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I am working on a system where a remote SBC monitors the health of all linked servers and issues fault reports when certain behaviors are observed. Everything is running RHEL 7. I have an interface on a server "eth1" with the ip "10.0.3.16" and a subnet mask "255.255.255.0".

If I run the command:

ip addr del 10.0.3.16/24 dev eth1

on the server, the remote SBC does not seem to notice that anything is wrong. However if I do:

ip link set eth1 down

Then I get the expected result. 10.0.3.16 is the only IP address assigned to the eth1 interface. I would like to understand the difference between the two commands, and why unassigning the IP address produces a different result than taking down the entire interface. Google has failed me so far, I can't seem to find anything relevant.

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ip addr del just removes the one IP address from the interface, i.e. tells the system to not communicate using that address. You could still have another address in use on the interface, either an IPv4 or an IPv6 one (or multiple). As far as the physical Ethernet link is concerned, that's no difference whatsoever, it only affects the network layer.

ip link set <iface> down on the other hand would shut down the whole interface on the physical layer, which would usually make it appear to the connected device as if the cable was unplugged. You could still have the IP address configured, and the system could still use it to communicate internally, or accept packets addressed to that IP address coming from other interfaces. (Unless you have settings to stop that.)

Now, as for why just cutting the link would trigger the monitor, it depends entirely on what the monitor is configured to watch for. If it e.g. checks only to see from the switch if the ports are up, then it will miss problems with that leave the link alive but the software unresponsive (because it crashed, or because you disabled the IP address).

As far as we can tell from the information you give, it could even be that the IP address 10.0.3.16 isn't used for anything critical, and isn't monitored. The system could have another address or another VLAN that actually runs the service that is being monitored.

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