In which cases is it needed to enable Proxy ARP function?

Unanswered Question
Jan 9th, 2009

I see 2 cases when it is needed to set proxy ARP in a router, but there might not be the good ones. Additionally I could be missing some other cases as well.

1) - when a DHCP server is not on the same LAN with the hosts, as otherwise the DHCP Discover messages (which are broadcast messages) will not pass the router and so the hosts will never get a reply (i.e. a DHCP offer).

2) - when the hosts are not configured with a Gateway, as otherwise the ARP messages with IPs from another subnets (ARP are broadcast messages) will not be replied.


Is there any exhaustive explanations/links regarding proxy arp?

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Jon Marshall Fri, 01/09/2009 - 16:22

Petru


1) This is not really to do with proxy-arp. This is to do with the "ip helper-address .." command. If you want a device to be able to get a DHCP address from a DHCP server on another network under the devices L3 interface you would configure an ip helper-address eg.


device is on subnet 192.168.5.0/24

router interface for this subnet is fa0/0 with address of 192.168.5.1


DHCP server is 192.168.10.10


so on router


int fa0/0

ip address 192.168.5.1 255.255.255.0

ip helper-address 192.168.10.10


2) Yes this is the more common use of proxy arp. Have a read of this link and come back if you have further questions -


http://www.cisco.com/en/US/tech/tk648/tk361/technologies_tech_note09186a0080094adb.shtml


Jon

Roberto Salazar Fri, 01/09/2009 - 16:23

1) - when a DHCP server is not on the same LAN with the hosts, as otherwise the DHCP Discover messages (which are broadcast messages) will not pass the router and so the hosts will never get a reply (i.e. a DHCP offer).

>> Not really, you need a DHCP relay for this, an IP helper-address is a relay which will let the DHCP request from clients which are bcast to be unicast to the DHCP server, it is not the function of proxy arp.


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