I've been having troubles with my Internet connection for the past 15 days, which I'm trying to work through in a different thread. This present issue has to do with the various computers on my local network.

My ISP has been apparently dropping my connection from 1-5 minutes in duration (and up to 1/2 an hour) intermittently, several times during the day. Connection drop-out occur from between several minutes up to several hours with the average interval being 2-3 hours. This could be the ISP, or it could be some other hardware issue.

My problem is this: when that happens. None of the computers on the LAN can see or talk to each other until Internet connection is restored and I can do no work on LAN hosts except on the computer I'm physically sitting in front of and logged into. If I'm ssh'd from one local computer into another, I cannot type anything. scp's from one local computer to another stop. Samba mounts are unavailable. Cellphones using wireless lose connection (but note that the computers themselves are hard-wired, not wireless).

At first I thought my ASUS RT-AC66U was getting old and hanging up, so I bought a new ASUS RT-N66U. That didn't solve the problem at all.

I don't get this, because the LAN hosts are 192.168.0.0/24 and they should all be able to see and talk to each other, even if they can't get to the internet.

The ASUS router is the DHCP server and gateway. As such it would route DNS resolution upstream to the ISP. Four of the primary hosts are configured in the ASUS with static IPs.

If the ISP drops connection, why would I not be able to still communicate within the LAN? I cannot do so even if I bypass name resolution and use the hosts' actual IP address. Could the ASUS, being the router/gateway, be the problem? If so, why? It should be routing LAN internally, not hanging the LAN because of no Internet connection.

What can I do to diagnose the problem and fix this?

I've got the same problem, but only with one of the computers of my network: A PowerMac G5 under Mac OS X.4. It's nonsense... The only explanations would be a bad routing table but in my case, it seems correct on the PowerMac...

You don't say if your devices are wired or wireless. If they are wired then there shouldn't be a problem as effectively your router should act as a normal switch. If they are wireless then you may find that the router is turning the wireless off when there's no internet access.

Yes, I did: "note that the computers themselves are hard-wired, not wireless".

Sorry, my bad.

The wired computers are all directly connected to the router, yes? And there are only four of them?

Are you using IP addresses (or /etc/host-provided names) for the in-network computers? That is, is there a name-resolution issue if the DNS resolution depends on the ISP's name servers?

Are you using static routes in the router?

I don't believe that the wan should affect the lan in a typical scenario.

All lan computers ought to be still on a DHCP lease. Unless you hosted some special task on these routers they should still be all able to send any data between.

So try three things. One is to set a reserved lease for each local computer. Asus has a way to match mac to ip. Second is to see if you are on some ipv4to6 deal or just take ipv6 off. Third is to remove wan connection to see if the same symptoms exist.

I get the feeling that the router is not really being disconnected but may be under load from some issue. Log on to the router and see if you have an performance metrics to view while removing wan.

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The computers are all first directly connected to a (dumb) switch which is then connected to the router. The IP addresses are assigned by the router DHCPD which has static IP addresses assigned based on MAC address. I can (normally) connect based on host name, which the ASUS router apparently resolves. /etc/hosts is not used. DNS resolution should depend in the ISP for external hosts (e.g. linuxquestions.org) but should have nothing to do with LAN host resolution.

Exactly why I posted this!
Lease time is set to 86400 (if that's what you're referring to). As mentioned above, the Asus is matching MAC to IP via the LAN > DHCP Server > Manually Assigned IP, page.

Disconnecting the WAN is a great idea! I should have though of that. I'll do that as soon as other users are off, likely much later today or tomorrow.
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I get the feeling that the router is not really being disconnected but may be under load from some issue. Log on to the router and see if you have an performance metrics to view while removing wan.[/QUOTE]
I've looked at some of the traffic tools on this ASUS, but so far I've only found intra-LAN monitoring. Still, I'll check that next time.

I've read a few similar posts mentioning intermittent connectivity problems with this router model. Using wireshark on a given LAN host would likely show that ARP requests aren't getting replies from the router intermittently, part of a bigger problem with the router I suspect.

OK, I tried pulling the ISP cable line and, as predicted by posters and expected by me, I was still able to maintain connection and response among LAN hosts. So, is this pointing to something internal? The router itself?
This is a new router, and a different model (old: ASUS RT-AC66U, new ASUS RT-N66U), but the same manufacturer. "Outages" have been far fewer than with the old router -- perhaps two or three time on two days over the past week.

Would wireshark tell me anything helpful? Maybe the solution is to get a different manufacturer's router!

Only that if no replies to ARP requests occur, it would confirm an issue with the router.

If that is an option, then I would give it a try.