What administrative distances does EIGRP use, and why is external higher than internal?
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90 for internal routes, 170 for external routes that were redistributed into EIGRP, and 5 for a summary the router creates itself, which points to Null0. External is high so that a prefix learned natively from another protocol, such as OSPF at 110 or RIP at 120, beats the same prefix coming back in through EIGRP after redistribution. That stops a common redistribution loop.
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How they look in the routing table
D 10.50.0.0/24 [90/3328] via 10.0.12.2, 00:14:02, GigabitEthernet0/0/0
D EX 172.16.0.0/16 [170/5376] via 10.0.12.2, 00:14:02, GigabitEthernet0/0/0
D 10.20.0.0/22 is a summary, 00:14:02, Null0D is EIGRP internal, D EX is EIGRP external. The summary line exists only on the router that
configured the summary.
Why the external value matters
Take two routers that both redistribute OSPF into EIGRP. R1 learns 172.16.0.0/16 from OSPF at 110 and redistributes it. R2 learns the same prefix through OSPF and also as an EIGRP external from R1. Because 110 beats 170, R2 keeps the OSPF route and does not loop traffic back through R1. If externals had AD 90, R2 would prefer the redistributed copy and you would build a loop.
Changing them
router eigrp 100
distance eigrp 90 170The first number is internal, the second external. I leave them alone unless I am fixing a specific redistribution problem.
They will ask next
What is special about an external route besides the AD?
It carries the router ID of the router that redistributed it, the source protocol, and a route tag. A router drops any external route that carries its own router ID, which is the loop check. The tag is what I match in route maps to stop routes being redistributed back.
Inside EIGRP, would an internal route lose to an external route with a lower metric?
No. EIGRP itself prefers an internal path over an external one for the same prefix, whatever the metrics, so only the internal route reaches the routing table. Its AD of 90 against 170 points the same way.
Traps
- Forgetting the summary AD of 5. It is why a summary's Null0 route beats almost everything on the router that made it.