Hello Sims
I’m assuming you’re asking about External type 1 and 2 for OSPF, correct? You can find out more information about those at the following lesson:
Now in a real-world scenario, when we redistribute, why would we choose E1 over E2 or E2 over E1? Well, first take a look at this post to remind us of how path cost is measured for E1 and E2:
Now take a look at this diagram:
Imagine that all links are of the same type, and have the same cost. If you were to redistribute the 5.5.5.5/32 destination into OSPF at both R2 and R4 as an E2 route, then the cost that R7 will see to that destination will be identical for both the red path as well as the green path, so both routes would be installed in the routing table. This is because E2 routes don’t take into account the distance to the ASBR, that is, the distances to R2 and R4 in this case.
If it was to be redistributed as an E1 route, then the cost to the ABSRs would be taken into account. Since the cost via the green route is less, that route would be installed in the routing table.
So it really comes down to, “how do you want your network to behave?” This is just one more way in which you can tweak routing to achieve the routing patterns and paths that you want.
I hope this has been helpful!
Laz