please see the picture
on R3, if you insert distance eigrp 90 109
what is this command going to accomplish ?
It does prevent the loop between two OSPF domain. Once the OSPF is redistributed into the EIGRP on R5, R3 receives it and compares the AD values 110>109. R3 prefers EIGRP external route over the OSPF 24 routes. So, it prevents loop here. If you didn't configure the EIGRP external AD lower than OSPF, it would have loop.
Hope this answers your query!
If the routes at R3 are not EIGRP then they won't make it to R4
Assuming no redis on R3
Yes, the purpose of using distance command in this scenario is to have R3
accept those eigrp external routes from R5. These will then be advertised to R4 so
it can have connectivity to ospf routes. Even if we have mutual redistribution on both
R3 and R5, both configured with distance eigrp 90 109 command, a loop will not occur.
If a link flaps one will alternate using the ospf O IA and the other external eigrp D EX route.
Inputs welcomed [:)]