Junos® OS Ethernet Interfaces Configuration ... - Juniper Networks

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OS 12.3 Ethernet Interfaces Configuration Guide
WARNING: If you configure unrestricted proxy ARP, the proxy router replies
to ARP requests for the target IP address on the same interface as the
incoming ARP request. This behavior is appropriate for cable modem
termination system (CMTS) environments, but might cause Layer 2
reachability problems if you enable unrestricted proxy ARP in other
environments.
When an IP client broadcasts the ARP request across the Ethernet wire, the end node
with the correct IP address responds to the ARP request and provides the correct MAC
address. If the unrestricted proxy ARP feature is enabled, the router response is redundant
and might fool the IP client into determining that the destination MAC address within its
own subnet is the same as the address of the router.
NOTE: While the destination address can be remote, the source address of
the ARP request must be on the same subnet as the interface upon which
the ARP request is received. For security reasons, this rule applies to both
unrestricted and restricted proxy ARP.
Topology Considerations for Unrestricted Proxy ARP
170
In most situations, you should not configure the router to perform unrestricted proxy ARP.
Do so only for special situations, such as when cable modems are used. Figure 16 on
page 170 and Figure 17 on page 171 show examples of situations in which you might want
to configure unrestricted proxy ARP.
In Figure 16 on page 170, the edge device is not running any IP protocols. In this case, you
configure the core router to perform unrestricted proxy ARP. The edge device is the client
of the proxy.
In Figure 17 on page 171, the Broadband Remote Access Server (B-RAS) routers are not
running any IP protocols. In this case, you configure unrestricted proxy ARP on the B-RAS
interfaces. This allows the core device to behave as though it is directly connected to the
end users.
Figure 16: Edge Device Case for Unrestricted Proxy ARP
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