How to connect to two wired networks

[cogitordi]

Glad you've got them pinging. If the default route is set to B, my guess is that the DHCP is doing that,

Recall that there are two DHCP servers. What is unexpected (to me) is that one DHCP server is consistently preferred to the other by Linux. I don't often laud M-Windows, but in this case, Redmond does well to select as the default route the DHCP server through which the OS can ping a server in the outside world.

Of course, I could write a script to do this very thing.

and you'd be better off disabling it or putting a static route there.

No, not in my case. Using DHCP on LAN B permits other devices to join LAN B without fuss. (No fuss now that I know how to do it, that is!)

What kind of "more fiddling" did you do to get them pinging? Did you get any luck with the tcpdump I suggested? Known good cable: but did you try a known good socket on the router?

The fiddling of a simpleton. (o: Cold boot to clear RAM of rubbish; examination of the eth information in dmesg, examination of port assignments using ifconfig; examination of "route". Pings all around. Then I fiddled with "ip route del" and "ip route add default via xxx.xxx.xxx.xxx" until everything was just right.

I didn't suspect a bad socket on the router or switch. The switch is in good condition; Ye Olde Router on LAN B only has one 8P8C socket. (o:

Just some thoughts.

I do appreciate your assistance. You helped me think through the problem and I learned what I needed to learn. I do most PC fiddling late at night, when I am worn, torn, and forlorn. (o:

Cheers and best wishes.

[Jonathan L]

Hi Cogito

Glad to hear you're winning.

Recall that there are two DHCP servers. What is unexpected (to me) is that one DHCP server is consistently preferred to the other by Linux.

I'd guess the one that is "preferred" is simply the later one, and it overwrites the default route of the previous one. Given that LAN B doesn't have an (actual) route to the internet, why not set the default route that the DHCP gives out to be Router A? Any device solely on LAN B will be no different in functionality, and those which are also on LAN A will get (or keep) a functioning route.

I don't often laud M-Windows, but in this case, Redmond does well to select as the default route the DHCP server through which the OS can ping a server in the outside world.

It's also an effective form of field monitoring. Another downside is that your presumed route has to be working at DHCP time, which might not be the case. In general, much better configuring something which is stable and known to work and doesn't depend on, for example, boot order or DHCP race conditions.


Just another couple of thoughts.

Kind regards,
Jonathan.