Traffic flow stops after 30 mins when proxy arp is disabled

[renju.zac]

We configuring a cisco 2811 router for internet access. One ethernet interface of this router connects to the lan and the other to the ISP. As part of hardening we disabled proxy arp on both ethernet interfaces.

We noticed that the traffic flow through the router stops after 30 minutes.

When the traffic is not flowing , users on the lan can ping the LAN ethernet interface , but not the Internet Ethernet Interface.

A simple ping to the Internet from the router works fine. However , an extended ping sourced from the LAN interface fails.

The same router configuration with proxy-arp enabled on both ethernet interfaces , the traffic flows smoothly even after 30 minutes.

Can disabling proxy arp cause such a behaviour ?

[pkhatri]

Hi,

One possible reason for this could be that your ISP has configured a subnet mask on its interface facing you to be different to what you have configured on your interface to the ISP. For example, say the ISP gave you the 10.1.1.0/24 network and you split it into 10.1.1.0/25 and 10.1.1.128/25. Also assume that the ISP used the 255.255.255.0 mask on its own interface to you. If you then assigned the first network to your WAN link and the second to your internet link, the following will happen: when the ISP wishes to get to an address in the second subnet, it will think it's on the same network as its interface to your router. It will then send out an ARP for it. WHen proxy-ARP is enabled, your router will respond with its own MAC address. If proxy-ARP is not enabled, this will fail.

Does this match your scenario ?

Hope that helps - pls rate the post if it does.

Regards,

Paresh.

[renju.zac]

Thank you for te suggestions. The scenario appears to be similar. Need to check if this is indeed causing the problem.

Regards,

Renju