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Example: Configuring Proxy ARP on an EX Series Switch

Requirements

• Junos OS Release 10.0 or later for EX Series switches
• One EX Series switch

Overview and Topology

This example shows the configuration of proxy ARP on an interface of an EX Series switch using restricted mode. In restricted mode, the switch does not proxy for hosts on the same subnet.

The topology for this example consists of one EX Series switch. When a host wants to communicate with a host that is not already in its ARP table, it broadcasts an ARP request for the MAC address of the destination host:

• When proxy ARP is not enabled, a host that shares the same IP address replies directly to the ARP request, providing its MAC address, and future transmissions are sent directly to the destination host MAC address.
• When proxy ARP is enabled, the switch responds to ARP requests, providing the switch’s MAC address—even when the destination IP address is the same as the source IP address. Thus, communications must be sent through the switch and then routed through the switch to the appropriate destination.

Configuration

To configure proxy ARP, perform the following tasks:

CLI Quick Configuration

To quickly configure proxy ARP on an interface, copy the following command and paste it into the switch terminal window:

Step-by-Step Procedure

1. To configure proxy ARP on an interface:
   [edit interfaces]
user@switch# set ge-0/0/3 unit 0 proxy-arp restricted

   Best Practice: We recommend that you configure proxy ARP in restricted mode. In restricted mode, the switch does not act as proxy if the source and target IP addresses are on the same subnet. If you use unrestricted mode, disable gratuitous ARP requests on the interface to avoid the situation of the switch’s response to a gratuitous ARP request appearing to the host to be an indication of an IP conflict: [edit interfaces] user@switch# set ge-0/0/3 no-gratuitous-arp-request

Results

Display the results of the configuration:

Verification

To verify that the switch is sending proxy ARP messages, perform these tasks:

• Verifying That the Switch Is Sending Proxy ARP Messages

Verifying That the Switch Is Sending Proxy ARP Messages

Purpose

Verify that the switch is sending proxy ARP messages.

Action

List the system statistics for ARP messages:

arp:
        198319 datagrams received
        45 ARP requests received
        12 ARP replies received
        2 resolution requests received
        2 unrestricted proxy requests
        0 restricted proxy requests
        0 received proxy requests
        0 proxy requests not proxied
        0 restricted-proxy requests not proxied
        0 with bogus interface
        0 with incorrect length
        0 for non-IP protocol
        0 with unsupported op code
        0 with bad protocol address length
        0 with bad hardware address length
        0 with multicast source address
        0 with multicast target address
        0 with my own hardware address
        168705 for an address not on the interface
        0 with a broadcast source address
        0 with source address duplicate to mine
        29555 which were not for me
        0 packets discarded waiting for resolution
        4 packets sent after waiting for resolution
        27 ARP requests sent
        47 ARP replies sent
        0 requests for memory denied
        0 requests dropped on entry
        0 requests dropped during retry
        0 requests dropped due to interface deletion
        0 requests on unnumbered interfaces
        0 new requests on unnumbered interfaces
        0 replies for from unnumbered interfaces
        0 requests on unnumbered interface with non-subnetted donor
        0 replies from unnumbered interface with non-subnetted donor

Meaning

The statistics show that two proxy ARP requests were received, and the proxy requests not proxied field indicates that all the unproxied ARP requests received have been proxied by the switch.