Hi guys, I’m doing a routing redistribution lab, the {U} in topology should be 20, the goal is redistribution ospf to eigrp, and eigrp to ospf. The network 172.16.20.0/24 is not in eigrp, and should be redistribute connected into eigrp.
In my understanding there should be a routing between R2 and R3, because they will both advertise a external network 172.16.20.0/24 and both of them should have O E2 172.16.20.0/24 in the routing table.
But after I config both eigrp and ospf redistribution, the R2 shows O E2 172.16.20.0/24 via 192.168.20.3, but R3 should D EX 172.16.20.0/24 via 10.20.13.1. I didn’t change AD or create route-map yet. Is my understanding right or does this lab just have a routing-optimizing problem?
Hello @royliu1995 ,
To understand these issues, take a look at these two lessons:
When dealing with these issues, it’s important to understand on paper what is going on step-by-step.
- When you redistribute 172.16.20.0/24 into EIGRP, it’s a D EX route, and R2 and R3 learn about it and install it in the routing table.
- Now when you redistribute this network from EIGRP into OSPF on R2, R3 learn about this 172.16.20.0/24 route through OSPF
- The OSPF E2 172.16.20.0/24 network has a lower AD than the 172.16.20.0/24 D EX route and R3 installs this instead of the D EX route.
Since 172.16.20.0/24 is no longer in R3’s routing table as an EIGRP route, it can’t redistribute it from EIGRP into OSPF.
That’s what you are seeing here. It can be vice versa, where R3 is the one doing the redistribution and R3 removes the D EX route to install the O E2 route. It depends who is doing things first.
Does this make sense? It’s an AD issue. If you want to ensure both R2 and R3 use the D EX route, you can change the AD so that the D EX route is better than the O E2 route.
Rene
1 Like
Thanks for your explanation, I understand it right now. It’s interesting an ASBR router doesn’t change D EX to O E2 on the routing table when advertising an external route
Hello Zongrui
I’m glad you found the explanation helpful! You’re correct in your understanding. An ASBR does not change the External Type 2 (E2) OSPF route to an OSPF Inter-Area (O IA) route when it advertises an external route.
This is because an ASBR is responsible for importing external routing information from other routing protocols or from static routes into OSPF, and then distributing them to the rest of the OSPF network. These routes are tagged as E2 by default, meaning they keep their original cost, regardless of how many OSPF routers they pass through.
Therefore, all the routers within the OSPF network will see the route as E2. This helps to maintain the original cost of the route and provides a clear indication that the route is external to the OSPF network.
I hope this has been helpful!
Laz