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Intermediate System – Intermediate System (IS-IS) Protocol

Intermediate System – Intermediate System (IS-IS) Protocol

IS-IS Standards History

  • ISO 10589 specifies OSI IS-IS routing protocol for CLNS traffic
  • Tag/Length/Value (TLV) options to enhance the protocol A Link State protocol with a 2 level
  • RFC 1195 added IP support
  • IS-IS runs on top of the Data Link Layer
  • Requires CLNP to be configured
  • RFC5308 adds IPv6 address family support to IS-IS
  • RFC5I20 defines Multi-Topology concept for IS-IS
  • Permits IPv4 and IPv6 topologies which are not identical


  • AIT: Authority and Format Identifier (the first octet of all OSI NSAP addresses—identifies format of the rest of the address)
  • CLNP: Connection-Less Network Protocol (ISO 8473—the OSI connectionless network layer protocol—very similar to IP)
  • ES: End System (the OSI term for a host)
  • IS; Intermediate System(the OSI term for a router)
  • ES-IS :End System to Intermediate System routing exchange protocol
  • (ISO 9542—OSI protocol between routers and end systems)
  • IS-IS : Intermediate System to Intermediate System routing exchange protocol (the ISO protocol for routing within a single routing domain)
  • IS-IS Hello :A Hello packet (defined by the IS-IS protocol)
  • LSP: Link State Packet (a type of packet used by the IS-IS protocol)
  • TLV: Type Length Value

ISIS features

  • Open standard (OSI)
  • First developed by DEC and It was standardized by the ISO in 1992 communication between network devices which are termed Intermediate Systems by the ISO.
  • Link-state routing protocol
  • Classless
  • VLSM and Manual summary
  • Work on SPF or Dijkstra algonthm
  • Incremental / trigger updates
  • Updates are send as unicast /multicast (Layer 2)
  • Metric= default (Cisco)
  • Administrative distance = 115
  • It is used by large ISP
  • Protocol independent i.e. Support IP, IPX. hippie Talk

Integrated or Dual IS-IS

  • Originally IS-IS was ECP for OSI layer, later developed to support TCP/IP called as DUAL IS-IS
    • Dual IS-IS works with this change features:
    • Function similar as any other IGP routing protocol
    • Fast convergence ( default hello = 10 sec . dead timer = 3 times of hello)
    • Stable
    • Make efficient use of bandwidth, memory &CPU resources


  • Cisco supports only default metric
  • e. Metric= 10 on any type of interface ( ex : serial/Ethemet/fastethemet/gigethemet)

  • Narrow metrics
  • Max value on interface can be 64 (6bit) and 1024 (10bit) per path in routing table
  • Other venders support different types of metrics (default, delay, expense, error)

IS IS Levels

  • ISIS has a 2 layer hierarchy
  • Level-2 (the backbone)
  • Level- 1 (the areas)
  • IS-IS support three type of router levels
  1. Level- 1 (LI) router (intra-area routing)
  2. Level-2 (L2) router (inter-area routing)
  3. Level-1-2 (L1L2) router (intra and inter-area routing)
  • Manually Need to define IS-type
  • Default is level 1 /Level 2

Level 1 Router

  • Establishes neighbor ship with only LI and Ll/12 routers
  • It acts like OSPF internal router (totally stub)
  • Has the Level 1 LSDB with all routing information for the area
  • It has a default route pointing towards nearest L1/L2 router
  • LI should be continuous within a area.

Level 2 Router

  • Establishes neighbor ship with only L2 and L1/L2 routers
  • It act like OSPF Backbone router
  • Has a Level 2 LSDB with all information about inter-area routing
  • L2 should be continuous within a domain with other L2 orLl/L2 router

Level 1/Level 2 Routers

  • Establishes neighbor ship with all LI , L2 and Ll/L2 routers
  • It act like OSPF ABR
  • Has two LSDBs:
  • Level 1 for the intra-area routing
  • Level 2 for file inter-area routing
  • if the router has adjacencies to other areas, lt will inform the Level 1 routers (intra-area) it is a potential exit point for the area
  • L1/L2 should be continuous within a domain with other L2 or Ll/L2 router

Backbone & Areas

  • ISIS does not have a backbone area as such (like OSPF)
  • Instead the backbone is the contiguous collection of Level-2/ or Level 1-2 capable routers
  • ISIS area borders are on links, not routers
  • Each router is identified with Network Entity Title (NET)
  • NET is an NSAP where the n-selector is 0

 Backbone must be L2 contiguous



An Addressing Example

Identifying Systems in IS-IS

  • The area address uniquely identifies the routing area and the System ID identifies each node
  • All routers within an area must use the same area address
  • An ES maybe adjacent to a level-1 router only if they both share a common area address
  • Area address is used in level-2 routing
  • System ID maybe the MAC address (CLNS) or IP address of an interface (IP world)
  • System ID used in level-1 routing and has to be unique within an area (and of same length)
  • System ID has to be unique within level-2 routers that form routing domain
  • General recommendation: domain-wide unique System ID



ISIS PDU is directly encapsulated in to data-link header. There is no CLNP or IP header.

  1. Hello
  2. LSP
  3. CSNP
  4. PSNP

 OSPF and ISIS Similarities

  • Both use the same method of topology abstraction
  • Both use [ practically}the same flooding algorithm
  • Both use [practically] the same Dijkstra algorithm
  • Both provide IP routing information

OSPF and ISIS Major differences

  • History
  • Transport protocol
  • PDU format
  • Details of hierarchical routing


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