ASUS RT-AC68U with Virgin Media Hub 3.0

[mmelbourne]

Hi,

I have recently purchased a RT-AC68U (h/w Rev. A2), and running Merlin 380.58.

I am connecting it toa Virgin Media Hub 3.0 in Modem Mode.

Running a constant ping to external destinations shows that traffic is dropped every few minutes (other traffic is also effected, e.g. DNS queries), and recovers after a few seconds to only fail again minutes later. (This is all over wired connections; I eliminated wireless as an issue early on).

If I replace the RT-AC68U with a Cisco ASA firewall, then the traffic drops do not occur. Nor do they occur if I connect the Hub 3.0 (Cable Modem) directly to a laptop in modem mode.

My next step is to probably span the ASUS WAN port, to check whether the packets make it onto the wire when connectivity is lost. There are no entries in the log on the router when connectivity is lost, but the Network Map does detect that connectivity is down.

This is completely baffling as I can't really blame the cable modem as another device works. I have also tried placing a dumb switch inbetween the cable modem and the ASUS router's WAN port in case it's some weird electrical interop issue.

Any ideas?

Kind regards,
Matt

 

[ColinTaylor]

Very strange. Are there any clues in the VM Hub's log (192.168.100.1)?

How about putting the Hub back into router mode and plugging the PC straight into it. Any change?

 

[mstombs]

I have been following a similar problem elsewhere - my N66 works fine - doesn't have this problem with my SH3 in modem mode

http://community.virginmedia.com/t5/Networking-and-wireless/Hub-3-0-port-flapping/m-p/3073990#M97654

Do you get more than 35 mins?

 

[mmelbourne]

I think my issue is subtly different. I don't get anywhere near 35 minutes - may be a couple max, then it drops for a few seconds, and comes back (and repeats...)

One interesting point, a continuous ping to 192.168.100.1 responds during those periods when all other traffic is dropped.

There are no errors in the VM Hub's logs. This might point to a cable modem issue, but another device instead of the ASUS works (I've actually tried a Cisco 877W, and that's fine too).

The Hub 3.0 in Router mode works reliably too. RF levels appear good.

I've looked at the thread referenced about - time for a managed switch and some extensive debugging - port mirrors and monitoring of line protocol. It shouldn't be this hard!

 

[ColinTaylor]

One interesting point, a continuous ping to 192.168.100.1 responds during those periods when all other traffic is dropped.

I was going to say that as you had eliminated everything else it must be something physical/electrical with the ASUS; but then you say this

So if it's something specific to the ASUS I can only think of two things to try;

1) If it's a firmware issue try another version; apparently there are known problems with the recent firmwares. http://www.snbforums.com/threads/rt-ac68u-random-crashes.31587/#post-251216

2) IIRC You can get connected to different CMTS's depending on your WAN MAC address. Try cloning the MAC of a device that works when directly connected to the modem (your PC) onto the ASUS. It might be a provisioning issue with VM.

 

[mstombs]

There are no errors in the VM Hub's logs.

This is a feature of VM SH3, the modem log doesn't even record anything on power cycling/reboot. The web gui also doesn't give any info about error rate before/after correction, but there are backdoor methods to get this info via http snmp queries. There is a known issue with ping response from/through the SH3 - something to do with the 16 downstream RF channels I understand - here's my ping monitor - the N66 responds with SH3 in modem mode, there are frequent 100ms spikes, but average is low.



This doesn't appear to affect usability, likely something just de-prioritizing ping traffic.

I'm sure normal CM rules apply re IP/MAC address - if you change the connected PC in modem mode you must power cycle the modem. i clone my laptop Ethernet MAC onto the N66 so I can swap for diagnostics easily.

 

[mmelbourne]

Well, with the ASUS router installed, and mirroring the traffic from its WAN port on a switch placed between the VM SH3 and the ASUS router, I see lots of TCP retransmissions coming from destination servers towards the router, and lots of Duplicate ACKs going from the router to the same destination (when downloading a test file via HTTP) - I see an ACK, then maybe up to 40 duplicate ACKs for the original transmitted ACK each time this occurs. This happened around 14 times in a ~3 minute window.

If I replace the ASUS with Cisco ASA, and perform the same test, the capture is clean; no retransmissions or duplicate ACKs - downloading the same file from the same origin server.

I can't really point the finger towards the VM SH3 here. I am going to have to try different ASUS firmware next.

I do agree that the VM SH3 is terrible for logging, and it's really annoying that you're stuck with a fixed LAN subnet (and WPS enabled by default

 

[mstombs]

Interesting - I don't have a clever switch like that, I used to use a 100Mbps hub with wireshark, or used tcpdump on the router which wasn't 100%.

I intended to look at using the SH3 in router mode, but never bothered due to the fixed LAN subnet!

 

[mmelbourne]

Interestingly, I've downgraded to Merlin 378.56_2 and the issues I was observing have gone away. More testing required, but it's looking positive. Thanks for all the suggestions!