Microsoft Azure To Cisco ISR Router Site to Site VPN


KB ID 0001220 Dtd 19/07/16


Last week I was having problems getting a VPN up from a client's Cisco ASA into Azure. This was because the Azure estate was using 'route-based' or a 'dynamic routing VPN'. See the following article;

Azure to Cisco VPN – ‘Failed to allocate PSH from platform’

So the firewall was a non-starter, but Cisco ISR routers are supported, and they can handle virtual tunnel interfaces (VTI's). So I used a Cisco ISR 1921 router, sat that beside the firewall, and gave that a public IP. Note: I did have to route the traffic to Azure, to use this router instead of the firewall but that's easy. 

Now we just need to  get the VPN Tunnel up.

Cisco Router to Azure VPN



OK, before you get started your router needs to be able to support crypto/VPN's. That means you should be running a 'security' license (show license should say you have a securityk9 licence installed and running, or K8 if you live in North Korea, or 1986). If you don't, the router will not recognise any of the crypto commands.

Log into the router and create an ACL to allow the traffic from your local LAN, to the Azure LAN.

Petes-ISR#conf terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Petes-ISR(config)#access-list 101 permit ip

To establish 'Phase 1' of the VPN tunnel we need an IKE proposal. Note I'm using IKEv2, that is a requirement for route-based, or dynamic routing from Azure.

Petes-ISR(config)#crypto ikev2 proposal IKE-PROP-AZURE
IKEv2 proposal should have atleast an encryption algorithm, an integrity algorithm and a dh group configured
Petes-ISR(config-ikev2-proposal)# encryption aes-cbc-256 aes-cbc-128 3des
Petes-ISR(config-ikev2-proposal)# integrity sha1
Petes-ISR(config-ikev2-proposal)# group 2
Petes-ISR(config-ikev2-proposal)# exit

Then add the proposal we created above to an IKEv2 Policy, (Note: a policy can have multiple proposals).

Petes-ISR(config)#crypto ikev2 policy IKE-POLICY-AZURE
IKEv2 policy should have atleast one complete proposal attached 
Petes-ISR(config-ikev2-policy)# proposal IKE-PROP-AZURE
Petes-ISR(config-ikev2-policy)# exit

Create a keyring, (in IKEv2 you can have multiple keys), and specify your VPN pre shared key, (PSK or shared secret).

Petes-ISR(config)#crypto ikev2 keyring KEYRING-AZURE
Petes-ISR(config-ikev2-keyring)# peer
Petes-ISR(config-ikev2-keyring-peer)# address
Petes-ISR(config-ikev2-keyring-peer)# pre-shared-key 1234567890asdfg
Petes-ISR(config-ikev2-keyring-peer)# exit
Petes-ISR(config-ikev2-keyring)# exit

Now all the 'Phase 1' settings get tied together in a Phase 1 profile. (Note: GigabitEthernet0/0 is the public facing port, yours may be different).

Petes-ISR(config)#crypto ikev2 profile PROFILE-PH1-AZURE
% IKEv2 profile MUST have match identity or match certificate statements
Petes-ISR(config-ikev2-profile)# match address local interface GigabitEthernet0/0
Petes-ISR(config-ikev2-profile)# match identity remote address
Petes-ISR(config-ikev2-profile)# authentication remote pre-share
Petes-ISR(config-ikev2-profile)# authentication local pre-share
Petes-ISR(config-ikev2-profile)# keyring KEYRING-AZURE
Petes-ISR(config-ikev2-profile)# exit

For 'Phase 2' (IPSEC) you create a 'transform set'.

Petes-ISR(config)#crypto ipsec transform-set TRANSFORM-AZURE esp-aes 256 esp-sha-hmac
Petes-ISR(cfg-crypto-trans)# mode tunnel
Petes-ISR(cfg-crypto-trans)# exit

Then you tie all the 'Phase 2' settings together with a 'Phase 2' profile, and link that back to the 'Phase 1' profile.

Petes-ISR(config)#crypto ipsec profile PROFILE-PH2-AZURE
Petes-ISR(ipsec-profile)# set transform-set TRANSFORM-AZURE
Petes-ISR(ipsec-profile)# set ikev2-profile PROFILE-PH1-AZURE
Petes-ISR(ipsec-profile)# exit

You then need to create a tunnel, that will use all these settings.

Note: Yes you can use 169.254.x.x (I know it's an APIPA address, but it will work fine).

Petes-ISR(config)#int tunnel 1
Petes-ISR(config-if)# ip address
Petes-ISR(config-if)# ip tcp adjust-mss 1350
Petes-ISR(config-if)# tunnel source GigabitEthernet0/0
Petes-ISR(config-if)# tunnel mode ipsec ipv4
Petes-ISR(config-if)# tunnel destination
Petes-ISR(config-if)# tunnel protection ipsec profile PROFILE-PH2-AZURE
Petes-ISR(config-if)# exit

Finally the router needs to 'know' that traffic destined for Azure is sent down the VPN tunnel.

Petes-ISR(config)#ip route tunnel 1

Do I Need To Worry About NAT?

No, (even if you are doing NAT Overload). Unlike an IPSEC VPN on a firewall you do not need to exempt the traffic for the VPN, from NAT translation. That's because it leaves the router through the tunnel interface and not the public facing interface.

Below are all the commands you can copy and paste and change accordingly;

Assumptions is behind the router is the Azure network is the Azure Gateway IP
1234567890asdfg is the pre shared key
GigabitEthernet0/0 is the ‘public facing interface on the router’

access-list 101 permit ip
crypto ikev2 proposal IKE-PROP-AZURE
 encryption aes-cbc-256 aes-cbc-128 3des
 integrity sha1
 group 2
crypto ikev2 policy IKE-POLICY-AZURE
 proposal IKE-PROP-AZURE
crypto ikev2 keyring KEYRING-AZURE
   pre-shared-key 1234567890asdfg
crypto ikev2 profile PROFILE-PH1-AZURE
 match address local interface GigabitEthernet0/0
 match identity remote address
 authentication remote pre-share
 authentication local pre-share
crypto ipsec transform-set TRANSFORM-AZURE esp-aes 256 esp-sha-hmac
 mode tunnel
crypto ipsec profile PROFILE-PH2-AZURE
 set transform-set TRANSFORM-AZURE
 set ikev2-profile PROFILE-PH1-AZURE
int tunnel 1
 ip address
 ip tcp adjust-mss 1350
 tunnel source GigabitEthernet0/0
 tunnel mode ipsec ipv4
 tunnel destination
 tunnel protection ipsec profile PROFILE-PH2-AZURE
ip route tunnel 1


Related Articles, References, Credits, or External Links

Microsoft Azure To Cisco ASA Site to Site VPN

Author: PeteLong

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  1. Did you try to peer BGP with the Azure gateway? If so, can you provide a config example of that from both the Cisco isr and the azure config.

    Post a Reply
    • Hi – I have not, If anyone else would like to comment, I’ll post their findings.


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