CCNP ENARSI Course: Techniques for Diagnosing Route Redistribution Failures

Route redistribution looks simple on paper. You type one command, two routing domains start talking, and everyone goes home happy. In real networks, it rarely works that smoothly. I have seen engineers lose hours chasing a missing prefix, only to find a forgotten keyword or a route map quietly dropping traffic. If you are working through the CCNP ENARSI course, learning to diagnose these failures is one of the most valuable skills you can build, both for the 300-410 exam and for day-to-day work.

Start With the Source Routing Table

Before touching the redistribution configuration, confirm the route actually exists where it should. Redistribution only moves routes that are installed in the routing table of the redistributing router as best paths from the source protocol. If the prefix is not there, nothing downstream will ever see it.

Run show ip route for the prefix and check which protocol owns it. A route can exist in the OSPF database but lose the best-path battle to a static route or an EIGRP route with a lower administrative distance. In that case, the OSPF version never gets redistributed, and the symptom looks like a redistribution bug when it is really a selection issue.

Check the Seed Metric

The seed metric is the most common culprit, and the ENARSI curriculum keeps coming back to it for good reason. Each protocol treats it differently:

• EIGRP needs a metric defined, either with the metric keyword or a default-metric command. Without one, the routes simply do not appear.

• OSPF assigns a default metric of 20 to redistributed routes, but it ignores subnetted networks unless you add the subnets keyword.

• BGP carries over the IGP metric as the MED, which can surprise people during path selection.

When a route is missing on the other side, show running-config | section router is a quick way to verify these details. It takes ten seconds and solves a surprising number of tickets.

Read the Protocol Output Closely

Once the configuration looks right, move to the protocol-specific views. For OSPF, show ip ospf database external shows whether Type 5 LSAs are being originated by your ASBR. If they are not there, the problem sits on the redistributing router. If they are there but missing from the routing table of a neighbor, look at filtering, forwarding addresses, or area types like stub and NSSA.

For EIGRP, show ip eigrp topology reveals whether external routes are present and what their original protocol was. External EIGRP routes carry an administrative distance of 170, which matters when the same prefix is learned another way.

Hunt for Filters and Route Maps

Route maps are powerful and also very easy to get wrong. A missing permit statement at the end creates an implicit deny, and suddenly half your prefixes vanish. Use show route-map to see match counters, since a counter that stays at zero tells you the match condition never fired. Prefix lists and ACLs deserve the same scrutiny, especially when they were written by someone else months ago.

A habit worth building is to read the route map line by line and ask what happens to a prefix that matches nothing. That single question catches most logic errors.

Watch for Loops During Mutual Redistribution

Redistributing in both directions between two domains is where things get dangerous. A route can leave one protocol, enter another, and bounce back in with a worse metric or a different administrative distance. This produces suboptimal paths, flapping, or outright loops.

The usual fix is route tagging. Tag routes as they enter the second domain, then deny those tags from coming back. Passing a tag through a route map is clean, simple, and easy to verify with show ip route <prefix>, which displays the tag value. Adjusting administrative distance on specific external routes is another option when tagging does not fit the design.

A Quick Troubleshooting Reference

Symptom	Likely Cause	Where to Look
Routes missing in EIGRP after redistribution	No seed metric defined	show run, show ip eigrp topology
Only classful networks appear in OSPF	Missing subnets keyword	show run, show ip ospf database external
Prefix exists but is not redistributed	Not best path in source table	show ip route <prefix>
Some prefixes disappear	Route map or prefix list deny	show route-map, show ip prefix-list
Routing loops or flapping	Mutual redistribution without tags	show ip route, debug ip routing
BGP routes not injected	Missing network statement or filter	show ip bgp, show ip protocols

Keep this kind of checklist handy. In the lab and in production, working through causes in the same order every time saves you from guessing.

Use Debugging Carefully

debug ip routing can confirm exactly when routes are added or removed, which helps with flapping. Use it sparingly on production devices, though, because verbose debugging on a busy router can cause more problems than the one you started with. Prefer show commands first, and use debugs with a narrow scope and a clear stopping point.

Build the Habit Through Practice

Reading about redistribution only gets you so far. The people who diagnose it quickly are the ones who have broken it on purpose in a lab. Build a small topology with OSPF, EIGRP, and BGP, redistribute between them, then introduce faults one at a time. Remove a metric, add a bad route map, create a loop with mutual redistribution, and practice finding each one without hints. Time yourself. The ENARSI exam rewards this kind of speed, and so do real outages at two in the morning.

It also helps to document what you find. A short note on the symptom, the cause, and the command that exposed it becomes a personal playbook you will reuse for years.

Final Thoughts

Redistribution failures feel mysterious until you realize they follow a small number of patterns. Confirm the route exists, check the seed metric, read the protocol databases, inspect filters, and watch for loops. That sequence, repeated with discipline, solves most cases. Combine it with regular lab practice and the techniques from your CCNP ENARSI course, and it will stop feeling like theory and start feeling like instinct. Next time a prefix goes missing, you will know exactly where to look first.