The key is to make sure the networks where the simulated routers are coming from are advertised to the DUT via the emulated routers. This will allow the DUT to know which next hop to use to forward traffic to destination addresses.
For example (use 2 ports):
Sim_Rtr1 is 10.0.0.2 Sim_Rtr2 is 18.104.22.168 DUT nearside interface is 22.214.171.124 DUT farside interface is 126.96.36.199 Emu_Rtr1 is 188.8.131.52 Emu_Rtr2 is 184.108.40.206 Sim_Rtr1/Emu_Rtr1 are on STC port 1 Sim_Rtr2/Emu_Rtr2 are on STC port 2
Steps to configure:
1). setup the BGP emulated routers on Port 1 and 2
2). ARP both router interfaces
3). add routes 10.0.0.0 and 220.127.116.11 to Emu_Rtr1 and Emu_Rtr2, respectively
4). bring up both emulated BGP sessions and make sure the 2 routes are advertised to DUT
5). on Port 1, create Sim_Rtr1 with IP address 10.0.0.2 and gateway 18.104.22.168
6). on Port 2, create Sim_Rtr2 with IP address 22.214.171.124 and gateway 126.96.36.199
7). create a bi-directional Stream Block and bound the simulated routers as endpoints
8). start traffic and should see Tx and Rx both sides going through
NOTE: The above configuration steps can also be used with other routing protocols or emulated hosts. If using emulated hosts instead of emulated routers, then static routes have to be configured on the DUT.