{
  "id": 19456919,
  "title": "How OSPF Interface Cost Is Calculated",
  "url": "https://urgent.news/2026/11/11/how-ospf-interface-cost-is-calculated",
  "topic": "tech",
  "section": "Tech",
  "published": "2026-11-11T06:16:47.000Z",
  "source": {
    "name": "Dev.to",
    "slug": "dev-to",
    "url": "https://dev.to/netengineerlabcom/how-ospf-interface-cost-is-calculated-5edj"
  },
  "original_language": "en",
  "account": "OSPF interface cost is an integer value determined using a specific formula. This cost is not a hop count, but rather calculated as reference bandwidth divided by interface rate, which is rounded and then clamped into a range of 1 to 65535. The OSPF Cost Calculator performs this calculation, requiring both reference and interface bandwidths to be in the same unit, preferably Mbit/s. A 10 Gbit/s link should be entered as 10000, not 10, to avoid misinterpretation. If the reference bandwidth is left blank, it defaults to 100000 Mbit/s, which explains why a 1 Gbit/s interface often receives a cost of 100 on modern equipment. The default rounding method is \"ceil,\" ensuring fractional costs are not used. A ratio of 1.01 would be rounded to 2 with ceiling rounding, and 1 with floor rounding. To illustrate, consider a path with one 10 Gbit/s and one 1 Gbit/s interface; the total cost would be 110. Disabled interfaces are not included in this total. The OSPF Cost Calculator also accounts for rounding differences, such as for an exact 33.3... ratio (100000 / 3000), ceiling rounding results in 34, while floor rounding yields 33. The calculator ignores interfaces faster than the reference, which always cost 1. Manual overrides can replace the calculated cost, with values of 0 or 65535 rejected. It's easy to make mistakes, such as entering 10 Gbit/s as 10, leading to a cost of 10000 for a 10 Mbit/s link. Mixing units in the reference and interface fields also causes errors. While a lower cost indicates a faster link, it does not guarantee equal load-sharing in the forwarding plane. A cost of 65535 signifies the link is not operational – the upper limit of the clamping range, not an indication of a down link. Lastly, this tool does not analyze vendor bandwidth CLI strings, nor does it generate LSAs or select areas or network types. Its sole purpose is to convert user-inputted numbers into integer costs, sum the calculated path costs, and present the lowest total. Readers can verify the calculations using the OSPF Cost Calculator.",
  "summary": "OSPF interface cost is a unitless integer, not a hop count. The usual formula is reference bandwidth divided by the interface rate. The result is rounded to an integer and then forced into the range 1 through 65535. That is the arithmetic used by the OSPF Cost Calculator . The formula and the units cost = clamp(1, 65535, round(B_ref / B_if)) B_ref and B_if must use the same unit. The calculator…",
  "key_points": [],
  "editors_take": null,
  "illustration": null,
  "coverage": {
    "outlets": 1,
    "also_reported_by": []
  },
  "ai_generated": true,
  "disclaimer": "Summaries, key points and the editor’s take are written by software from other outlets’ reporting and may contain errors — always check the linked original."
}