SRT - NAT for RIP unicast
I learned about this Stupid Router Trick to use NAT for unicast of RIP updates. The scenario is that we would not be allowed to use the neighbor statement in say RIP to unicast updates. We could then use NAT instead. So if we have two routers connected to each other R1 and R2 with a subnet of 188.8.131.52/24. We are told that R2 must send unicasts to R1. We then configure the following on R2.
ip nat outside
ip nat outside source static udp 184.108.40.206 520 220.127.116.11 520
R2 will translate the RIP packets with a destination of 18.104.22.168 to a destination fo 22.214.171.124 and R1 installs the route perfectly. What I'm not understanding is why there is no translation in the reverse direction, why is R2 not trying to translate R1's packet with a source of 126.96.36.199 to a source of 188.8.131.52. I know that a packet can never have a multicast address as a source but what is preventing this from happening? Is it a builtin check or is it the NAT order of operations that prevents it? Any input would be appreciated.