Azure No Arp

L5 Sessionator

Azure No Arp

Hey All,


I'm coming across a weird issue here.


We have two subents in Azure. Let's call them Subnet1 and Subnet2


Subnet1 has a UDR to point traffic to the internal interface of the firewall.


This works, we see the traffic come into the firewall. We don't see any return traffic from the server in subnet 2. There is a static route pointing to the azure fabric .1 address.


When I do a flow basic, the firewall is unable to send the traffic to the gateway (azure .1 address) because there is no ARP.


Route found, interface ethernet1/2, zone 2, nexthop

Resolve ARP for IP on interface ethernet1/2

ARP pending

Packet dropped, no ARP



L3 Networker

Re: Azure No Arp

Do you have a corresponding route in subnet 2 pointing to the firewall for subnet 1?  If you do not, azure will assymetrically return the traffic directly to the server rather than return routing through the firewall due to the VNET route.

L5 Sessionator

Re: Azure No Arp

Hi @jmeurer 


Thanks for your input. Yes the UDR for subent 2 is there.


Any other ideas?




L3 Networker

Re: Azure No Arp

I assume the firewall has corresponding routes for both subnets pointing to the first IP of the internal subnet the firewall is attached two.


Also, check to ensure the interface has IP Forwarding enabled on the azure side.  If you do need to change this setting.  Reboot the firewall.  I have seen it not apply until after reboot.


From there, double check your NSGs.


Since this intrazone traffic, it should be allowed, but you may not be logging it due to the inherent rule.  Override logging on intrazone, it may give you some further information in the Monitor.

L5 Sessionator

Re: Azure No Arp

Hey @jmeurer 


Firewall routes exist and are correct.


IP forwarding enabled on all interfaces. I've also rebooted the firewall.


nSGs are all allowed.


I've done logging on the policies but just show "bytes received: 0"


I've tried with a NAT rule to source NAT and not, makes no difference.

L3 Networker

Re: Azure No Arp

You have hit all of the usual culprits.  Time to get a TAC case open.

L4 Transporter

Re: Azure No Arp

Is Subnet 2 a gateway subnet?

L5 Sessionator

Re: Azure No Arp

Hey @dmaynard 


Nope! It's not a gateway subnet.

L5 Sessionator

Re: Azure No Arp

Found out the issue.. the static routes on the firewall were pointing to each .1 address of the subnet rather than the .1 address of the address range assigned to the VNET

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