Question of the day - OSPF Cost Caulcation

rakemrakem Member Posts: 800
This thread needs more technical discussions!
So here is my attempt at stimulating something....

How does an OSPF router, that isn't an ABR or ASBR, calculate it's cost to an inter-area subnet?
CCIE# 38186


  • al3kt.R***al3kt.R*** Member Posts: 118
    Nice one!

    Router calculates (applying SPF to its LSDB), for each type3 LSA in its LSDB, the intra-area cost from itself to reach the ABR listed in a given type3 LSA and adds to it the cost value listed in this type3 LSA (the actual cost from this ABR to the dest subnet)

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  • MrBrianMrBrian Member Posts: 520
    Ditto what the previous poster said.

    However, I think the way ospf routers calculate the cost to reach redistributed routes, that were also redistributed in a different area than its own, is the money question.

    Like what if I redistributed routes into area 10, let's say. How would an internal ospf router, that's part of the backbone area, determine the cost to those routes?? That's the one that took me the longest to get down!

    The type 3 lsa's are low-hanging fruit for a ccie thread imo.. icon_wink.gif

    Love the quick hit question though! Got me thinking about something I hadn't thought of in awhile.
    Currently reading: Internet Routing Architectures by Halabi
  • rakemrakem Member Posts: 800
    Yep good answers.

    I guess the main point is the router trusts the cost that is received from the ABR. It then calculates its cost to reach the ABR then adds that value to the cost received from the ABR.
    CCIE# 38186
  • vishaw1986vishaw1986 Member Posts: 40 ■■□□□□□□□□
    Per hop cost as added to the particular route when moving in the OSPF area .
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