Understanding Proxy ARP on EX Series Switches
You can configure proxy Address Resolution Protocol (ARP) on your Juniper Networks EX Series Ethernet Switch to enable the switch to respond to ARP queries for network addresses by offering its own Ethernet media access control (MAC) address. With proxy ARP enabled, the switch captures and routes traffic to the intended destination.
Proxy ARP is useful in situations where hosts are on different physical networks and you do not want to use subnet masking. Because ARP broadcasts are not propagated between hosts on different physical networks, hosts will not receive a response to their ARP request if the destination is on a different subnet. Enabling the switch to act as an ARP proxy allows the hosts to transparently communicate with each other through the switch. Proxy ARP can help hosts on a subnet reach remote subnets without configuring routing or a default gateway.
What Is ARP?
Ethernet LANs use ARP to map Ethernet MAC addresses to IP addresses. Each device maintains a cache containing a mapping of MAC addresses to IP addresses. The switch maintains this mapping in a cache that it consults when forwarding packets to network devices. If the ARP cache does not contain an entry for the destination device, the host (the DHCP client) broadcasts an ARP request for that device's address and stores the response in the cache.
Proxy ARP Overview
When proxy ARP is enabled, if the switch receives an ARP request for which it has a route to the target IP address, the switch responds by sending a proxy ARP reply packet containing its own MAC address. The host that sent the ARP request then sends its packets to the switch, which forwards them to the intended host.
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Note: For security reasons, the source address in an ARP request must be on the same subnet as the interface on which the ARP request is received. |
Proxy ARP Modes
EX Series switches support two modes of proxy ARP, unrestricted and restricted. Both modes require that the switch has an active route to the destination address of the ARP request.
- unrestricted—The switch responds to all ARP requests for which it has a route. This is the default mode (due to compatibility with other Juniper Networks JUNOS Software configurations).
- restricted—The switch responds to ARP requests in which the physical networks of the source and target are different, and does not respond if the source and target IP addresses are in the same subnet. In this mode, hosts on the same subnet communicate. This mode is recommended for most situations.
Best Practices for Proxy ARP on EX Series Switches
When configuring proxy ARP on EX Series switches, the following best practices are recommended:
- Disable gratuitous ARP requests on each interface on which proxy ARP is enabled.
- When configuring proxy ARP on a routed VLAN interface (RVI), we recommend that you use unrestricted mode because in restricted mode, ARP resolution will not take place between two hosts on the VLAN with which the RVI is associated.
- We recommend setting proxy ARP to restricted mode in most situations outside of a routed VLAN interface.
