Mikrotik IPSec VPNs with multiple destination Networks/Policies and SA(s) management.

While configuring Multiple Networks VPNs (Multiple policy and destination SubNets reached via the same remote IPSec VPN Peer) between Mikrotik and other Firewalls, traffic would randomly stop for certain destinations.

Packet forwarding and encryption only works for one destination (the first matched IPSec Policy) and any other destination (and only the first matched one) will be reachable by performing a ‘SA Flush’.

This (apparently a routing misconfiguration) happens because some other firewalls have been configured to create a separate SA for each network policy.

Setting Mikrotik IPSec Policy with the ‘require’ level (default option) causes the router to create a single SA with the remote peer.
In this configuration, all traffic sent to any policy (in other words to any destination SubNet) for a specific peer, will use the same SA, regardless of the Source or Destination addresses.

For this reason this works only for the first SubNet routed trough the IPSec Tunnel.
As soon as the Mikrotik Firewall tries to send traffic for another destination SubNet, using a policy with same SA Peer, it will use the already existing SA and the remote peer starts complaining because of SA sequence numbers.

To solve this problem, Mikrotik side, it is necessary to change the policy Level to ‘unique‘, to allow both endpoints to share a unique SA for each Policy and Destination SubNet.
If the shared SA option is preferred, it is possible to check if the other EndPoint supports per-policy SA Settings.

 

Policy Level, as Mikrotik Manual and Wiki suggest, specifies what to do if some of the SAs for this policy cannot be found:

  1. use – skip this transform, do not drop packet and do not acquire SA from IKE daemon.
  2. require – drop packet and acquire SA (DEFAULT OPTION SET).
  3. unique – drop packet and acquire a unique SA that is only used with this particular policy.

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