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Sim 05: Mutual Redistribution, a Routing Loop and Route Tags

Blueprint 1.1, 1.2, 1.3, 1.4

Difficulty Time target Devices Faults
Hard 30 minutes 4 x IOSv 3

Scenario

The head office runs OSPF (R1). A newly acquired company runs classic EIGRP AS 100 (R4), which also carries two partner networks that R4 redistributes as EIGRP externals. R2 and R3 sit between the domains and both perform mutual redistribution. Two redistribution points give redundancy, but they also create the risk of route feedback and loops, so the design uses three controls:

  1. Tags. Routes redistributed from EIGRP into OSPF get tag 90. Routes redistributed from OSPF into EIGRP get tag 110. Each redistribution point denies routes carrying the other domain's tag, so nothing is redistributed back into the domain it came from.
  2. Administrative distance. EIGRP externals have AD 170 and OSPF externals have AD 110, so a border router would prefer the OSPF copy of an EIGRP-domain external and route the long way round. distance ospf external 175 on R2 and R3 makes them prefer the EIGRP copy.
  3. Seed metric. OSPF routes enter EIGRP with an explicit metric.

Download the lab

CML lab file · EVE-NG lab file

4 nodes. The lab boots already broken (the faults are in the start-up configs, so skip Break it). CML: Import a lab, then start it. EVE-NG: import the .unl from the lab list; if your image folder names differ, edit each node's image. After all routers have converged (about 3 minutes), shut down Loopback44 on R4 to trigger the fault.

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