Many times have I seen engineers frustrated with getting a VPN tunnel between a Cisco and Juniper device. Often it is the Juniper side which is left to resolve the problem because the Cisco guy has never had a problem before, It should just work!!
All very nice however not all VPN devices are created equal. Know your device!
Let's take two security devices: Juniper SRX and a Cisco ASA
The ASA has one method for IPsec which is cryptomap. If we decode this into an easier to understand term it means Policy based VPN.
The Juniper SRX has two methods:
Typically you are going to find the Juniper end configuring a route based VPN because it's nice and easy to route whatever you want into the tunnel. Here lies the problem as route based VPN and policy based VPN don't always play nice. If we go back to basic in IPsec you realize every Phase 2 security association needs a proxy id pair; a remote network and a local network. A route to policy VPN with a single proxy id pair; no problem. A policy to route VPN with multiple proxy id pairs; tears.
What is typically overlooked on the ASA side is how many networks are put into the ACL associated with the cryptomap. ACL lines equal proxy id pairs.
For this ACL:
access-list 123 permit ip 192.168.1.0 0.0.0.255 18.104.22.168 0.0.0.255
access-list 123 permit ip 192.168.2.0 0.0.0.255 22.214.171.124 0.0.0.255
access-list 123 permit ip 192.168.3.0 0.0.0.255 126.96.36.199 0.0.0.255
We end up with 3 IPsec security associations. On the Juniper side we should configure 3 policies (with vpn action) to match the network pairs. You can try to use a route based VPN but be careful if you don't control both ends of the VPN. An extra ACL will break your VPN.
The recommendation for SRX to ASA VPN's is Policy based VPN.
If we had a Cisco router with the IP Security IOS we could use a VTI or virtual tunnel interface with a Juniper route based VPN. They are essentially the same thing. This would do away the need for 3 IPsec SA's and reduce overhead. Sending traffic through the VPN is then just a matter of routing.
I guess the question is, Why doesn't the ASA have the VTI option? One would think on a device with relativley low throughput you would want to make the best of it. They are used frequently for site to site VPN's that carry multiple subnets. Cisco ASA users should be screaming for this feature.