To run multiple jails on a single host and provide different routing tables (FIBs) to each jail, you have to compile a custom kernel.

Update: Compiling a custom kernel is no longer needed with FreeBSD 12. Start at step 4 for the same results.

In this example, we will recompile the kernel to provide 15 additional FIBs.

1. Create a new kernel file:

   mkdir -p /root/kernels
   touch /root/kernels/ROUTING
   cd /usr/src/sys/amd64/conf
   ln -s /root/kernels/ROUTING
   

2. Edit the new kernel config file (vim /root/kernels/ROUTING). In this example, we're adding the option for additional FIBs while regerencing the GENERIC kernel settings for the rest:

   include GENERIC
   ident ROUTING
   options		ROUTETABLES=16
   

3. Compile:

   cd /usr/src
   make -j20 buildkernel KERNCONF=MYKERNEL
   make installkernel KERNCONF=MYKERNEL
   

4. Update the boot loader config vim /boot/loader.conf.local:

   net.fibs="16"
   

5. Update the sysctl config vim /etc/sysctl.conf:

   net.add_addr_allfibs="0"
   

6. Reboot

Here's how to keep the custom kernel up-to-date.

Update: 2018-08-18: Fixed errors in step 1.