Well, after some time, I have returned to this matter.
I have done one more test that I think is is conclusive. As I said, this
Debian GNU/Linux firewall PC has two network cards:
* eth0: RealTek RTL8139 (to the ADSL router)
* eth1: VIA Rhine II (to the local network)
The problem was with the RTL8139 card connected to the router. This only
worked when setting up it to 10 Mbps. The VIA card connected to the
switch of the local network never gave problems.
So I now exchanged the configuration of the cards. Now the eth0 Realtek
card connects to the internal network. It runs smoothly at 100 Mbps,
full duplex and auto negotiation. So I think it was not a problem with
the Realtek card. In fact, now the VIA card (that previously worked
properly connected to the internal network switch) only works against
ADSL router when it is set to 10 Mbps.
All this makes me think the problem is in the ethernet interface of the
ADSL router.
I have requested a new router to Telefonica. I hope they send me a good
one. This Zyxel router worked smoothly for quite some time. I also hope
it have a default password to access the router and make the necessary
settings (such as SNMP monitoring).
Thanks for your reply.
Kind regards,
Daniel