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https://wiki.mikrotik.com/wiki/Manual:IP/IPsec

Start off by creating new Phase 1 profile and Phase 2 proposal entries using stronger or weaker encryption parameters that suits your needs. It is advised to create separate entries for each menu so that they are unique for each peer in case it is necessary to adjust any of the settings in the future. These parameters must match between the sites or else the connection will not establish.

/ip ipsec profile
add dh-group=modp2048 enc-algorithm=aes-128 name=ike1-site2
/ip ipsec proposal
add enc-algorithms=aes-128-cbc name=ike1-site2 pfs-group=modp2048

Continue by configuring a peer. Specify the address of the remote router. This address should be reachable through UDP/500 and UDP/4500 ports, so make sure appropriate actions are taken regarding the router's firewall. Specify the name for this peer as well as the newly created profile.

/ip ipsec peer
add address=192.168.80.1/32 name=ike1-site2 profile=ike1-site2

The next step is to create an identity. For a basic pre-shared key secured tunnel, there is nothing much to set except for a strong secret and the peer to which this identity applies.

/ip ipsec identity
add peer=ike1-site2 secret=thisisnotasecurepsk

Lastly, create a policy which controls the networks/hosts between whom traffic should be encrypted.

/ip ipsec policy
add src-address=10.1.202.0/24 src-port=any dst-address=10.1.101.0/24 dst-port=any \
tunnel=yes action=encrypt proposal=ike1-site2 peer=ike1-site2

Office 2 configuration is almost identical as Office 1 with proper IP address configuration. Start off by creating new Phase 1 profile and Phase 2 proposal entries.

/ip ipsec profile
add dh-group=modp2048 enc-algorithm=aes-128 name=ike1-site1
/ip ipsec proposal
add enc-algorithms=aes-128-cbc name=ike1-site1 pfs-group=modp2048
</code
Next is the peer and identity.
<code>
/ip ipsec peer
add address=192.168.90.1/32 name=ike1-site1 profile=ike1-site1
/ip ipsec identity
add peer=ike1-site1 secret=thisisnotasecurepsk

When it is done, create a policy:

/ip ipsec policy
add src-address=10.1.101.0/24 src-port=any dst-address=10.1.202.0/24 dst-port=any \
tunnel=yes action=encrypt proposal=ike1-site1 peer=ike1-site1

At this point, the tunnel should be established and two IPsec Security Associations should be created on both routers:

/ip ipsec
active-peers print
installed-sa print

At this point if you try to send traffic over the IPsec tunnel, it will not work, packets will be lost. This is because both routers have NAT rules (masquerade) that is changing source address before packet is encrypted. Router is unable to encrypt the packet, because source address do not match address specified in policy configuration. For more information see IPsec packet flow example.

To fix this we need to set up IP/Firewall/NAT bypass rule.

/ip firewall nat
add chain=srcnat action=accept  place-before=0 \
 src-address=10.1.202.0/24 dst-address=10.1.101.0/24

/ip firewall nat
add chain=srcnat action=accept  place-before=0 \
 src-address=10.1.101.0/24 dst-address=10.1.202.0/24

It is very important that bypass rule is placed at the top of all other NAT rules.

Another issue is if you have IP/Fasttrack enabled, packet bypasses IPsec policies. So we need to add accept rule before FastTrack.

/ip firewall filter
add chain=forward action=accept place-before=1
 src-address=10.1.101.0/24 dst-address=10.1.202.0/24 connection-state=established,related
add chain=forward action=accept place-before=1
 src-address=10.1.202.0/24 dst-address=10.1.101.0/24 connection-state=established,related

However, this can add significant load to router's CPU if there is a fair amount of tunnels and significant traffic on each tunnel.

Solution is to use IP/Firewall/Raw to bypass connection tracking, that way eliminating need of filter rules listed above and reducing load on CPU by approximately 30%.

/ip firewall raw
add action=notrack chain=prerouting src-address=10.1.101.0/24 dst-address=10.1.202.0/24
add action=notrack chain=prerouting src-address=10.1.202.0/24 dst-address=10.1.101.0/24