No IP Duplicate logs

[Emil Naklicki]

Hello all,

Today I tried to do a quick experiment. I added two hosts on the same access switch under vlan 20. I gave both hosts the same IP address. When I checked to core switch logs, I never received any logs indicating there are duplicates on my network. Logging is set to debug. Is there something I'm missing in my config?

[Matt Delony]

Without knowing more details, The core switch would usually only care about duplicate addresses if a host is using an IP address already assigned to the core switch. Try the experiment again by assigning a host an IP address that is already assigned to the core switch.

[Emil Naklicki]

I was under the impression that my switch should get IP duplicate errors based it's arp table. A switch will see that that an IP address has multiple mac addresses associated to it and log "IP Duplicate". Is this not the case with Cisco Switches? If not, is the only way see duplicates is to manually check the arp table?

[Matt Delony · Accepted solution]

Hello Emil Naklicki,

The duplicate IP address error messages are for when the switch detects that another host is using an IP address assigned to the switch. The switch doesn't normally monitor for duplicate IP addresses between other hosts. Normally hosts have their own method of detecting duplicate IP address assignment using ping or ARP.

Depending on the switch model, it could support optional config for detecting duplicate IP address when handing out IP address, if it is configured as DHCP server.

I'm not sure of another method for detecting duplicate IP addresses from the switch side.

[Emil Naklicki]

Ahh thank you for clearing that up Matt.