User`s guide

Customer Domain Features
2-3
8000-A2-GB21-10
June 1997
Proxy ARP (Theory of Operation)
An Address Resolution Protocol (ARP) request is used to dynamically bind an IP
address to a MAC address. Proxy ARP is a technique by which a router answers
ARP requests intended for another machine by supplying its own MAC address
(also referred to as the physical address). By answering for another device, the
router accepts responsibility for forwarding packets to that device.
ARP is supported by the MCC and DSL cards, and the HotWire 5446 RTU. Proxy
ARP allows the end users to appear to be directly connected to the router or
VLAN switch providing access to the ISP network. This is an advantage because
routers connected to a device running proxy ARP require less configuration. The
following scenarios show why this is an advantage.
Scenario 1: Without Proxy ARP
In this scenario, a router does not have proxy ARP software and the networks of
the default router for workstation 1 (175.1.2.3) and workstation 2 (135.1.3.45) are
different.
97-15458-01
Router
175.1.2.6
135.1.3.9
135.1.3.45
LAN A LAN B
WS2
Default
Router
(for WS1)
175.1.2.3
WS1
Workstation 1 (WS1) needs to send a packet to workstation 2 (WS2). For the
packet to arrive successfully at WS2:
 Configure a static route on the default router for WS2 (the next hop being the
router and the destination being WS2).
 WS1 sends a packet to the default router.
 The default router consults its routing table to determine the next hop
address (i.e., router IP address) for WS2 because WS2 is on another network
(135.1.0.0).
Now that it knows the next hop address to the router, the default router then
ARPs for the router. The router receives the ARP request for its IP address and
does an ARP reply with its MAC address (also known as the physical address).
After the default router receives the ARP reply, it sends the packet to the router
which, in turn, forwards it to WS2.