EIGRP topology table - unexpected behavior

Question

Prepping for CCNP - In my topology the path R1 takes to a Loopback Ip is via R2 by default. My goal is to have a feasible successor route through R3. I've attached a diagram of topology.

http://imgur.com/a/6KZPz#4

Problem is: R1 only has one entry for the loopback ip of 3.3.3.3 in it's topology table. When I look at R3, it has two entries for this network, but somehow it's not sharing it..

I had planned to manipulate delay to add the FS route in R1, so I tried that on all links from R3 to the loopback (reduced delay to 10 usec), and for one update I saw R1 have both entries.. after another update though it removed it..

My understanding was that even if it wasn't a FS the topology table (using the 'detail' switch) would show all possibilities? What am I missing?

Answer 1

Try using the command

show ip eigrp topology all-links

This should allow you to view all routes including non-successor/non-feasible routes

Answer 2

The default view of the EIGRP topology table actually does not contain ALL paths to a network. You'll see a next-hop if it is either a successor, or a feasible successor. If a certain path doesn't qualify as either of those, then it doesn't go in the topology table.

EDIT: As another answer correctly points out, you can use the "all-links" argument to get the topology table to list every potential next-hop. EIGRP will still need to re-compute DUAL to guarantee a loop-free topology, but this way you can at least see the non-feasible and non-successor routes.

This is different from OSPF (at least within an area), where the "topology table" (the OSPF Link-State Database) contains all information about the network.

When a route goes active in the topology table, it is because it has neither an S or a FS. An active route is actively asking the network for valid next-hops to reach said network. In this case, the metric to your loopback network from R1's perspective through R3 is considered not good enough to qualify as a FS, so it merely installs the single route through R2 into the routing table as a successor. If this path were to fail, the route to 3.3.3.0 would go active, and R3 would respond, saying that it is able to offer reachability to this network the long way around.

R3 uses both potential paths because your metric weights make it so that only delay is used in the metric computation. So, from R3's perspective, the two paths to the loopback are equal-cost, so both are successors (see your topology table?)