Kevin Wallace's Dijkstra Calculations confusion

dontstop

Just watchin some of Kevin Wallace's videos on OSPF and SPF (Dijkstra) calculations. I'm really confused with some of the numbers he's getting.

For instance from node: R1->10.3.3.0 from the perspective of 10.1.1.0 he gets 3? I would've thought this would be 2.

I'm not sure but it seems he's adding both outgoing and incoming interface values. I thought that when calculating Dijkstra with a weight graph like this we only used 1 directions value.

rob42

Maybe I'm looking at this in the wrong way, but the way I see it is, starting at R1 (on Network 10.1.1.0 /24) there's a cost of '1' to the Switch plus another cost of '1' from the Switch to R2 and another cost of '1' to the next Switch (using Network 10.3.3.0 /24). Total Cost '3'.

dontstop

I was under the impression that we don't add costs of Layer 2 devices when calculating Layer 3 paths We only consider the outgoing interfaces cost until the next router.

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rob42

I understand what you're saying, and I concur. I've not seen the video, so I'm surmising, but if he's talking about Dijkstra's algorithm, it could be that he's not talking in a literal way about OSPF Costs, but more in a conceptual way. Or it could be that he's just plain wrong!