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nagylzs
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Joined: Sun May 26, 2019 2:08 pm

RBSXTR&R11e-LTE6 random connection errors

Thu Nov 04, 2021 8:01 pm

I have a RouterOS 6.49 on RBSXTR&R11e-LTE6.

The mobile connection is like this (some information is replaced with * characters):
[gandalf@lte.lacinet] /system package update> /interface lte info 0
           pin-status: ok
  registration-status: registered
        functionality: full
         manufacturer: "MikroTik"
                model: "R11e-LTE6"
             revision: R11e-LTE6_V028
     current-operator: 12345
                  lac: 1350
       current-cellid: 999999
               enb-id: 2906
            sector-id: 13
           phy-cellid: 309
    access-technology: LTE (CA2)
       session-uptime: 1h55m50s
                 imei: ***************
                 imsi: ***************
                 uicc: *******************
         primary-band: B1@10Mhz earfcn: 100 phy-cellid: 309
              ca-band: B20@20Mhz earfcn: 6350 phy-cellid: 411
                 rssi: -72dBm
                 rsrp: -108dBm
                 rsrq: -15dB
                 sinr: -3dB
                   ri: 2
Signal strength is not the best, but that is because the device is indoors. I plan to move it outside on the roof later. The connection is very stable. If I don't reboot the device, then I can easily get serveral days of uptime.

I experience random connection errors. In most cases, I get ICMP admin prohibited messages.

Here is an example:
 /ping google.com
  SEQ HOST                                     SIZE TTL TIME  STATUS                                                                      
    0 10.254.49.226                              56 126 29ms  admin prohibited                                                            
    1 10.254.49.226                              56 126 25ms  admin prohibited                                                            
    2 10.254.49.226                              56 126 25ms  admin prohibited                                                            
    3 10.254.49.226                              56 126 25ms  admin prohibited                                                            
    4 172.217.20.14                              56 116 25ms
    5 172.217.20.14                              56 116 30ms
    6 172.217.20.14                              56 116 35ms
    7 172.217.20.14                              56 116 27ms
    8 172.217.20.14                              56 116 26ms
    9 172.217.20.14                              56 116 25ms
   10 172.217.20.14                              56 116 29ms
   11 172.217.20.14                              56 116 27ms
   12 172.217.20.14                              56 116 29ms
   13 172.217.20.14                              56 116 28ms
   14 172.217.20.14                              56 116 28ms
   15 172.217.20.14                              56 116 29ms
   16 172.217.20.14                              56 116 31ms
   17 172.217.20.14                              56 116 26ms
   18 172.217.20.14                              56 116 27ms
   19 172.217.20.14                              56 116 23ms
    sent=20 received=16 packet-loss=20% min-rtt=23ms avg-rtt=27ms max-rtt=35ms
The above example shows ICMP ping problems, but the very same is happening with TCP connections. I have tried this with wireshark (from a computer on the LAN). It begins with TCP SYN, then sometimes "ICMP admin prohibited" comes back instead of TCP SYN-ACK.

I spent a lot of time, trying to analyze this problem. I have ascertained that all of these messages are coming from a specific IP address: 10.254.49.226. It is always the same. This address is a reserved class A address. I'm almost sure that it belongs to the mobile service provider. The errors are random. Sometimes there are no errors for some minutes, sometimes 30% of the packets are dropped, and sometimes 90% are dropped (although it is rare). I'm not sure about packet loss of TCP connections, TCP retransmits lost packages, there are no obvious problems with established TCP connections.

These errors are very annoying, they make the connection unusable. Consider the case when a web browser wants to load a web page - it usually requires multiple HTTP requests: one for the main HTML page, some requests for CSS files, and some for javascript files. One or more of these may fail. In some cases, the DNS lookup for the request will fail the same way, and it manifests as a "no route to host" error:
[adm@router] > /system ssh [/resolve www.google.com] port=80
connectHandler: No route to host
The browser won't retry, and some pages will half-load with missing CSS/styles or with missing javascript codes etc.

(Tried to change the MTU but it did not help - the default ping requests are 56 bytes long, I think it cannot be an MTU issue anyway.)

This is an older problem, and I did not ask about it on this forum, because everything I wrote so far points to the direction of the ISP. The errors are coming from a remote IP, and they are intermittent. I thought that this cannot be a fault of routeros or the modem. I have also tried to replace the SIM card with a different one (from a different ISP) and there are no errors with those. There is a specific ISP where these errors appears.

Because of this, I have called the ISP several times. This is a big company, they did not let me explain the details of the problem. I didn't have the chance to explain the ICMP admin prohibited messages, or anything. I think that the helpdesk person who created the ticket for me, didn't know anything about mobile or IP networks... They just created a ticket, and after some hours they replied: connection was checked, and they concluded that the error is on my side, probably a device error. They did not find anything unusual on their side (or at least this is what they said). Actually, I reported this problem at least 3 times now, and they (hopefully) checked their side three times, always with the same conclusion: the error is on my side. (But they did not let me explain the details, and I'm affraid I cannot change this.)

Then I made more tests. Yesterday I put the same SIM card into a mobile phone, and used its hotspot function to share the mobile connection. Surprise: there are no errors. Not even one packet loss. I was so surprised, that I tried a second phone, and then a third one. Only RBSXTR&R11e-LTE6 produces these errors, nothing else.

No I'm completely clueless. Here are the facts:

* the same SIM card works with any other phone, except this MikroTik device
* different SIM cards work with this MikroTik device (I have tried two), except this one
* the ISP says that everything is good on their side, and I cannot do anything about this (I can report the problem again, but they will say the same again and again)

Does anybody have an idea about what is going on here? Where should I look for a solution?

Right now I only see one workaround: use an old used phone with wifi-hotspot or USB thetering instead of the RBSXTR&R11e-LTE6 - but that would be very sad, because I bought it specifically for this purpose. I won't get great reception and good speeds from indoors, and I cannot put a phone on the roof...

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