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Example: Configuring Proxy ARP on an EX Series Switch

Requirements

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

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 act as a 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 a 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 a situation wherein the switch’s response to a gratuitous ARP request appears 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:
         90060 datagrams received
         34 ARP requests received
         610 ARP replies received
         2 resolution request  received
         0 unrestricted proxy requests
         0 restricted proxy requests
         0 received proxy requests
         0 unrestricted proxy requests not proxied
         0 restricted proxy requests not proxied
         0 datagrams with bogus interface
         0 datagrams with incorrect length
         0 datagrams for non-IP protocol
         0 datagrams with unsupported op code
         0 datagrams with bad protocol address length
         0 datagrams with bad hardware address length
         0 datagrams with multicast source address
         0 datagrams with multicast target address
         0 datagrams with my own hardware address
         0 datagrams for an address not on the interface
         0 datagrams with a broadcast source address
         294 datagrams with source address duplicate to mine
         89113 datagrams which were not for me
         0 packets discarded waiting for resolution
         0 packets sent after waiting for resolution
         309 ARP requests sent
         35 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. The unrestricted proxy requests not proxied and restricted proxy requests not proxied fields indicate that all the unproxied ARP requests received have been proxied by the switch.