Mutual Redistribution: The Route That Comes Home
A maintenance link comes back and the server LAN's traffic takes the long way round. Fix mutual redistribution on four real routers, right below.
Symptom
A maintenance link comes back and traffic to the server LAN takes the long way round. Fix mutual OSPF and EIGRP redistribution on four real routers without losing redundancy.
Your mission
Prove the cause, then fix it
Format
Four real routers
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Lab debrief
Turn the session into a repeatable troubleshooting pattern.
Use this reference after your attempt: first explain the symptom, then verify the evidence, then confirm the repair.
The problem
A merger joined an OSPF campus to an EIGRP site through two border routers, R2 and R3, each redistributing both ways. R3's link to the EIGRP site was down for maintenance. As soon as it comes back, R3 keeps sending traffic for the server LAN through the OSPF campus instead of straight across its new link, and R4 starts hearing its own LAN advertised back to it by a border router.
What you'll practice
- Read administrative distance and route source in the routing table to explain which copy of a route wins
- Prove route feedback from R4's EIGRP topology table
- Tag routes by origin with route-maps and deny them on the way back out
- Change administrative distance so each domain prefers its own native path
- Prove the fix keeps OSPF as the backup path when a border's EIGRP link fails
1. Why the OSPF copy wins
A route redistributed into OSPF is an external E2 route at administrative distance 110. The same route inside EIGRP is external at 170. Lower distance wins, so a border that already holds the OSPF copy keeps it even when a direct EIGRP path appears.
2. How feedback happens
Once a border installs the OSPF copy of an EIGRP route, its own OSPF-to-EIGRP redistribution advertises that route back into EIGRP. R4 then hears its own LAN from a border, which you can see directly in its topology table.
3. Tags mark where a route came from
Setting a tag as routes enter each domain, and denying that tag on the way back out, stops a route from re-entering the domain it started in. Tags stop the loop but do not change which copy a border prefers.
4. Distance changes the preference
Raising the OSPF external distance above 170 lets the native EIGRP route win at both borders, while OSPF still installs the route the moment the EIGRP path disappears.
5. Redundancy is part of the fix
Filtering routes can hide the symptom and remove the backup at the same time. The lab shuts a border's EIGRP link to prove the OSPF path still takes over.
The topology
Four real routers. R1 sits in OSPF area 0 with loopback 1.1.1.1. R4 runs EIGRP AS 100 and owns the server LAN 192.168.4.0/24. R2 and R3 each connect to both R1 and R4 and redistribute between OSPF and EIGRP in both directions. R3's link to R4 starts shut for maintenance.
Commands to run yourself
The real diagnostic commands for this lab, copy-paste ready. Run these in the same order to reproduce the investigation on your own session.
show ip route 192.168.4.0show ip eigrp topology 192.168.4.0/24show ip ospf database external 192.168.4.0traceroute 192.168.4.1 source Loopback0Frequently asked
Are these real routers or a simulation?
Real routers. Pressing Start boots four routers just for you, with full command sets: any valid command works, not only the ones the guide suggests. The first start can take a minute or two while the lab server wakes up.
Who can run this lab?
It is included with the Exam Pass and the Year Pass. Each session lasts up to three hours, and you can run one live lab at a time.
Does my exact fix have to match the answer?
No. The lab is scored from what the routers actually do (which route wins, the path traffic takes, the tags on each route and whether R4 hears its own LAN back), never from which commands you typed. Any correct fix passes, and the tempting partial fixes do not.
Which ENARSI topics does it cover?
Administrative distance, route-maps and route tagging, loop prevention and mutual redistribution between OSPF and EIGRP, from the Layer 3 technologies section of the 300-410 blueprint.
Ready to run this lab yourself?
No setup, no image sourcing. Book a session or ask for a live demo.