VPNs to GCP using IKEv2 when your Cisco router is behind NAT

J5

I was able to follow this tutorial but had to make a few adjustments.  The main one is to configure the public IP address in the IKEv2 profile (see step 3 below). 

Remember of course that the router will need UDP ports 500 & 4500 forwarded by the firewall, which also must support ESP passthrough.

1) Configure a global IKEv2 proposal and policy.  

crypto ikev2 proposal MY_IKEV2_PROPOSAL 
 encryption aes-cbc-256 aes-cbc-192 aes-cbc-128
 integrity sha512 sha384 sha256
 group 24 16 14
!
crypto ikev2 policy MY_IKEV2_POLICY 
 proposal MY_IKEV2_PROPOSAL

2) And add the peer to the keyring:

crypto ikev2 keyring MY_KEYRING
 peer gcp1
  address 35.212.226.126
  pre-shared-key abcdef1234567890

3) Create a custom IKEv2 profile.  Note the highlighted public IP address and also the lifetime and DPD interval settings.

crypto ikev2 profile GCP_IKEV2_PROFILE
 match address local interface GigabitEthernet0
 match identity remote address 0.0.0.0
 ! If router is behind NAT, set this to the public IP
 identity local address 203.0.113.222
 authentication remote pre-share
 authentication local pre-share
 keyring local MY_KEYRING
 lifetime 36000                       ! 10 hour SA lifetime
 dpd 60 5 periodic                    ! 1 minute keepalives
!

4) Configure a custom IPSec transform set and profile.  This is 128-bit AES encryption with SHA-256 integrity:

! IPsec Settings
crypto ipsec transform-set ESP_AES128_SHA256 esp-aes esp-sha256-hmac
!
crypto ipsec profile GCP_IPSEC_PROFILE
 set security-association lifetime kilobytes disable
 set security-association lifetime seconds 10800
 set transform-set ESP_AES128_SHA256
 set pfs group14                          ! 2048-bit
 set ikev2-profile GCP_IKEV2_PROFILE
!

5) Finally, create the tunnel interface.  Unlike the IKEv2 profile, this simply references the External interface, not the public IP:

interface Tunnel1
 ip address 169.254.0.2 255.255.255.252
 ip mtu 1460
 ip virtual-reassembly in 
 ip tcp adjust-mss 1420
 tunnel source GigabitEthernet0
 tunnel mode ipsec ipv4
 tunnel destination 35.212.226.126
 tunnel protection ipsec profile GCP_IPSEC_PROFILE
!

Troubleshooting

The SAs should look like this:

Router#show crypto ikev2 sa
IPv4 Crypto IKEv2 SA

Tunnel-id Local Remote fvrf/ivrf Status 
2 192.168.1.123/4500 35.212.226.126/4500 none/none READY 
Encr: AES-CBC, keysize: 128, Hash: SHA256, DH Grp:14, Auth sign: PSK, Auth verify: PSK
Life/Active Time: 36000/1226 sec

Router#show crypto ipsec sa peer 35.212.226.126

interface: Tunnel1
    Crypto map tag: Tunnel1-head-0, local addr 192.168.1.123

   protected vrf: (none)
   local  ident (addr/mask/prot/port): (0.0.0.0/0.0.0.0/0/0)
   remote ident (addr/mask/prot/port): (0.0.0.0/0.0.0.0/0/0)
   current_peer 35.212.226.126 port 4500
     PERMIT, flags={origin_is_acl,}
    #pkts encaps: 45, #pkts encrypt: 45, #pkts digest: 45
    #pkts decaps: 58, #pkts decrypt: 58, #pkts verify: 58
    #pkts compressed: 0, #pkts decompressed: 0
    #pkts not compressed: 0, #pkts compr. failed: 0
    #pkts not decompressed: 0, #pkts decompress failed: 0
    #send errors 0, #recv errors 0

     local crypto endpt.: 192.168.1.123, remote crypto endpt.: 35.212.226.126
     path mtu 1500, ip mtu 1500, ip mtu idb GigabitEthernet0
     current outbound spi: 0x962EDB69(2519653225)
     PFS (Y/N): N, DH group: none

     inbound esp sas:
      spi: 0x10B829B(17531547)
        transform: esp-aes esp-sha-hmac ,
        in use settings ={Tunnel UDP-Encaps, }
        conn id: 5, flow_id: Onboard VPN:5, sibling_flags 80000040, crypto map: Tunnel1-head-0
        sa timing: remaining key lifetime (sec): (14259)
        Kilobyte Volume Rekey has been disabled
        IV size: 16 bytes
        replay detection support: Y  replay window size: 1024
        Status: ACTIVE(ACTIVE)

     inbound ah sas:

     inbound pcp sas:

     outbound esp sas:
      spi: 0x962EDB69(2519653225)
        transform: esp-aes esp-sha-hmac ,
        in use settings ={Tunnel UDP-Encaps, }
        conn id: 6, flow_id: Onboard VPN:6, sibling_flags 80000040, crypto map: Tunnel1-head-0
        sa timing: remaining key lifetime (sec): (14259)
        Kilobyte Volume Rekey has been disabled
        IV size: 16 bytes
        replay detection support: Y  replay window size: 1024
        Status: ACTIVE(ACTIVE)

     outbound ah sas:

     outbound pcp sas:

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