3Com Switch 7754, Configuration Manual, Page: 583 / 940

55
P
ROXY
ARP C
ONFIGURATION
Proxy ARP Overview
Proxy ARP allows hosts that have IP addresses of the same network segment but
reside on different physical networks to communicate with each other through
ARP.
Figure 148 Work mechanism of proxy ARP
As shown in Figure 148:
From the perspective of the switch, Host A and Host D reside on different
networks. However, when Host A (192.168.0.22/16) needs to send packets to
Host D (192.168.1.30/16), it finds they are on the same network 192.168.0.0/16,
and thus Host A will broadcast an ARP request to request the MAC address of
Host D.
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When the proxy ARP feature is not enabled on the switch, because Host A and
Host D are in different VLANs, the ARP request sent by Host A cannot reach
Host D, and the two hosts cannot communicate.
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With proxy ARP enabled on the switch, when VLAN-interface 3 receives the
ARP request, if the switch finds a route to the destination IP address
(encapsulated in the ARP request) in the routing table, the switch sends host A
the MAC address (00-00-0e-12-33-33) of VLAN-interface 3 in an ARP response
(with the source IP address being the destination IP address of the ARP
request). After receiving the ARP response, Host A creates an ARP entry, in
which the destination IP address is the IP address of Host D (192.168.1.30/16),
and the MAC address is that of VLAN-interface 3 (00-00-0e-12-33-33). The
subsequent packets sent from Host A to Host D will all be sent to
192.168.0.22/16
00-00 -0e-12 -33-34
Host A
192 .168.1.29/16
00 -00-0 e-14-34 -34
Host B
Host C
Host D
192.168.0.23/16
00-00-0 e-12-33 -35 192 .168.1.30/16
00 -00-0 e-14-34 -35
Vlan -int3
192 .168 .0.27 /24
00-00 -0e-12 -33-33
Vlan -int4
192 .168 .1.27 /24
00 -00-0e -14-34 -33
Switch