[CentOS] network connectivity lost after reboot/upgrade

[Gordon Messmer]

On 03/04/2013 05:02 PM, Kai Schaetzl wrote:
> The tcpdump shows a lot of arp requests
> who-has <IP> tell <IP>
> As I understand these are requests for MAC addresses? And tell is the asking
> IP number?

The arp request will have both the source IP address and the Ethernet 
address of the requesting host.  tcpdump will only print the IP unless 
you use the -e flag.

If the layout of your network is such a closely guarded secret that you 
can't share the information that we need to help, you're mostly on your 
own here.

At this point, the problem could be almost anything.  A bad switch port, 
or a bad switch, or a bad cable seem very likely.  Try a new cable to a 
new switch port and reboot the switch if the problem continues.  Try a 
full power down (as in, remove the power cable) for the affected system 
and with the switch.  It sounds like your system is receiving packets 
but unable to send them to other hosts.

 From any other host on the network, you should be able to:
   tcpdump -nn -e ether host <mac>
where <mac> is the Ethernet address of the system with no connectivity. 
  If you try to ping any address at all, the other system should see it 
broadcasting ARP requests for the local destination or the default 
gateway.  If you don't see ARP requests on the other host, then you know 
that the affected system isn't able to sent out traffic.