Nexus 7000 HSRP Gratuitous ARP

[Mokhalil82]

Hi

We have 2 x Nexus 7000 switches in the DC and currently they are far from ideal in the way they are setup. So Switch 2 terminates both out internal links and all L3 activity also happens on this switch. Sw1 is only really passing traffic to switch 2 at L2. 

We plan to implement HSRP (we are already using a VPC to the firewalls and to some servers) as well as other things to get to a best practice setup. When configuring HSRP, we expect the MAC will change as a virtual MAC will be used. By default the Nexus switches send a gratuitous ARP when a switch becomes HSRP active. 

I am not nsure how our VMWare environment accepts the GARP, will the hosts update their ARP caches. I was advised by our VMWare guy that the hosts need a reboot to update the ARP cache but surely the hosts should have the ability to auto update the ARP cache or accept a GARP

Thanks

[Mark Malone]

Hi

I would have thought the hosts would update themselves why would they not did the systems guy give any reason? , the arp has a timer even on hosts and if its sent a new arp there's no reason it should not update unless they have specifically set it not too ?

The garp is sent to all ffs broadcast so everyone should get it as its flooded , the whole point of it is its sent before the end system has to ask for it so having to reboot to me makes no sense really , I just asked a VMware guy in our systems team and hes on board too that it should automatically update as the timer will either run out or it will pushed to the cache at host level (well ours will anyway), no reply is even expected for a garp when there sent

This is just opinion im not a VMware guy but there supposed to act generally like physical hosts so unless something has been put in place to prevent the update I cant see why it would require a reboot , I would push back on systems see exactly what his reasoning is behind this it could be valid or just a mistake

EXTRACT.................

Send gratuitous ARP when interface goes up to notify other hosts about new MAC/IP bindings in advance so that they don't have to use ARP requests to find out

[Mokhalil82]

Thanks Mark, I'm thinking the same, can't see why hosts would not update their ARP cache. Il ask further questions to see if there is a specific reason he believes why they may not update their cache. As the change will be done out of hours when no VMWare guy is available, I will have to make sure the vms will update their cache.

[Richard Burts]

I believe that whether the VM updates its arp cache is not the crucial point. When you configure HSRP you are introducing a new IP address. Are you going to change the default gateway on the VM to use the new virtual IP as its gateway? Assuming that you are going to change the default gateway on the VM then it will not have an arp entry for that address and the VM will send an arp request. So whether the VM processes the gratuitous arp or not does not impact whether the VM will have an arp entry for the HSRP address.

HTH

Rick

[Mokhalil82]

The gateway address will not change, the gateway address the hosts use currently will be set as the HSRP Virtual address to save changing the gateway on all hosts.

[Richard Burts]

Thank you for the clarification. I believe that you will be able to make the change to implement HSRP with some assurance that there will not be problems. I agree with Mark that it is normal for hosts to periodically time out the entries in their arp table and send an arp request to refresh the content of the arp table. So I would expect that the VM would learn the HSRP MAC address. And even if the VM does not do this then things should still work. The VM arp table currently has the MAC address of the physical interface in its arp table. After you make the changes to implement HSRP the VM will still have a valid MAC address to reach its gateway. You will not have the redundancy functional till the VM updates it arp entry. But you would have connectivity through the original switch.

HTH

Rick

[Mokhalil82]

Thanks Rick. That seems spot on. Even if the hosts do not update the ARP entries straight away, they will still get to the gateway using the physical MAC they have already stored.

Didn't think of that, but that's good enough I suppose to know that I should not get any issues during my changes in respect to HSRP. 

[Richard Burts]

Yes. I really believe that the VM will learn the new MAC. The concept of timing out arp entries and re-learning them seems so common that I would be very surprised if VM does not do it. And in the worst case the VM will retain the original MAC and your connectivity with HSRP will be no worse that it was before HSRP.

HTH

Rick