[Gllug] Problem with new Virgin 50Mbps Modem

James Courtier-Dutton james.dutton at gmail.com
Wed Mar 28 08:01:48 UTC 2012


On 28 March 2012 06:54, Christopher Hunter <cehunter at gb-x.org> wrote:
> On Tue, 2012-03-27 at 18:52 +0100, Alain Williams wrote:
>> You may be interested in the solution for a customer of mine who recently
>> installed (upgraded to) the Virgin 50Mbps Super Hub.
>>
>> The symptoms were lost packets, this badly affected incoming emails (we run an
>> MTA) especially, for some reason, those from google/gmail. There was (I think)
>> also a problem with an SSL session.
>>
>> The fix is to set the MTU to 1460. We did that with an immediate improvement.
>>
>> The sad thing is that we have been beating our heads on the table for a week,
>> with various Virgin support droids blaming our config.
>>
>> On a redhat system edit /etc/sysconfig/network-scripts/ifcfg-eth0 and
>> insert the line:
>>     MTU=1460
>>
>> After fixing it you will see 1460 as in below:
>>
>> # ifconfig eth0
>> eth0      Link encap:Ethernet  HWaddr 00:0D:56:64:3A:C6
>>           inet addr:192.168.10.2  Bcast:192.168.10.255  Mask:255.255.255.0
>>           inet6 addr: fe80::20d:56ff:fe64:3ac6/64 Scope:Link
>>           UP BROADCAST RUNNING MULTICAST  MTU:1460  Metric:1
>>           RX packets:31171623 errors:4 dropped:13 overruns:0 frame:0
>>           TX packets:19810324 errors:0 dropped:0 overruns:0 carrier:0
>>           collisions:0 txqueuelen:1000
>>           RX bytes:4150314049 (3.8 GiB)  TX bytes:3814195185 (3.5 GiB)
>>           Interrupt:201
>>
>>
>
> I'm very surprised that this has only just made it to this list - it's a
> problem that's been around on Virgin ("on the ridiculous") since they
> "upgraded" the network they inherited from Telewest.
>
> One thing that hasn't changed - they don't understand the problem and
> won't accept that there could be anything wrong with their network.
>

This is not just a Virgin problem. I have seen the problem present on
Cable&Wireless and Global Crossing (Level 3) WAN networks. The
difference there is at least they fix the problem when you tell them
what it is.
The problem can be very difficult to spot, as you can get black hole
packets on only certain paths, and also only in one direction. So, for
a WAN link, you own both directions so can send packets from each end,
but for Internet, you only own one end. The only way to send packets
from the other end is if you have a person across the internet who can
help, or a host on the internet that can help.
For example, if you told me your IP address, I could tell if there was
a black hole from me to you, but not the other way round. That is also
assuming that I can do ping (icmp echo) requests to you, although it
can be tested with UDP and TCP packets if needed.
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