Troubleshooting VPN from Fedline Fortigate through a Cisco ASA Firewall

 

Recently I was troubleshooting an issue with a customer who was having issues with their VPN connection from a Fedline Fortigate appliance through a Cisco ASA firewall. After taking a packet capture I saw that the Fortigate was responding to the ASA’s ARPs, but the ASA was ignoring them:

I turned on ARP debugging on the ASA and saw the following:

arp-in: response at dmz from 10.x.x.10 085b.xxxx.xxxx for 10.x.x.254 885a.xxxx.xxxx having smac 085b.xxxx.xxxx dmac 885a.xxxx.xxxx

arp-in: src ip is same as one of nat mapped address 10.x.x.10 .Consuming the packet

 

A search led me to an email in the cisco-nsp mailing list email and Cisco Bug ID CSCuc11186 (Cisco login required). If you don't have a Cisco account, the relevant line from the bug report is:

NOTE: The fix for this issue may cause the ASA to not reply to ARP requests if the Source IP in the ARP request overlaps with a NAT rule on the ASA. This may occur when the NAT configuration line is overly broad (such as an all zeros configuration, or any. To workaround this, add the keyword "no-proxy-arp" to the nat config line.

 

The issue wasn’t due to the bug itself, but due to how the bug was fixed. Similar to the post in teh cisco-nsp group proxy ARP was disabled on the DMZ interface. However, proxy ARP was not turned off on the NAT rule. 

 

Changing the NAT rule to ‘nat (inside,dmz) source static any unidirectional no-proxy-arp’ fixed the problem.

Networking Fortigate ASA Fedline