User`s guide

Customer Domain Features
2-4
8000-A2-GB21-10
June 1997
Scenario 2: With Proxy ARP
In this scenario, a router is running the proxy ARP software, and WS2 and the
default router for WS1 are on the same network (135.1.0.0).
97-15459-0
1
Router
135.1.2.6/24
135.1.3.9/24
135.1.3.45/24
LAN A LAN B
Default
Router
(for WS1)
135.1.2.3/16
WS2
WS1
WS1 again needs to send a packet to WS2. This time, however, the router is
running proxy ARP and knows that WS2 lies on LAN B on the same logical
subnetwork as the default router (135.1.0.0). The router uses proxy ARP to
maintain the illusion that only one physical network exists. The router keeps the
location of WS2 hidden from the default router, allowing the default router to
communicate as if directly connected to WS2.
NOTE:
The default router does not need a static route entry for the WS2 route
because the two LANs appear to be one.
Therefore, when WS1 needs to send a packet to WS2, this is the sequence of
events:
 WS1 sends a packet to its default router.
 The default router invokes ARP to map the WS2’s IP address into a MAC
address.
 The router running proxy ARP software receives the broadcast ARP request
from the default router, knows that WS2 is on LAN B, and responds to the
default router’s ARP request with its own MAC address.
 The default router receives the ARP reply, then sends the packet to the MAC
address of the router.
 The router then forwards the packet destined for WS2 on LAN B.
The proxy ARP capability is card or system dependent and detailed examples for
the MCC card, DSL card, and HotWire 5446 RTU are given in Chapter 4.