Downtime when moving default Gateway

[Stefan E.]

Hi,

we are working on replacing our old Nexus 5k and Nexus 7k Enrivonment to new Cisco ACI Environment.

Currently it's mixed. The Servers are already moved to Nexus 9k leaf Switches in the ACI Fabric, but the old N7k are still handling the routing and are still the default gateway for the connected VLANs.

We now need to move the L3 also to the new ACI environment.

Means: all the Servers will get a new Default GW (at least from L2 perspective).

The IP will not be changed, but the MAC will be different as it will be on a completely different Hardware.

Now my question: how long is the downtime, we need to expect?

We plan to shutdown the L3 Interfaces on the old Box and directly enabled the new ones.

Now the question is: what will happen? Do we have to wait, till the ARP Cache on the Windows and Linux Servers is expired?

Will the new Nexus environment sent a GARP directly after enabling the interface?

Or is there any other mechanism for the Servers, to identifiy that the Default GW IP has been changed to a new MAC address?

Thanks for your support!

Stefan

[Sergiu.Daniluk]

Hi @Stefan E. 

Yes - there will be impact in moving the gateway, and yes - bringing up an BD in ACI will generate leafs to send GARP, which technically will reduce the impact of changing the gateway. However, most likely there will be some servers/VMs which will not update their ARP entries. For these type of endpoints, you will either have to ping them or manually clear their ARP table.

Stay safe,

Sergiu

[Stefan E.]

Hi Sergiu,

thanks for your response.

Good to know, that this action will generate GARP.

But why do you think some Servers or VMs will not update the ARP entries? Shouldn't that be a standardized behaviour?

Would a ping to the Broadcast IP help here as well?

I also tried to find some more information but that's not easy. So i also would like to know the default ARP Timeouts of Windows and Linux.

Or at least an information, what would be the maximum timeout we need to expect?

Do you have any information here?

Best regards

Stefan