R1 ╚════════════╝ R2 ╚════════════╝ R3 ╚════ R3 LAN ════╝ 

In OSPF each router knows the full topology, so it uses the Dijkstra algorithm to calculate the path to the destination. But in EIGRP (and any other distance vector routing protocol I believe), routers only know about routes to their neighbours. How will R1 know how to reach, when this is not a neighbour network?

The EIGRP and distance vector routing protocol tutorials I've studied do not seem to cover this.

2 Answers 2


There seems to be a confusion here : in a distance-vector routing protocol, the routers don't just get routes to their neighbors (which would be trivial in most cases) they get their neighbor's full routing table. A router can then compare the various routes it has gotten from its neighbors for a given destination network and select which appears to be the shortest to add to its own table.

So in your example, R2 receives R3's routing table, and will update its own table accordingly, with a route to, and then pass that on to R1 as part of its own routing table.

  • But doesn't that mean that ultimately, every router will know about the full topology, since every router is constantly sharing their full routing table with neighbours? That's sort of weird, since I've read that one of the fundamental differences between link state- and distance vector routing protocols is that link state knows about the full topology, and distance vector does not. Jan 24, 2016 at 6:34
  • 2
    There's a difference between knowing the full topology (i.e. what nodes exist and how they are linked) and having a complete list of routes (what destinations are reachable through the network, regardless of the detailed path you'll take to get there). With link state you get both, at the cost of greater complexity. Jan 24, 2016 at 7:04

EIGRP is kind of hybrid protocol who act as distance vector as well as link state protocol. EIGRP doesn’t send link-state packets as OSPF does instead, it sends traditional distance-vector updates containing information about networks plus the cost of reaching them from the perspective of the advertising router. And EIGRP has link-state characteristics as well, it synchronizes routing tables between neighbors at startup and then sends specific updates only when topology changes occur. This makes EIGRP suitable for very large networks.

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