OSPF Intra-Area External v.s. Inter-Area External Path Selection

ahmed1206ahmed1206
edited September 2019 in OSPF

Hi,
I'm stuck understanding this concept. INE CCIE ATC 44 (Controlling NSSA Redistribution). At 22:45, when R1 traces to 150.1.9.9, it prefers the path to R6 -> R7 -> R9 (Inter-Area external).

From R1's perspective, it's only in Area 0. So the path through R6 is Inter-Area external. Since the Type 5 LSA has a forward address of NOT 0.0.0.0 (so goes from Type 5 to Type 3 to Type 1 LSA aka Inter-Area).

But, R1 should also be learning 150.1.9.9 in Area 0 since R2 is also doing redistribution (which should be an Intra-Area external). This Type 5 external LSA has a forward address of 0.0.0.0 (so goes from Type 5 to Type 1 LSA aka Intra-Area External).

I understand the metric is lower via R6 because of Tunnel/DMVPN interfaces, but I'm under the impression that metric isn't a factor in this.

"We always have to prefer our intra-area external over inter-area external. Simply because when we are looking at the LSA preference, we would always choose the shortest path to someone in our area before we choose the shortest path to the ABR that knows reachability to another area" (11:58 ATC 44).

Please help me. Thanks :smiley:

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