H3C Technologies H3C S10500 Series Switches - Manual (Page 26)

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Proxy ARP configuration
Proxy ARP overview
Proxy ARP includes common proxy ARP and local proxy ARP.
• Common proxy ARP allows communication when a sending host considers the receiving host to be
on the same subnet, but the receiving host actually resides on a different subnet.
• Local proxy ARP allows communication between hosts that reside on the same subnet but are
isolated at Layer 2.
In both cases, a device located between the two hosts must respond with the MAC address of the
receiving interface to allow Layer 3 communication between the two hosts. This is achieved by proxy ARP,
which hides the physical details of the network.
NOTE:
The term proxy ARP in the following sections of this chapter refers to common proxy ARP unless otherwise
specified.
Proxy ARP
A proxy ARP enabled device allows hosts that reside on different subnets to communicate.
As shown in
Figure 7 , a switch connects to two subnets through VLAN-interface 1 and VLAN-interface 2.
The IP addresses of the two interfaces are 192.168.10.99/24 and 192.168.20.99/24. Host A and Host
B are assigned the same prefix 192.168.0.0. Host A connects to VLAN-interface 1 and Host B connects
to VLAN-interface 2.
Figure 7 Application environment of proxy ARP
Because Host A and Host B have the same prefix 192.168.0.0, Host A considers that Host B is on the
same network, and it broadcasts an ARP request for the MAC address of Host B. However, Host B cannot
receive this request because it is in a different broadcast domain.
You can enable proxy ARP on VLAN-interface 1 of the switch so that the switch can reply to the ARP
request from Host A with the MAC address of VLAN-interface 1, and forward packets sent from Host A
to Host B. In this case, the switch acts as a proxy of Host B.
A main advantage of proxy ARP is that you can enable it on a single switch without disturbing routing
tables of other routers in the network. Proxy ARP acts as the gateway for hosts that are not configured with
a default gateway or do not have routing capability.