ISIS Crib Notes

These are my ‘crib notes’ that I’ve made to serve as a last minute refresher. Please forgive the grammer / spelling as I did not develop these notes with publishing in mind.



  • Route summarisation – always do it on the ABR (L1/L2) router. Not from the advertising router. Also do it from router process
  • AD = 115, metric = 10, i in routing table code
  • ISIS treats NBMA as broadcast (not good)
  • Routing = Checks packet for area ID > If same > Routes via L1
    • If not same > send to L2 or L1/L2 router > L2 Routes via L2
    • Relationship =Hello (IIH) >Neighbour forms > CSNP (Complete sequence number packet) sent to each other to synchronise Link State Database (LSD) > sends hello to keep relationship alive
    • DIS (Designated Intermediate System) – Like the DR in OSPF, floods changes on broadcast segment. There is no Backup DIS.  Election via interface priority. Default is 64. If tied, highest subnet number (SNPA) is used. DIS creates pseudonode and this forms relationship with each router.
    • NSAP – Is the address of the CLNS packets. Each router only has one NSAP address. Is assigned at the router level (you can set up to 3)
      • Area ID (AFI+IDI+DSP)+ SYSTEM ID (L1) + NSEL (Set to 0 for router)
      • If the AFI is 49 = Private address. Area ID must be same for the area. System ID is unique, usually the MAC address
      • 49.0001.4356.1234.1245.00 (to break it down stared from right to left)
        • 00 is NSEL.
        • 4356.1234.1245 is the System ID (Next 12 numbers) – L1 routing with the area
        • 49.0001 is the Area ID (remaining numbers) – L2 routing between areas
        • NET (Network Entity Title)–Part of the NSAP address. NET refers to the host portion. NSEL is set to 0 to indicate a router. NET is assigned at the router level not interface.
        • SNPA (subnet point of attachment) – Layer 2 address e.g. MAC or highest DLCI
        • CLNS (Connectionless Network Service) – Own protocol and uses NSAP as it address
        • CLNP (Connectionless Network Protocol)
        • ES (End System) – Is a end system e.g. host. IS is intermediate System which is a router
        • 4 levels of routing
          • L1 = in single area and connects to L1/L2 router as default gateway. L1 has it own hello type and forms relationships with other L1 in same area
          • L2 = has it own hello type and can form relationships with any L2 in any area and L1/L2
          • L1/L2 = Maintains 2 databases.  L1 database & L2 database. L1/L2 is default setting on router. Can form relationship with any L1 in same area and L2 in any area.
          • Packets aka PDUs (Protocol Data Units)
            • Hello – sent every 3.3 seconds (fast)
            • Link State Packet – is used to build link state database (lsd) and is sent when a network change occurs
            • CSNP (Complete Sequence Number Packet) – every 10 seconds. Sends complete list of all link state packets (lsp) to neighbour. Helps synchronise between routers
            • PSNP (Partial Sequence Number Packet) – requests an LSP & Acks that a requested LSP has been received
            • Area
              • Backbone – group of L2 and L1/L2 routers
              • Metric = Cost & Default (Default), Delay, Expense, Error
              • OSPF vs ISIS
                • Ospf = ip & ISIS = CLNS
                • OSPF = 30 different process & ISIS = 1
                • ISIS = more default times. So more fine tuning
                • OSPF = Has more area types. So can limit route advertisement better
                • OSPF = LSU contain LSA & ISIS = LSP contain TLV
                • Both use cost as default metric & MD5 + clear text link authentication


Ip router isis

  • Use this to enable ISIS on an interface

Show clns neighbours (detail)

  • Shows neighbours and SNPA addresses for all routers

Show clns interface

  • Shows circuit type, adjcancies, DIS

Summary-address x.x.x.x x.x.x.x

  • Manual summarisation under routing process

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