How does EIGRP choose its router ID, and what breaks if two routers share one?
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Say this first
Same order as OSPF: the configured eigrp router-id, otherwise the highest IPv4 address on an up
loopback, otherwise the highest IPv4 address on an up interface. The router ID is stamped on external
routes, and a router discards any external route carrying its own router ID as a loop check. So if
two routers share an ID, each silently drops the other's redistributed routes.
Go deeper
Set it, do not let it float
router eigrp 100
eigrp router-id 10.255.0.1Named mode puts the same eigrp router-id under the address family. Check it:
R1# show ip protocols | include Router-ID
Router-ID: 10.255.0.1The duplicate ID failure
R2 and R6 were both cloned from one template with router ID 10.255.0.2. R6 redistributes a static route to 192.0.2.0/24. R2 receives the external, sees its own router ID as the originator, and drops it. Every router behind R2 also never sees it. Nothing is logged, the neighbor stays up, and the route is simply missing in part of the network.
Seeing the originator
show ip eigrp topology 192.0.2.0/24 on a router that does have the route lists
"Originating router" and the external protocol. That is where you spot two routers with the same ID.
They will ask next
Does EIGRP for IPv6 need a router ID?
Yes, it is still a 32-bit value. On a router with no IPv4 address at all you must configure one by hand, or the process will not start.
Does the router ID have to be an address the router owns?
No. It is just a 32-bit identifier written like an address. Using the loopback address is a convention that makes it easy to find the router.
Traps
- Saying a duplicate router ID stops the neighbor forming. It does not; the damage is silent loss of external routes.