BT Business Hub -- ARP broadcast problems

[ROJones]

Hi all,

We have a BT Business Hub 3.0 (Version 4.7.5.1.83.8.82.1.1 [Type A]) with several wireless and Ethernet devices connected.

I've having some trouble with connecting from wired devices to some embedded wireless modules we're trying out: I think that this is down to the Hub not passing on ARP messages from the Ethernet side to the wireless side. For example, if I ping one of the wireless modules from my PC I don't get any replies but as soon as I ping the PC from the wireless module I can ping it back from the PC with no trouble. After a while the IP/MAC address is dropped from the ARP cache table on my PC and I can no longer ping the module. If I add a static ARP entry for the module to the ARP cache on the PC then I can always ping it.

With Wireshark on the Ethernet I can see my PC sending an ARP request for the wireless module but it does not get a reply. Running Wireshark on the wireless side I can't see the PC's ARP requests at all.

I've tried the same test with both an old Netgear and Thomson access points and these work as expected -- the ARP broadcasts go in both directions and I can connect at will from the PC to the module.

Is there some hidden setting on the Business Hub that I've missed or may have messed up? Is this a fault or bug on the Hub that anyone else has spotted?

EDIT: Just tried a BT Home Hub 2.0 (Version 8.1.H.U[Type A]) and a BT Business Hub 2700HGV, both of which work without a hitch.

[markp]

Hi ROJones,

it is not an issue I am familure with, I have spoken to a colleague and he has advised that is should not be an issue doing this with the hub 3.

Have you tried a factory reset on the hub3 to see if it makes a difference?

Markp

[ROJones]

Hi markp,

Thanks for the reply -- I've not tried a reset yet but I'll see if I can do one this evening when there are less people in the office (otherwise I'll get shouted at 😉 ) and get back to you. I'll grab a backup of the Hub settings before doing a reset anyway so I can restore them later on, would a copy of that file be of any help at all?

I was also thinking, the company across the car-park also have a Business Hub 3 so I could ask them if I could borrow their network for a quick test to see if it's just ours that's doing it (which I suspect is the case as no one else seems to be having a problem).

Thanks,

[Burkem5]

Hi @ROJones 

What happens if you use the command "arp -a" on the PC when the ping times out
and does the IP of Access point then show up in "arp -a" when you ping from the AP to the PC?

Can you set the access point to a static internal IP and see if this stops the issue as i would imagine the lease on the IP expiring is what causes the issue to come back after a time.

Thanks

Burkem5

[ROJones]

Hi Burkem5,

Thanks for the suggestions.

When pings from the PC to the module fail, "arp -a" does not show the module IP. As soon as I ping the PC from the module "arp -a" shows the module IP address and everything works for a few moments but as soon as I stop using the address for, say, 5 minutes the module get's cleared from the ARP cache and vanishes from "arp -a".

Using "arp -s <module IP> <module MAC>" on the PC to add a static entry for the module works, until I reboot the PC. Using a static IP for the module does not help as the PC still can't find a route to the module without some sort of intervention on my part.

Listening to network traffic on both the "wired" and "wireless" side of the network I can see the PC sending out ARP broadcasts for the module's IP address but they are not showing up on the wireless side. When I ping the PC from the module I can see it's ARP broadcasts on both sides and the PC's reply comes back (as it's now routed, not broadcast). I believe I can also replicate the problem using a laptop on the wireless side so it's not just these modules.

I've not had the chance to try resetting the Hub yet as I had other work to be getting on with and I can work around the problem for now by using a second accesspoint (old BT 2700HGV) on the network or "arp -s" to add a static route.

Thanks again,

Roger.

[Burkem5 · Accepted solution]

Hi @ROJones 

What happens if you set a static route within the hub its self to a local static applied to the wireless device?

that's in settings > advanced settings > Broadband > routing

Let me know how ou get on with that

Burkem5

[ROJones]

Hi Burkem5,

How very odd... I went to the routing page on the Hub but could not add a new static route as there was one alredy in the list for the subnet (192.168.0.0/255.255.0.0; gw=0.0.0.0; if=br0). I noticed a section at the bottom for "Dynamic Routing" that was set to "No". I though "why not" and changed it to "Yes" and suddenly everything works as it should. Don't know if it was trying to add the route or enabling Dynamoc Routing that did it.

The weird thing is that I then changed the Dynamic Routing back to "No" and it still works so I'm very confused -- unless changing the setting back and forth and saving it corrected something that was corrupt in the Hub's flash before.

I'll keep testing it for the next few days, not sure if this is a soulution as such but I'll mark the thread as solved if it's still working tomorrow

Thanks.

[Burkem5]

0_0   That's really strange,  let me know if this keeps up and i will see if we can do this in a test enviroment to see if that happens on our end.

Thanks

Burkem5

[ROJones]

Just to let you know that I've been testing the modules for a couple of days now and have not seen a recurrence of the problem since fiddling with the "Dynamic Routing" setting on the Hub. In fact, I can't even force it to go wrong now, so I don't even know if that's what solved the issue...

I'm going to accept your post about adding static routes as a solution as it got me to that page on the Hub which seemed to have fixed the issue.

Thanks again.

Roger.