If you are reading this blog then it is more likely you are looking to get some assistance with configuring a VPN on the ASA. What if I tell you that configuring site-to-site VPN on the Cisco ASA only requires around 15 lines of configuration. Better yet, you may only need less than 10 lines if you already have another tunnel configured.
This blog post assumes prior knowledge of Cisco ASA CLI syntax and site-to-site VPN fundamentals.
Our ultimate goal here is to set up a site-to-site VPN between the Branch Office and the Headquarters. Let's assume the client-pc (10.10.60.10) in the branch office needs to access a web server (192.168.10.10) in the headquarters.
Cisco ASA non-VPN Configurations
Branch Office ASA ----------------- interface GigabitEthernet0/1 nameif USERS security-level 100 ip address 10.10.60.1 255.255.255.0 interface GigabitEthernet0/6 nameif OUTSIDE security-level 0 ip address 126.96.36.199 255.255.255.248 route OUTSIDE 0.0.0.0 0.0.0.0 188.8.131.52 1 object network ho-server-subnet subnet 192.168.10.0 255.255.255.0 object network user-subnet subnet 10.10.60.0 255.255.255.0 nat (USERS,OUTSIDE) dynamic interface
The interface configuration is self-explanatory, ASA has two interfaces, one for the user and another one for the Internet. The default route is pointing to the ISP router with a static route. There are two objects, one for the branch user subnet and another one for the HQ webserver subnet.
The NAT part is quite important because we will talk about that later. As you can see above, I have a dynamic PAT configuration for the user subnet. So, when the user traffic leaves the ASA, the source IP is translated to the IP address of the ASA's outside interface (184.108.40.206)
interface GigabitEthernet0/1 nameif SERVERS security-level 50 ip address 192.168.10.1 255.255.255.0 interface GigabitEthernet0/6 nameif OUTSIDE security-level 0 ip address 220.127.116.11 255.255.255.248 route OUTSIDE 0.0.0.0 0.0.0.0 18.104.22.168 1 object network branch-user-subnet subnet 10.10.60.0 255.255.255.0 object network ho-server-subnet subnet 192.168.10.0 255.255.255.0 nat (SERVERS,OUTSIDE) dynamic interface
Same as above except for the object names and the IPs.
ASA VPN configurations IKEv1
Please note that if you already have another VPN tunnel then most likely most of the configurations are already done for you. So, please make sure not to change or override them.
- Enable IKEv1 on the outside interface (if not enabled already)
crypto ikev1 enable OUTSIDE
2. Create an IKEv1 policy that defines the algorithms/methods to be used for hashing, authentication, DH group, lifetime, and encryption. Please note that these policies should match on both sides. If you already have a policy then you don't need to create them. You can check whether there are any policies by running
show run crypto ikev1 command.
crypto ikev1 policy 5 authentication pre-share encryption aes-256 hash sha group 5 lifetime 3600
3. Create a tunnel-group and configure the peer IP address alongside the tunnel pre-shared key (PSK). Please note that the PSKs should match on both sides.
tunnel-group 22.214.171.124 type ipsec-l2l tunnel-group 126.96.36.199 ipsec-attributes ikev1 pre-shared-key Cisco123
4. Configure the Transform Set which is a combination of security protocols and algorithms that define the way the VPN peers protect data.
crypto ipsec ikev1 transform-set AES-HMAC esp-aes-256 esp-sha-hmac
5. Configure a Crypto Map and apply it to the outside interface. A crypto map defines an IPSec policy that includes an ACL to identify the interesting traffic, peer IP and IKEv1 transform-set that we created in the previous step. If there is already a crypto map applied on the ASA, you only need to add a new entry with the same crypto map name and increment the number instead of creating a new crypto map.
access-list BRANCH-TO-HO extended permit tcp object user-subnet object ho-server-subnet crypto map VPN-MAP 1 match address BRANCH-TO-HO crypto map VPN-MAP 1 set peer 188.8.131.52 crypto map VPN-MAP 1 set ikev1 transform-set AES-HMAC crypto map VPN-MAP 1 set security-association lifetime seconds 3600 crypto map VPN-MAP interface OUTSIDE
6. The most crucial part is NAT exemption. As we discussed before, any traffic that is initiated from the user subnet going out to the Internet is NATed to the outside interface's public IP. However, we want the traffic from the branch office to the headquarters to be exempted from the dynamic PAT. So, we need to tell the ASA that if the traffic is initiated from 10.10.60.0/24 and is going out to 192.168.10.0/24, the IP address shouldn't be NAted. The following command just does that.
nat (USERS,OUTSIDE) source static user-subnet user-subnet destination static ho-server-subnet ho-server-subnet
We just need to mirror the configuration in terms of the IP addresses. Phase-1 and Phase-2 policies should be identical.
crypto ikev1 enable OUTSIDE crypto ikev1 policy 5 authentication pre-share encryption aes-256 hash sha group 5 lifetime 3600 tunnel-group 184.108.40.206 type ipsec-l2l tunnel-group 220.127.116.11 ipsec-attributes ikev1 pre-shared-key Cisco123 crypto ipsec ikev1 transform-set AES-HMAC esp-aes-256 esp-sha-hmac access-list HO-TO-BRANCH extended permit tcp object ho-server-subnet object branch-user-subnet crypto map HQ-MAP 10 match address HO-TO-BRANCH crypto map HQ-MAP 10 set peer 18.104.22.168 crypto map HQ-MAP 10 set ikev1 transform-set AES-HMAC crypto map HQ-MAP 10 set security-association lifetime seconds 3600 crypto map HQ-MAP interface OUTSIDE nat (SERVERS,OUTSIDE) source static ho-server-subnet ho-server-subnet destination static branch-user-subnet branch-user-subnet
That's all, let's see if the client-pc can access the webserver.
