Dive into Proxy ARP

Proxy ARP allows a host with not routing capability to reach remote subnets without the default gateway configuration. The hosts assume the network they reside as a flat network in which they can reach any hosts after the ARP resolution process. Proxy ARP is defined in RFC 1027.

Below are some of the disadvantages of using Proxy ARP:
1) It increases the amount of ARP traffic on the network.
2) A host requires larger ARP table for handling IP-to-MAC address mappings.
3) Security threat – spoofing, where a host claims to be another for intercepting packets.

Sample Proxy ARP Network

Below shows the ARP table on PC1 when RT1 providing Proxy ARP service and PC1 is not configured with any default gateway:

PC1#sh arp
Protocol  Address          Age (min)  Hardware Addr   Type   Interface
Internet  10.10.10.10             -   cc00.0f28.0000  ARPA   FastEthernet0/0
Internet  10.10.10.1              0   cc01.0f28.0000  ARPA   FastEthernet0/0
Internet  192.168.1.10            0   cc01.0f28.0000  ARPA   FastEthernet0/0
Internet  172.16.1.2              0   cc01.0f28.0000  ARPA   FastEthernet0/0
PC1#
RT1#sh ip redirects
Default gateway is not set

Host               Gateway           Last Use    Total Uses  Interface
ICMP redirect cache is empty
RT1#