IS-IS

Intermediate System to Intermediate System (IS-IS, also written ISIS) is a link-state interior gateway protocol (IGP), typically deployed within a single autonomous system, and is used in large enterprise and service-provider networks to exchange routing information. The IS-IS protocol is defined in ISO/IEC 10589:2002 as an international standard in the Open Systems Interconnection (OSI) model.

IS-IS operates by flooding link-state information through a network of routers. Each IS-IS router builds its own link-state database (LSDB) by collecting the flooded link-state information from other routers. Like Open Shortest Path First (OSPF), IS-IS uses Dijkstra's algorithm for computing the shortest paths through the network. Packets (datagrams) are then forwarded along the computed shortest paths to their destinations.

History

IS-IS was developed by Digital Equipment Corporation as part of DECnet Phase V. The Internet Engineering Task Force (IETF) published an IS-IS specification in 1990, but that RFC was retracted and marked "historic" because it republished a draft rather than a final version of the International Organization for Standardization (ISO) standard (causing confusion).

The protocol was standardised by ISO in 1992 as ISO 10589 for use between Intermediate Systems rather than end systems or hosts. It was intended to make the routing of datagrams possible using the ISO-developed OSI protocol stack Connectionless-mode Network Service (CLNS). IS-IS was developed at roughly the same time that the IETF was developing a similar protocol, OSPF. It was later extended to support the routing of datagrams in the Internet Protocol (IP), the network-layer protocol of the global Internet. This extension is known as Integrated IS-IS. By 2005, IS-IS had become the de facto standard for large service-provider network backbones.

Terminology

The ISO IS-IS standard defines its terminology for network components, some of which differs from terms usually found in the industry:

Packet types

IS-IS operates over a broadcast (LAN) and point-to-point network links. Adjacencies are formed between neighboring routers to exchange routing information.

IS-IS Hello PDU (IIH)
IS-IS Hello PDUs are exchanged periodically to establish and maintain adjacencies. On broadcast networks, a Designated Intermediate System (DIS) is elected based on interface priority and system ID. This hello packet will be sent separately for Level 1 or Level 2. There are three IS-IS hello packets, depending on the circuit type:
  • LAN L1 (PDU type 15)
  • LAN L2 (PDU type 16)
  • P2P (PDU type 17) On point-to-point links, there are no separate hello packets per level like there are on broadcast links. Unlike OSPF, IS-IS does not require matching hello intervals, although significant mismatches may affect adjacency stability.
This contains the routing information. The LSP contains fields (type–length–values, or TLVs), which contain the routing data. Each LSP has an LSP ID, and consists of a System ID, Pseudonode ID and Fragment ID. In an LSP with an ID of 1921.6820.0002.02-01,
  • 1921.6820.0002 is the System ID (which generated this LSP),
  • 02 is the Pseudonode ID, and
  • 01 is the Fragment ID.
If the Pseudonode ID is zero, it represents a real intermediate system. Any non-zero value means that the LSP is generated by a DIS (Pseudonode).
If an LSP exceeds the maximum transmission unit (MTU), it is fragmented into multiple LSPs distinguished by a Fragment ID. Fragment numbering begins at zero; an unfragmented LSP consists of a single fragment (ID 0), and additional fragments are assigned incrementing identifiers.
Complete Sequence Number PDU (CSNP)
The Complete Sequence Number PDU (CSNP) is sent by the Designated Intermediate System (DIS) at regular intervals, typically every 10 seconds. It contains a summary of Link State PDUs (LSPs), including sequence numbers and checksums.
Partial Sequence Number PDU (PSNP)
If a router detects discrepancies between its link-state database and a received CSNP, it sends a PSNP requesting the missing or updated LSPs.

Addressing and NET

Unlike most IP routing protocols, IS-IS operates directly over Layer 2 rather than relying on Layer 3 for transport and does not use IP addresses to identify interfaces. It identifies interfaces using Layer 2 addresses such as Mac addresses on Ethernet, and identifies routing nodes using ISO network addresses. Each Intermediate System is assigned a Network Entity Title (NET), which is a NSAP address used to identify the router in the IS-IS routing domain. A NET is an NSAP used as a Network Entity Title where the NSEL field is set to zero. The System ID field is commonly set to a unique IPv4 address from one of the router’s loopback interfaces. An intermediate system can have up to three NET addresses. The NET consists of Area, System ID and NSEL fields. The Area field consists of an AFI (Address Family Identifier) and an Area ID, and can be one to 13 bytes long. The System ID is six bytes long, and the NSEL is one byte. The fields of the ISO Network Address "49.0100.1921.6821.1138.00" are:

