Why does the Internet disappear on vlan in ubuntu?

Question

ubuntu

Dear colleagues good day. I have a network built on board: Clients --> cisco (4 pcs) ---> ubuntu 12 ---> inet On ubuntu, a firewall on squid is raised, vlans are set up like on ciscos ... So more than once I already encounter a problem, that from time to time on all vlan (from 1-15) the Internet disappears and only a complete reboot of ubunta helps. on vlan "default" the Internet continues to run. what is the problem where to look. I don’t understand what other log to look into and see what’s going on.
vlanov 17.
Thanks in advance for your help.