ARP with the switch.
ARP-Incapable Devices
To configure the switch to respond to ARP Requests on behalf of devices that are incapable of doing so,
you must configure the IP address and MAC address of the ARP-incapable device using the use the
following command:
config iparp add proxy <ipaddress> {<mask>} <mac_address> {always}
Once configured, the system responds to ARP Requests on behalf of the device as long as the following
conditions are satisfied:
• The valid IP ARP Request is received on a router interface.
• The target IP address matches the IP address configured in the proxy ARP table.
• The proxy ARP table entry indicates that the system should always answer this ARP Request,
regardless of the ingress VLAN (the
always parameter must be applied).
Once all the proxy ARP conditions are met, the switch formulates an ARP Response using the
configured MAC address in the packet.
Proxy ARP Between Subnets
In some networks, it is desirable to configure the IP host with a wider subnet than the actual subnet
mask of the segment. Proxy ARP can be used so that the router answers ARP Requests for devices
outside of the subnet. As a result, the host communicates as if all devices are local. In reality,
communication with devices outside of the subnet are proxied by the router.
For example, an IP host is configured with a class B address of 100.101.102.103 and a mask of
255.255.0.0. The switch is configured with the IP address 100.101.102.1 and a mask of 255.255.255.0. The
switch is also configured with a proxy ARP entry of IP address 100.101.0.0 and mask 255.255.0.0, without
the
always parameter.
When the IP host tries to communicate with the host at address 100.101.45.67, the IP hosts
communicates as if the two hosts are on the same subnet, and sends out an IP ARP Request. The switch
answers on behalf of the device at address 100.101.45.67, using its own MAC address. All subsequent
data packets from 100.101.102.103 are sent to the switch, and the switch routes the packets to
100.101.45.67.