Monday 19 April 2010

BSCI - IS-IS - Differences between IS-IS and OSPF

  • OSPF supports 50 routers per area
  • IS-IS can support up to 1000 routers per area
  • OSPF includes more features, including route tags, stub/nssa configuration, on demand circuits
  • OSPF requires many LSA's to advertise and withdraw routes.
  • IS-IS only requires one LSP per IS-IS router in a given area.
  • IS-IS doesn't support a Backup designated router (BDR).i.e. you only have the primary Designated Intermediate System (DIS).
  • IS-IS repeats the DIS election process every time a new router is added to an area.
  • If the new router has the same of higher priority it will become the new DIS.
  • IS-IS is designed to be deterministic so that the same set of IS-IS routers always produce the same DIS.
  • Every time a DIS changes a new set of LSP's are flooded to the neighbours.
  • IS-IS form adjacencies with all routers in the area not just the DIS.
  • OSPF uses a scaled metric by default where as IS-IS uses a default metric of 10 for all links.


  1. Nice notes you have there. Good to see you are working hard as well. As for the first statement "OSPF supports 50 routers per area", this is Cisco recommendation when it comes to designing OSPF networks but OSPF works fine even when the number of routers go beyond 50 per area. I have worked for a MNC with more 1000 routers in area 0 and so far don't have any convergence issues.

  2. Cheers for the feed back Pete! :o)

    I work on a modest OSPF set up so I'm having to take Cisco on it's word here.