Service Domain Features
2-3
8000-A2-GB21-20
November 1997
WS1 sends a packet to Router A.
Router A 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 Router B, Router A then ARPs for
Router B. Router B receives the ARP request for its IP address and does an ARP
reply with its MAC address. After Router A receives the ARP reply, it sends the
packet to the Router B which, in turn, forwards it to WS2.
Scenario 2: With Proxy ARP
In this scenario, Router B is running the proxy ARP software, and WS2 and
Router A for WS1 are on the same network (135.1.0.0).
97-15459-02
Router B
135.1.2.6/
255.255.255.0
135.1.3.9/
255.255.255.0
135.1.3.45/
255.255.255.0
LAN A
LAN B
Router A
135.1.2.3/
255.255.0.0
WS2
WS1
WS1 again needs to send a packet to WS2. This time, however, Router B is
running proxy ARP and knows that WS2 lies on LAN B on the same logical
subnetwork as Router A (135.1.0.0). Router B uses proxy ARP to maintain the
illusion that only one physical network exists. Router B keeps the location of WS2
hidden from Router A, allowing Router A to communicate as if directly connected
to WS2.
NOTE:
Router A 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 Router A.
Router A invokes ARP to map the WS2’s IP address into a MAC address,
because WS2 appears to Router A to be on the same 135.1 subnet.
Router B running proxy ARP software receives the broadcast ARP request
from Router A, knows that WS2 is on LAN B, and responds to Router A’s
ARP request with its own MAC address.
Router A receives the ARP reply, then sends the packet to the MAC address
of Router B.
Router B then forwards the packet destined for WS2 on LAN B.
