R2511-HP MSR Router Series Layer 3 - IP Services Configuration Guide(V5)

10
Configuring proxy ARP
Overview
Proxy ARP enables a device on a network to answer ARP requests for an IP address not on that network.
With proxy ARP, hosts on different broadcast domains can communicate with each other as they do on
the same network.
Proxy ARP includes common proxy ARP and local proxy ARP.
• Common proxy ARP—Allows communication between hosts that connect to different Layer-3
interfaces and reside in different broadcast domains.
• Local proxy ARP—Allows communication between hosts that connect to the same Layer-3 interface
and reside in different broadcast domains.
Common proxy ARP
A proxy ARP enabled device allows hosts that reside on different subnets to communicate.
As shown in Figure 4, R
outer connects to two subnets through Ethernet 1/1 and Ethernet 1/2. 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 Ethernet 1/1 and Host B connects to Ethernet
1/2.
Figure 4 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 Ethernet 1/1 of the router so that the router can reply to the ARP request
from Host A with the MAC address of Ethernet 1/1, and forward packets sent from Host A to Host B. In
this case, the router acts as a proxy of Host B.
A main advantage of proxy ARP is that you can enable it on a single router 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.
Local proxy ARP
As shown in Figure 5, Host A and Host B belong to VLAN 2, but are isolated at Layer 2. Host A connects
to Ethernet 1/3 while Host B connects to Ethernet 1/1. Enable local proxy ARP on Router to allow Layer
3 communication between the two hosts.