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Network Warrior, 2nd Edition by Gary A. Donahue

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Redistribution Loops

Things can get interesting when multiple routers are redistributing. Routes redistributed from one routing protocol into another can be redistributed back into the originating protocol, which can cause some pretty strange results. All of the original metrics will have been lost, so the route will inherit whatever metric was configured during redistribution.

Figure 11-6 shows a network with three routers. R3 has a network attached that is being advertised in EIGRP 100 by way of the redistribute connected command (50.50.50.0/24). R1 is redistributing from OSPF into EIGRP (from left to right in the figure), and R2 is redistributing from EIGRP to OSPF (from right to left in the figure).

Redistribution loop

Figure 11-6. Redistribution loop

The network 50.50.50.0/24 will be advertised from R3 to R1 and R2 through EIGRP. R2 will in turn redistribute the route into OSPF 100. R2 now has an entry for 50.50.50.0.24 in the OSPF database as well as the EIGRP topology table. Because the route was originally redistributed into EIGRP, it has an administrative distance of 170 when it gets to R2. R2 advertises the route to R1 via OSPF, which has an administrative distance of 110. So, even though R1 has also learned of the route from R3, where it originated, it will prefer the route from R2 because of the more attractive administrative distance.

Here are the IP routing tables from each router. Router R1 ...

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