A city clerk’s office sees both an external EIGRP route and an OSPF route for the same water-SCADA prefix. Which path does IOS prefer by default, and why?
Select an answer to reveal the explanation.
Short Explanation
You’ll trust external EIGRP less than OSPF because OSPF’s AD is 110 and external EIGRP sits at 170. Same SCADA prefix, OSPF gets the install.
Full Explanation
When the city clerk’s office sees both an external EIGRP route and an OSPF route for the same water-SCADA prefix, IOS compares default administrative distances. OSPF uses AD 110 for intra-area and inter-area routes (and for external OSPF routes unless distance is changed). EIGRP internal routes default to AD 90, but EIGRP external routes—those injected into EIGRP via redistribution—default to AD 170. Because 110 is lower than 170, the OSPF path is installed by default for that SCADA prefix. The claim that the external EIGRP path wins because EIGRP always outranks OSPF is true only for internal EIGRP (90 versus 110). External EIGRP is intentionally less trusted so redistributed noise does not automatically override a native OSPF path in a mixed municipal core. The claim that the external EIGRP path wins because redistributed routes inherit AD 20 misstates inheritance: redistributed routes into EIGRP take the external EIGRP AD of 170 (unless distance is tuned); they do not inherit external Border Gateway Protocol’s administrative distance of 20. The claim that neither path enters the routing information base without a floating static is false; external routes enter the routing information base routinely when their administrative distance wins. No floating static is required for an external EIGRP or OSPF route to be eligible. Exam caveat: always separate 'EIGRP' into internal versus external on exam language. If the stem says 'EIGRP route' without 'external', it often implies AD 90. Here the stem explicitly says external EIGRP, so 170 applies. Also note OSPF external Type-1/Type-2 metrics affect cost within OSPF, not the AD comparison against EIGRP. Distance manipulation under router eigrp or router ospf can invert this outcome in production water-SCADA designs. Operational check: show ip route <scada-prefix> and confirm the OSPF route code (or external OSPF route code if the OSPF path is external) with AD 110 beating any external EIGRP route code entry at 170. show ip eigrp topology <prefix> may still list the external candidate. show ip protocols helps verify whether distance eigrp or distance ospf overrides are present that would change the clerk’s expected preference.