So this week I was tasked with bringing up a site-to-site VPN between a Cisco ISR and Cisco ASA.   Every site-to-site VPN I’ve done over the last 10 years or so has been route-based.  But the older Cisco ASA code won’t support those, so it was back to isakmp profiles and crypto-maps.  The real trick was the ISR was using both frontdoor and interior VRFs, and HSRP for failover for added complexity.

Here’s was the information:

  • Site A (Cisco ISR 4431, IOS-XE version 16.6.8):
  •   public IP: 198.51.100.52/29, HSRP IP: 198.51.100.54, VRF: EXTERNAL
  •   private IP IP: 10.111.111.1/24, VRF: INTERNAL
  • Site B (Cisco ASA, software version 9.4):
  •   public IP: 203.0.113.105
  •   private network: 10.222.222.0/24

The configuration for the ISR starts with the ISAKMP policy, which basically sets encryption parameters.  This is global, and not VRF-aware:

crypto isakmp invalid-spi-recovery
crypto isakmp keepalive 10
!
crypto isakmp policy 1
 encr aes 256              ! 256-bit AES encrytpion
 authentication pre-share
 group 2                   ! 1024-bit key
 lifetime 86400            ! default setting
 hash sha                  ! default setting
!

The pre-shared key is set in the crypto keyring, with needs to be mapped to the frontdoor VRF (the VRF set on the public interface)

crypto keyring MY_KEYRING vrf EXTERNAL
 local-address 198.51.100.54 EXTERNAL
 pre-shared-key address 203.0.113.105 key xxxxxxxxxxxxxxx

Unlike route-based VPNs, an ISAKMP profile is required, which is VRF-aware.  Note the presence of the iVRF (internal one) on the “vrf” line:

crypto isakmp profile MY_ISAKMP_PROFILE
 vrf INTERNAL
 keyring MY_KEYRING 
 match identity address 203.0.113.105 255.255.255.255 EXTERNAL
 local-address 198.51.100.54 EXTERNAL
!

For IPSec / Phase 2, a transform set of course should be defined.   Here I’m using Encapsulation Security Payload, AES 128-bit encryption with SHA-1 hashing, operating in IPSec tunnel mode:

crypto ipsec transform-set ESP_AES-128_SHA_TUNNEL esp-aes esp-sha-hmac 
 mode tunnel
!
crypto ipsec df-bit clear

Since this is not route-based, an ACL defines the “interesting” traffic to send via the tunnel:

ip access-list extended 101
 permit ip 10.111.111.0 0.0.0.255 10.222.222.0 0.0.0.255
!

Then a crypto map links this ACL with the ISAKMP profile and IPSec settings:

crypto map MY_CMAP 1 ipsec-isakmp 
 set peer 203.0.113.105
 set transform-set ESP_AES-128_SHA_TUNNEL 
 set pfs group5
 set isakmp-profile MY_ISAKMP_PROFILE
 match address 101
 reverse-route
 lifetime 3600                ! default setting
!

Finally, the crypto map is applied to the public interface.  If HSRP is being used between a pair of routers, a name for the HSRP group must be set, then added to the crypto map statement:

interface Port-channel1
 description Public Interface
 vrf forwarding EXTERNAL
 ip address 198.51.100.52 255.255.255.248
 ip nat outside
 standby delay minimum 30
 standby 1 ip 198.51.100.54
 standby 1 priority 254
 standby 1 preempt
 standby 1 name VPNHA
 load-interval 30
 no negotiation auto
 crypto map MY_CMAP redundancy VPNHA
!
ip route vrf EXTERNAL 0.0.0.0 0.0.0.0 198.51.100.49

I had missed the “redundancy” option on the crypto map in my initial config, which probably cause the traffic to source from the primary IP address rather than the HSRP one.  Running debug crypto isakmp, I would see this:

May 15 17:15:35.892: ISAKMP: (0):SA request profile is MY_ISAKMP_PROFILE
May 15 17:15:35.892: ISAKMP: (0):Created a peer struct for 203.0.113.105, peer port 500
May 15 17:15:35.892: ISAKMP: (0):New peer created peer = 0x80007FAD0B334DB8 peer_handle = 0x8000000080000012
May 15 17:15:35.892: ISAKMP: (0):Locking peer struct 0x80007FAD0B334DB8, refcount 1 for isakmp_initiator
May 15 17:15:35.892: ISAKMP: (0):local port 500, remote port 500
May 15 17:15:35.892: ISAKMP: (0):set new node 0 to QM_IDLE      
May 15 17:15:35.893: ISAKMP: (0):insert sa successfully sa = 80007FAD0B75C470
May 15 17:15:35.893: ISAKMP: (0):Can not start Aggressive mode, trying Main mode.
May 15 17:15:35.893: ISAKMP-ERROR: (0):No Cert or pre-shared address key. 
May 15 17:15:35.894: ISAKMP-ERROR: (0):construct_initial_message: Can not start Main mode

Since the debug wasn’t showing the source IP for this traffic, I spent several hours troubleshooting the ISAKMP profile and VRF statements, only to conclude they were in fact correct.