Cisco ASA to Fortigate VPN (Properly!)

KB ID 0001721


A while ago I did a run through on site to site VPNs from Cisco ASA to Fortigate firewalls. Back then I said that the default settings were a bit ‘shoddy‘ and that I’d revisit it once I had more time.

What do you mean shoddy? Well, Cisco and Fortinet are both guilty of enabling ‘Everything’ to make the tunnel come up, so people can just use a wizard and not put to much thought into the process, for most people thats absolutely fine. However I’ve found ‘Many Times‘ I’ve been trying to put a VPN into third party and it’s like a game of ‘Encryption Bingo‘ e.g. ‘Can you change it from AES128 to AES256 and change the hash to SHA512‘, or ‘Do you not support elliptical curve’. Who are these people? Do they expect Tom Cruise  to come rappelling out of a skylight to steal the details of their 2016 Christmas golf event!

I digress, so here’s how to set up a site to site VPN using IKEv2 with some weapons grade encryption. Here’s a pretty picture of what it will look like;

Fortigate to Cisco ASA VPN

And here’s what my test bench topology looks like in EVE-NG.

Fortigate to Cisco ASA VPN EVE-NG

Configuring the Fortigate for Site to Site VPN

After saying don’t use the wizard, I’m going to use the wizard to do the Fortigate end, then I’ll edit the tunnel it creates and make it a bit more ‘fit for purpose’.

From the web management portal > VPN > IPSec Wizard  > Give the tunnel a name > Change the remote device type to Cisco > Next.

Give it the ‘public’ IP of the Cisco ASA > Set the port to the ‘outside’ port on the Fortigate > Enter a pre-shared key, (text string, you will need to enter this on the Cisco ASA as well, so paste it into Notepad or something for later) > Next.

Fortigate Site to Site VPN

Local interface will be in the ‘inside’ interface on the Fortigate > Enter the local subnet(s) > Enter the remote (behind the ASA) subnet(s) > Next.

Fortigate VPN Policy

Review the settings > Create.

Fortigate VPN Review

Select IPSec Tunnels > Select the new tunnel  > Edit.

Fortigate VPN Not Established

In the Authentication Section > Edit > Change the IKE Version to 2 > Edit the Phase 1 Proposal.

Note: If you can’t see this option, change the tunnel type to custom.

Fortigate VPN Enable IKEv2

Delete all the factory ones and add one for;

Then under Phase 2 Selectors click the pencil icon to edit.

Fortigate VPN Edit Phase 1


Fortigate VPN Edit Phase 2

Remove any existing Phase 2 proposals, add a new one

Encryption: AES256GCM 

Fortigate VPN IPSEC Phase 2

Click OK to close.

Manually Configuring the Cisco ASA For Site to Site VPN

Manual VPN via CLI

We all know real men work at command line, paste this in, boom done!

WARNING: If your ASA already has a crypto map then use the name of that map rather than CRYPTO-MAP (as below) or all your existing VPNs will break!

crypto ikev2 policy 5
 encryption aes-gcm-256
 integrity null
 group 21
 prf sha512
 lifetime seconds 86400
crypto ikev2 enable outside
object network OBJ-SITE-B
object network OBJ-SITE-A
access-list VPN-INTERESTING-TRAFFIC extended permit ip object OBJ-SITE-B object OBJ-SITE-A
nat (inside,outside) source static OBJ-SITE-B OBJ-SITE-B destination static OBJ-SITE-A OBJ-SITE-A no-proxy-arp route-lookup
tunnel-group type ipsec-l2l
tunnel-group ipsec-attributes
ikev2 remote-authentication pre-shared-key 123456
ikev2 local-authentication pre-shared-key 123456
isakmp keepalive threshold 10 retry 2
crypto ipsec ikev2 ipsec-proposal VPN-FORTIGATE
 protocol esp encryption aes-gcm-256
 protocol esp integrity null
crypto map CRYPTO-MAP 1 match address VPN-INTERESTING-TRAFFIC
crypto map CRYPTO-MAP 1 set peer
crypto map CRYPTO-MAP 1 set ikev2 ipsec-proposal VPN-FORTIGATE
crypto map CRYPTO-MAP interface outside

Manual VPN via ASDM

You can do the first couple of steps together, but I like to do the Phase1 and Phase 2 proposals first, then tie it all up at the end. Configuration > Site to Site VPN > Advanced > IKE Policies > IKEv2 Policies > Add.

  • Priority: 5
  • D-H Group: 21
  • Encryption: AES-GCM-256 
  • Integrity Hash: null (GCM protocols don’t need an integrity hash)
  • Pseudo Random Function (PRF) Hash: sha512

OK > Apply

Cisco ASA VPN ISAKMP Phase 1

Now for Phase 2 (On a Cisco ASA that’s defined with a ‘transform  set’). IPsec  Proposals (Transform Sets)  > IKEv2 > Add.

OK > Apply

Cisco ASA VPN IPSEC Phase 2

Connection Profiles > Tick IKEv2 on the OUTSIDE interface to enable it.

Cisco ASA Enable IKEv2

Connection Profile > Add

  • Peer IP: (Public address of the Fortigate)
  • Local Network: Add in the network behind the ASA.
  • Remote Network: You may need to add an object-group for the remote network (behind the Fortigate).
  • Group Policy Name: FORTIGATE_VPN
  • Local Passphrase: An alphanumeric string of characters (it’s a pre-shared key it must match the one you set on the Fortigate).
  • Remote Passphrase: Set the same at the local passphrase.

Scroll down.

Cisco ASA VPN Config Manually

Make sure your IKE  Phase 1 policy is in the list, (you may have many) > IPSec proposal > Select > locate yours and add it in > OK > OK > Apply.

Cisco ASA VPN Config Manually

The last thing to do is make sure that the traffic travelling over the VPN DOES NOT get NAT translated > Firewall > NAT Rules > Select the top one > Add  >Add Nat Rule Before.

  • Source Interface: inside
  • Destination Interface: outside
  • Source Address: any
  • Destination Address: {The group you created above for the network address behind the Fortinet}
  • Source NAT Type: Static
  • Disable Proxy ARP :  Tick
  • Lookup Route Table: Tick

OK > Apply

Cisco ASA ASDM Static NAT

Don’t forget to save the changes > File > Save Running configuration to flash

Finally send some interesting traffic across the VPN to bring up the tunnel.

Test VPN Fortinet to Cisco

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Author: PeteLong

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