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EIGRP, OSPF and external prefixes – different redistribution scenarios with different outcomes

Redistribution proces between IGP routing protocols especially with  externally redistributed networks may cause problems like routing loops or suboptimal routing. In thic article we will look closer on particular scenarios of redistribution between them.

Before we go over to redistribution, let’s recall Administrative Distance values for EIGRP and OSPF. EIGRP has AD 90, but for prefixes redistributed into EIGRP, AD is 170. In case of OSPF both values are the same and comes to 110

 

 

EIGRP into EIGRP  (no redistribution)

When we redistribute prefix from one EIGRP process into another, the prefix will have AD 170. Originally prefix in EIGRP 1 has AD 90, so won’t be redistributed back into EIGRP 1, because in EIGRP 2 the same prefix has AD 170

EXTERNAL PREFIX into EIGRP into EIGRP (redistribution)

This time we are redistributing prefix from external network into EIGRP 1 so will have AD 170 in EIGRP 1. The same AD 170 will have in EIGRP 2, and will be redistributed back. It will takes place because EIGRP uses the lowest bandwidth as one of the “K” values in metric calculation. So, because there is a slow serial link, into EIGRP 1 redistributed prefix will have a high metric. In EIGRP 2 metric is calculated from the scratch, because there is no slow link will have a lower metric.

EXTERNAL PREFIX into EIGRP into OSPF (redistribution)

Prefix that will get into OSPF from EIGRP will have AD 110, becaue in EIGRP AD is 90, prefix will not be redistributed back.

OSPF into OSPF (no redistribution)

We have 110 AD on both sides, but on routers that take part in redistribution the same prefix in RIB table will we marked as Intra Area in original OSPF 1 and Inter Area 2  in OSPF 2. Intra Area is always prefered over Inter Area and prefix will not be redistributed back.

EXTERNAL OSPF into OSPF (no redistribution) 

Exsternal prefix is being redistributed into OSPF 1 with AD 110, next into OSPF 2 with AD 110, so why is not redistributed back into OSPF 1? Let’s check RIB table on R1 (the router at the bottom, that blocks redistribution) for external prefix 52.0.0.0

 

AS we see in both processes OSPF 1 and OSPF 2 the prefix is marked as External with cost of 20 but there is one difference. In OSPF 1 “fwd cost” is 1 whereas in OSPF 2 is equal 2. The lower forwarding cost is prefered.

 

EXTRENAL into OSPF into EIGRP (no redistribution)

Prefix has AD 110 is OSPF 1, when gets into EIGRP 1 will have AD 170 so will not be redistributed back

OSPF into EIGRP (no redistribution)

Again prefix has AD 110 in OSPF 1 in EIGRP 1 the same prefix will have AD 170 and will not be redistributed back

EIGRP into OSPF (no redistribution)

In OSPF 1 prefix has AD 90, after redistribution will have AD 110 and will not be redistributed back

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