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Adding second modem (VMG3925-B10B) without DHCP - time is incorrect

  • 1 kesäkuu 2020
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Hei, 

I’ve added a second modem of the same type (as in subject) with DHCP disabled, to cover the other end of the home, everything seems work except that it does not seem to sync time with ntp.saunalahti.fi and always starts with 1970-01-01, while the primary router works and sync time. Second modem at times does not connect to internet, however works after a restart.  Just thinking, could this time sync issue be the cause for the disruption?  Is it expected behaviour that the non-dhcp modem cannot connect to ntp server?

From the modem page, diagnostic → ping ntp.saunalahti.fi throws Unknown Host or Unreachable Network!. Any ideas what else to check.

Appreciate any pointers.

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Vastauksen tähän kysymykseen jakoi irritus 1 kesäkuu 2020, 21:48

Is it expected behaviour that the non-dhcp modem cannot connect to ntp server?

Yes, it is.

Using a modem as just an access point by turning the dhcp server off is a hack. Under these conditions the modem itself cannot connect to the internet, as it is designed to act as the default gateway in the network.

Most consumer modems do not understand a situation where it would have to use another modem as gateway. And worse than that, you cannot even edit the default gateway ip address for LAN side.

As VMG3925-B10B does not have built in IPsec nor OpenVPN support, which may use certificates, having an incorrect date does not have any functional implications. The only effect is that the dates on log files will be incorrect.

Second modem at times does not connect to internet, however works after a restart.

Did you remember to change the lan ip address of the second modem, too?

If you have two VMG3925-B10B both at 192.168.100.1, you will end up with a mayhem.

 

Should you like to have the both modems show correct time, you will have to use the second modem as a router. Check which ethernet port is labelled WAN on the second modem, connect the cable coming from the first modem to the WAN-port of the second modem. Now, either reset the second modem to factory defaults or check the configuration of the modem how to set it up to use Ethernet WAN-port as WAN instead of DSL.

 

If you are concerned about double-NAT, set the second modem to be a real non-nat router.

For example, set the second modem like this:

  • Select Ethernet WAN to be the WAN connection.
  • Set the lan address to be 192.168.101.1
  • Enter wan ip address manually like 192.168.100.2, mask 255.255.255.0, gateway 192.168.100.1, dns 192.168.100.1.
  • Turn nat off

Set the first modem like this:

  • Add route for network 192.168.101.0, mask 255.255.255.0, gateway 192.168.100.2. Metric really doesn’t matter in this case, but you can use 0 to be sure.
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Is it expected behaviour that the non-dhcp modem cannot connect to ntp server?

Yes, it is.

Using a modem as just an access point by turning the dhcp server off is a hack. Under these conditions the modem itself cannot connect to the internet, as it is designed to act as the default gateway in the network.

Most consumer modems do not understand a situation where it would have to use another modem as gateway. And worse than that, you cannot even edit the default gateway ip address for LAN side.

As VMG3925-B10B does not have built in IPsec nor OpenVPN support, which may use certificates, having an incorrect date does not have any functional implications. The only effect is that the dates on log files will be incorrect.

Second modem at times does not connect to internet, however works after a restart.

Did you remember to change the lan ip address of the second modem, too?

If you have two VMG3925-B10B both at 192.168.100.1, you will end up with a mayhem.

 

Should you like to have the both modems show correct time, you will have to use the second modem as a router. Check which ethernet port is labelled WAN on the second modem, connect the cable coming from the first modem to the WAN-port of the second modem. Now, either reset the second modem to factory defaults or check the configuration of the modem how to set it up to use Ethernet WAN-port as WAN instead of DSL.

 

If you are concerned about double-NAT, set the second modem to be a real non-nat router.

For example, set the second modem like this:

  • Select Ethernet WAN to be the WAN connection.
  • Set the lan address to be 192.168.101.1
  • Enter wan ip address manually like 192.168.100.2, mask 255.255.255.0, gateway 192.168.100.1, dns 192.168.100.1.
  • Turn nat off

Set the first modem like this:

  • Add route for network 192.168.101.0, mask 255.255.255.0, gateway 192.168.100.2. Metric really doesn’t matter in this case, but you can use 0 to be sure.

Did you remember to change the lan ip address of the second modem, too?

Yes! they are running on different IP addresses. If there is no functional implication with the wrong date, may be I need enable the logging to see whats really happening.

Should you like to have the both modems show correct time, you will have to use the second modem as a router. Check which ethernet port is labelled WAN on the second modem, connect the cable coming from the first modem to the WAN-port of the second modem. Now, either reset the second modem to factory defaults or check the configuration of the modem how to set it up to use Ethernet WAN-port as WAN instead of DSL.

Thanks, this is interesting, I need to digest this, is there any example on paper how this can be done,  any youtube videos.

Are there any advantages / improvement of using this kind of setup over the one I have which is running second modem as non-dhcp mode? 

 

If you are concerned about double-NAT, set the second modem to be a real non-nat router.

For example, set the second modem like this:

  • Select Ethernet WAN to be the WAN connection.
  • Set the lan address to be 192.168.101.1
  • Enter wan ip address manually like 192.168.100.2, mask 255.255.255.0, gateway 192.168.100.1, dns 192.168.100.1.
  • Turn nat off

Set the first modem like this:

  • Add route for network 192.168.101.0, mask 255.255.255.0, gateway 192.168.100.2. Metric really doesn’t matter in this case, but you can use 0 to be sure.

Thanks.

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