Route map and tagging to prevent routes coming back in

MrXpert

I setup this network and wanted to prevent routes being redistributed back into the OSPF Process they originated from. I believe it would cause routing loops otherwise?
can anyone confirm or deny whether my config looks OK please? pic attached

mattau

redistribution and tags are always brain teasers : ) . Your logic looks good though. You set 2 tags for each way on both routers. Eg on R2 process 2 to process 1 you gave it tag 221 then denied tag 221 from being redistributed back in to process 2 on R4 which is correct.

The connected links wont have this problem of AD as directly connected implies AD of 0, but yeah for all routes in the table that are ospf the tags look good to me.

Was just having a quick read of
OSPF Redistribution Among Different OSPF Processes* [IP Routing] - Cisco Systems

about half way down they did it another way by using distribute lists to deny the tag, as the route has to the be in the table to be up for redistribution and if its not in the table then it cant be redistributed in the first place.

from cisco site

The OSPF route selection rule is that intra-area routes are preferred over inter-area routes, which are preferred over external routes. However, this rule should apply to routes learned via the same process. In other words, there is no preference between external routes from one process compared to internal routes of other process.
The preference rule between a given OSPF process and any other process (be it OSPF or another routing protocol) should follow the administrative distance rule. However, because different OSPF processes will have the same administrative distance by default, OSPF distance should be configured explicitly for different OSPF processes, to achieve the desired behavior.

so for a more complete solution you might have to change AD's aswell as I believe for example when R2 process 2 gets redistributed into process 1 , on R4 even though you are denying tag 221 back into ospf process 2. R4 itself will have 2 ways to get to 150.5.5.0/24 prefixes.
I am studying switch now so cannot confirm : )

Nate--IRL--

I would have thought your solution a tad over elaborate - I can't remember where I read it but it's a great idea - tag routes from both routers going from OSPF into EIGRP with a tag matching OSFPs AD of 110 and Block 90 - then tag EIGRP routes into OSPF with a tag matching EIGRPs AD of 90, on both routers, and block 110.

Much easier method to remember and to decipher when fixing problems.

Nate