IS-IS Routing

Adjacency requirements:

  • Areas must match (for level1)
  • Levels must match
  • System ID’s must be unique
  • Authentification must succeed
  • Interface MTU’s must match

Abbreviations:

  • TLV – Type-Length-Value
    Variable-length modular dataset
  • LSP -Link State PDU
    Carry TLVs encompassing link state information
  • SNP – Sequence Number Packet
    Used to request and advertise LSPs
  • DIS – Designated Intermediate System pseudonode responsible for emulating point-to-point links across a multi-access segment

Example ISIS address:

46.0004.7off.f800.000 .000.0c00.1324 .00

46.0004.7off.f800.000 = area (address used for routing)
000.0c00.1324 = system id (unique router identifier – often derived from mac address)
00 = Sel/NSEL (network selector) identifies network layer service, always 0x00 in a NET address

Attributes:

  • Link State
  • Dijkstra alogirithm
  • Default 10 metric
  • Administrative distance 115
  • Protocols IP/CLNS
  • Layer 2
  • Authentification plain, MD5

Levels:

  • Level 0 used to locate end systems
  • Level 1 Routing within an area
  • Level 2 Backbone between areas
  • Level 3 Inter-AS routing

DIS ( Designated Intermediate System) election :

  • Highest priortiy interface elected
  • Default interface priotiy 64
  • Current DIS can be preempted