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I am looking a bit into mutual redistribution and I am trying to find out how a router decides which routes to redistribute and why.

R1 --ospf-- R2 --rip-- R3

Imagine the above scenario, in which R2 is learning about a number of remote networks from both R1 and R3. If on R2 I redistribute RIP routes into OSPF, RIP redistributed routes will appear as type 5 in R2's LSDB and it will advertise them towards R1. But since OSPF has lower AD than RIP, what stops R2 from simply overwriting RIP routes that it originally learnt from R3, with the same routes it has in the LSDB due to redistribution?

I've searched through a lot of documents and there are some rules redistribution follows, but I couldn't find an answer to this one.

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Found the answer, through testing and some Cisco documentation. First thing to mention is that route redistribution is not a standardized functionality. The following points apply to Cisco equipment:

  1. for a route to be redistributed from protocol A into protocol B, the route must first be in the RIB installed by protocol A.
  2. redistribution is not a transitive operation: if redistribution is configured from protocol A to protocol B, and then from protocol B to protocol C, when a route is learned by the router via protocol A and redistributed into B, it will not be automatically redistributed into protocol C. It goes back to the first rule mentioned. If protocol A has the route installed in the RIB and redistributes it into protocol B, protocol B does not have the route installed in the RIB and therefore does not redistribute into protocol C.
  3. a router is able to differentiate between redistributed routes and routes learned via protocols' operations. For instance, if redistribution is configured from protocol A into protocol B, and a route is learned via A which is then redistributed into B, the route that now exists into B's database due to redistribution will not replace the original route learned via A, even in the situations where B has lower administrative distance than A.

References: https://community.cisco.com/t5/ipv6/routing-loops-at-ipv6-mutual-redistributon/td-p/5134469

https://www.cs.cmu.edu/~4D/papers/rr-icnp07.pdf

https://netacad.fit.vutbr.cz/wp-content/uploads/ccnp/ce2/ENARSI_Chapter_16.pdf

https://www.cisco.com/c/en/us/support/docs/ip/enhanced-interior-gateway-routing-protocol-eigrp/8606-redist.html

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