(08-28-2020, 08:59 PM)nguessan_yao Wrote: R3
router ospf 1
redistribute eigrp 1 subnets route-map SET-TAG



!
route-map SET-TAG permit 10 set tag 1

R4
router eigrp 1
redistribute ospf 1 metric 2000000 1 255 1 1500 route-map FILTER-TAG

route-map FILTER-TAG deny 10 match tag 1

route-map FILTER-TAG permit 20



You may need the moderator or some else with more contiguous years of experience on the subject to answer your question here but it sounds like you may need to read up more on or watch some tutorials on route maps and redistribution to understand whats happening here. 

R3 OSPF is redistributing  EIGRP learned routes

R4 EIGRP is redistributing OSPF but with a Route Map FILTER-TAG
FILTER-TAG route map is ensuring that OSPF will not be redistributed back into EIGRP potentially causing a Routing Loop

At the end of every access list/filter list/distribute list is an implicit Deny. To allow all other routes not to be effected by this Filter List there must be a Permit statement at the end of the filter list.

The purpose of this configuration was to prevent a routing loop between Routers 3 &4 

If you are router 3 and I am router 4

You just informed me (Router 4) of attach subnets you have

Why would you as Router 3 want to hear me as Router 4 tell you about information you were on record telling me first? Because these are devices and not humans this information would keep getting passed back and forth in an infinite loop. This configuration prevents that