How to route OPT traffic to IPSec tunnel?

Hello list,

My config is:

  ---- Router A ----        ---- Router B ----
  WAN: 64.64.46.65/29       WAN: 86.86.68.31/29
  LAN: 192.168.12.1/24      LAN: 192.168.13.1/24
  OPT: 123.123.123.1/24     OPT: 110.110.110.1/24

The LANs of both routers are connected via a IPSec tunnel, so:

  RouterA/LAN $ traceroute 192.168.13.13
  traceroute to 192.168.13.13, 30 hops max, 60 byte packets
  1  192.168.132.1 0.194 ms  0.180 ms  0.230 ms
  2  * * *
  3  192.168.13.13 52.103 ms  56.046 ms 61.009 ms

...pinging works fine. The problem is trying to pass any traffic
from OPT to the VPN does not work:

  RouterA/OPT $ traceroute 192.168.13.13
  traceroute to 192.168.12.12, 64 hops max, 52 byte packets
  1  123.123.123.1  0.670 ms  0.505 ms  0.510 ms
  2  * * *
  3  * * ^C

What is the correct way to route any (not just ICMP) traffic
from the OPT interface to hosts through the tunnel? Thanks.

Regards,
Michael