IP selection when a single mac address is mapped with two IP

[mestasew1]

Hello Cisco community,

I have a situation where I configured the end windows node with dual ip address and as a result the switch connected to the node has two IP arp entries  for a single mac address. 

when this node make an outbound connection, I wanted to use one of the IP as preferred address . how does the switch(3850) select from the two IP addresses for layer 3 traffic? I would guess IP from static arp entries might get prioritized. other than manipulating the arp table on the node or switch  or would the switch select an IP that was mapped first ? or how the switch determine which ip to select  ?

[Reza Sharifi]

Hi,

Assuming you use the Advanced TCP/IP setting in windows to add to IP addresses, the first IP (top) will be used first and than the second one.

HTH

[mestasew1]

Hi Reza ,

Thanks for your response. Yes ,I used the advanced settings in windows.  I don't want to turn this into windows discussion but the windows server states both ip as preferred for ipconfig result. And in this case I thought the switch has to select one of the IP as primary. Currently I am constrained to force change the order of the IP. 

assuming the order in adv setting alone determine the ip selection, what determines the order ?editing on the setting did not save the change. it revert back to the previous order after reboot.  

[Reza Sharifi]

Hi,

Not sure about the host side, but I don't think you can make any changes on the switch side, as the switch simply forward traffic based on later-2 (Mac address) and in this case, the switch learns one MAC address even though the host has 2 IPs. 

[ngkin2010]

Yes, Switch have nothing to do with IP selection on Windows Server.

@mestasew1 you may read the below link for more information about 1) when priroity is not set, how windows decide which IP is used; 2) how to set primary IP

https://secura.cloud/technical-insight/windows-server/set-primary-ip-address-windows-server-2012#.VOe_0EfF98E