ISIS Levels

ISIS has a 2 layer hierarchy:
  • Level-2 (the backbone).
  • Level-1 (the areas).

Level of routing:
  • Level 0 : Between an End System (a host or a PC) and IS (a Router).
  • Level 1 : IS to IS in the same area.
  • Level 2 : IS to IS in different areas in the same AS.
  • Level 3 : IS to IS in different AS or routing domain. Like routing through ASBR in OSPF.

An ISIS router can be either:
  • Level-1 (L1) router.
  • Level-2 (L2) router.
  • Level-1-2 (L1/L2) router(default type - Cisco).

Level-1 router:
  • Has neighbors only in the same area.
  • Has a level-1 LSDB with all routing information for the area.

Level-1-2 router:
  • May have neighbors in any area.
  • Has two separate LSDBs: level-1 LSDB & level-2 LSDB.
  • By default, a router becomes L1/L2 router unless specifically changed the role to L1-only or L2-only.

Level-2 router(kind of a backbone area):
  • May have neighbors in the same or other areas.
  • Has a Level-2 LSDB with all routing information about inter-area.