Route Redistribution Explained (and Its Dangers)
Route redistribution injects routes learned by one routing protocol into another — essential when a network runs OSPF in one part and EIGRP in another. Powerful, but a classic source of routing loops if done carelessly.
Why and how
Mergers, migrations and multi-vendor networks often run more than one protocol. Redistribution lets them share reachability. Because protocols measure "best" differently, you must set a seed metric when redistributing (e.g. a default-metric into OSPF/EIGRP) or routes may be ignored.
The loop danger and the fix
With redistribution at two points between two protocols, a route can flow out one protocol and back in the other — a loop. The professional fix: tag routes on the way out and filter tagged routes from coming back with route-maps. Redistribution and route-maps are core CCNP ENARSI skills — build on the routing fundamentals.
Frequently asked questions
What is route redistribution?
Injecting routes from one routing protocol into another so different parts of a network running different protocols can reach each other.
Why do redistributed routes sometimes not appear?
Often a missing seed/default metric — protocols measure cost differently, so redistributed routes need an explicit metric to be usable.
How do you prevent redistribution loops?
Tag routes as they're redistributed out and use route-maps to deny those tagged routes from being redistributed back in.
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