I need to give an example in order to understand Passive interface:
Let s say I have a router R2 which is an ABR between OSPF are 0 and area 3 and EIGRP AS 10.
it has S0/1 connecting to area 0 and E0 connecting to area 3 and S0/2 connecting to EIGRP AS 10
now if I configure S0/1 and E0 as passive interfaces , then area 0 and area 3 will not get any routes from EIGRP 10 and the other way around I guess will also be true,EIGRP10 will not get any OSPF routes from area 0 and area 3.
My question is if later, I will try to redistribute OSPF into EIGRP 10 and EIGRP 10 into Area 0 and Area 3,will still be possible even if the afore-mentioned interfaces are configured as passive ?
Thank you
What I have concluded from reading on the Net, is some routers version will advertize EIGRP with the classfull IP adress by default when you redistribute them into other other Routing Protocols such as OSPF. so the work around is to block the advertizement manually by making the interfaces that connect the ABR to the OSPF as Passive, then for instance, create Access-List that encompass the classless subnets of the EIGRP and Tag them, so that later when you redistribute the EIGRP routes you can use the Route Map to mach the Tag.
*** I might be going too far, but I wanted to make sure that Passive interface will come handy in the case where the router version will advertise the classful network ie, 172.16.10.0 as 172.16.0.0 even when you specify the wild card mask of 0.0.0.255.