Yes, the client-pc can. There are a few ASA commands that you can use to verify the tunnel status.
show crypto isakmp sa to verify the currently active phase - 1 tunnels. As you can see below, branch office ASA initiated the tunnel.
Role: initiator using IKEv1.
branch-01# show isa sa IKEv1 SAs: Active SA: 1 Rekey SA: 0 (A tunnel will report 1 Active and 1 Rekey SA during rekey) Total IKE SA: 1 1 IKE Peer: 22.214.171.124 Type : L2L Role : initiator Rekey : no State : MM_ACTIVE There are no IKEv2 SAs
To check the status of the phase - 2 IPSec tunnels, you can use
show crypto ipsec sa command.
#pkts encrypt and
#pkts decrypt are a very good indicator if you run into any issues. If you see the 'number of packets' encrypted increasing but the 'number of packets' decrypted stays the same then the issue is with receiving the packets, more likely an issue on the other side. If you see the 'number of packets encrypted' stays the same then our side of the ASA is not sending any traffic through the tunnel.
branch-01# show crypto ipsec sa interface: OUTSIDE Crypto map tag: VPN-MAP, seq num: 1, local addr: 126.96.36.199 access-list BRANCH-TO-HO extended permit tcp 10.10.60.0 255.255.255.0 192.168.10.0 255.255.255.0 local ident (addr/mask/prot/port): (10.10.60.0/255.255.255.0/6/0) remote ident (addr/mask/prot/port): (192.168.10.0/255.255.255.0/6/0) current_peer: 188.8.131.52 #pkts encaps: 31, #pkts encrypt: 31, #pkts digest: 31 #pkts decaps: 22, #pkts decrypt: 22, #pkts verify: 22 #pkts compressed: 0, #pkts decompressed: 0 #pkts not compressed: 31, #pkts comp failed: 0, #pkts decomp failed: 0 #pre-frag successes: 0, #pre-frag failures: 0, #fragments created: 0 #PMTUs sent: 0, #PMTUs rcvd: 0, #decapsulated frgs needing reassembly: 0 #TFC rcvd: 0, #TFC sent: 0 #Valid ICMP Errors rcvd: 0, #Invalid ICMP Errors rcvd: 0 #send errors: 0, #recv errors: 0 local crypto endpt.: 184.108.40.206/0, remote crypto endpt.: 220.127.116.11/0 path mtu 1500, ipsec overhead 74(44), media mtu 1500 PMTU time remaining (sec): 0, DF policy: copy-df ICMP error validation: disabled, TFC packets: disabled current outbound spi: 7FB999E3 current inbound spi : D9F696BC
ASA VPN configurations IKEv2
I'm going to remove all the IKEv1 related configurations and then re-configure the VPN using IKEv2. The configuration is almost identical to IKEv1.
crypto ikev2 enable OUTSIDE crypto ikev2 policy 20 encryption aes-256 integrity sha256 group 14 prf sha lifetime seconds 3600 tunnel-group 18.104.22.168 type ipsec-l2l tunnel-group 22.214.171.124 ipsec-attributes ikev2 remote-authentication pre-shared-key Cisco123 ikev2 local-authentication pre-shared-key Cisco123 crypto ipsec ikev2 ipsec-proposal VPN-EXAMPLE protocol esp encryption aes-256 protocol esp integrity sha-256 crypto map VPN-MAP 1 match address BRANCH-TO-HO crypto map VPN-MAP 1 set peer 126.96.36.199 crypto map VPN-MAP 1 set ikev2 ipsec-proposal VPN-EXAMPLE crypto map VPN-MAP 1 set security-association lifetime seconds 3600 crypto map VPN-MAP interface OUTSIDE
crypto ikev2 enable OUTSIDE crypto ikev2 policy 20 encryption aes-256 integrity sha256 group 14 prf sha lifetime seconds 3600 tunnel-group 188.8.131.52 type ipsec-l2l tunnel-group 184.108.40.206 ipsec-attributes ikev2 remote-authentication pre-shared-key Cisco123 ikev2 local-authentication pre-shared-key Cisco123 crypto ipsec ikev2 ipsec-proposal VPN-EXAMPLE protocol esp encryption aes-256 protocol esp integrity sha-256 crypto map VPN-MAP 1 match address HO-TO-BRANCH crypto map VPN-MAP 1 set peer 220.127.116.11 crypto map VPN-MAP 1 set ikev2 ipsec-proposal VPN-EXAMPLE crypto map VPN-MAP 1 set security-association lifetime seconds 3600 crypto map VPN-MAP interface OUTSIDE
show crypto isakmp sa command.
branch-01# show crypto isakmp sa There are no IKEv1 SAs IKEv2 SAs: Session-id:2, Status:UP-ACTIVE, IKE count:1, CHILD count:1 Tunnel-id Local Remote Status Role 6440241 18.104.22.168/500 22.214.171.124/500 READY INITIATOR Encr: AES-CBC, keysize: 256, Hash: SHA256, DH Grp:14, Auth sign: PSK, Auth verify: PSK Life/Active Time: 3600/97 sec Child sa: local selector 10.10.60.0/0 - 10.10.60.255/65535 remote selector 192.168.10.0/0 - 192.168.10.255/65535 ESP spi in/out: 0x35d1507/0xe46501d5
IKEv2 has many new features that make it more reliable and secure but there are many companies that still use IKEv1. One of the benefits of IKEv2 is that it exchanges fewer messages to establish a tunnel compare to IKEv1. IKEv2 uses four messages whereas IKEv1 uses either nine messages (in the main mode) or six messages (in aggressive mode).
Please let me know in the comment section which one did you end up using. As always your comments and feedback are always welcome. If you want to learn about ASA VPN filters, please check out my post here.