Lab: two routing domains, joined at two points
Redistribute at one point and nothing can loop. Redistribute at two — which is what redundancy means — and this is what you have to do about it.
An EIGRP domain and an OSPF domain, joined at two routers for redundancy. That redundancy is also a loop, and this lab is about closing it without giving it up.
Do this
- Redistribute both ways at R1 only. Confirm everything is reachable.
- Add the second redistribution point at R2.
- Look at
show ip routeon a router in each domain and find a route that - Tag on the way out, filter on the way in.
R1(config)# route-map TO-OSPF permit 10
R1(config-route-map)# set tag 100
R1(config)# router ospf 1
R1(config-router)# redistribute eigrp 100 subnets route-map TO-OSPF
The point
The failure here is not loss of connectivity — it is a path that works and is wrong, until something flaps and it becomes a black hole. That is why it survives testing: everything pings.
Two commands matter more than they look. `subnets`, without which OSPF redistributes only classful networks and silently drops every /30. And E1 versus E2 — E2 keeps the same metric everywhere, so the nearest exit is not necessarily chosen; E1 adds the internal cost, which is usually what you want with two exits.
Try breaking it
- Leave out
subnetsand count how many routes disappear. - Switch the external type and watch which exit the traffic picks.
- Raise the administrative distance of external routes and see the loop close
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