Hostname resolution

When administering large networks, using IP addresses directly can be inconvenient. In large networks, routers are often identified using hostnames such as "if-bundle-22-2.qcore1.pye-paris.as6453.net", which may encode location and topology information. Routers are identified by IP addresses and hostnames are resolved externally using the Domain Name System (DNS) in many routing protocols. Because IS-IS operates directly over the data link layer rather than over IP, hostname mappings can be distributed within the protocol. IS-IS link-state PDUs may include Type-Length-Value 137 (TLV 137), which advertises a hostname associated with an IS-IS System ID derived from the router's NET.

Areas and levels

Similar to OSPF, IS-IS employs the concept of areas to divide the network. This reduces the overall burden on routers in the network by only requiring them to have complete link-state information for their area. In IS-IS, ISs operate at Level 1, Level 2, or Level 1/Level 2. Level 1 routers are internal to an area, and only maintain a link-state database (LSDB) for that area. Level 2 routers form the backbone of an IS-IS network, and route traffic between areas. They maintain a separate Level 2 LSDB for inter-area routing. Level 2 routers must be contiguous; their network must be fully internally routable, without crossing into different areas. Level 1/Level 2 routers are on the boundary between Level 1 and Level 2 routers; they participate in intra- and inter-area routing, maintaining separate Level 1 and Level 2 LSDBs.

When a Level 1 router needs to send traffic to a destination outside its area, it directs it to a Level 1/Level 2 router. Level 1/Level 2 routers advertise their status as boundary routers by setting the Attached Bit (ATT) in its Level 1 LSP. Routers receiving this LSP will add a default route to the origin of the LSP. External routes may be redistributed into Level 1 areas via Level 1/Level 2 routers. By default, external routes are not advertised into the Level 2 backbone; redistribution into Level 2 must be configured on Level 1/Level 2 routers.

Attribute bits in LSPs

IS-IS LSPs contain information about the LSP in the attribute block of the eight-bit LSP header:

Metrics

When IS-IS was introduced, the default type–length–value (TLV) formats for IS reachability (TLV 2) and IP reachability (TLVs 128 and 130) used a six-bit link metric field with a maximum value of 63 and a 10-bit path metric field with a maximum value of 1023. As network scale and link speeds increased, these metric limitations became restrictive for larger topologies. To address this, new TLVs were introduced: TLV 22 for extended IS reachability and TLV 135 for extended IP reachability. These extensions increase the supported link metric to 24 bits (maximum value 16,777,215) and the path metric to 32 bits (maximum value 4,294,967,295). Metrics without TLV 22 and 135 are known as "narrow", and metrics that include them are known as "wide". Wide and narrow metrics may be configured independently on a per-level basis.

Adjacency formation

Compared to OSPF, IS-IS adjacency formation rules are simpler and depend primarily on the router's level. Level 1 routers form adjacencies only with other Level 1 or Level 1/Level 2 routers in the same area. Level 2 routers form adjacencies with other Level 2 or Level 1/Level 2 routers, regardless of area. Level 1/Level 2 routers may form Level 1 adjacencies with Level 1 or Level 1/Level 2 routers in the same area and Level 2 adjacencies with Level 2 or Level 1/Level 2 routers, regardless of area. Level 1 adjacencies require matching area addresses, but Level 2 adjacencies are formed independently of area membership.

Broadcast segments and designated intermediate system

Like OSPF, all routers in a broadcast domain need to form adjacencies and exchange LSPs so there are adjacencies for each router in the domain. To overcome this issue, a designated intermediate system (DIS) is elected on each LAN segment. The router with the highest priority and System ID is elected as the DIS; if another router is connected with a higher priority (or higher System ID, if the priorities are equal), it will be elected as the new DIS. Instead of each router forming an adjacency with every other router in the broadcast domain, each router forms an adjacency with the DIS, and the DIS becomes responsible for relaying LSPs to the subordinate routers in a hub-and-spoke topology.

An elected DIS router is a pseudonode, which uses the resources (including System ID) of one router. LSPs originated by a DIS always have a non-zero Pseudonode ID field. The DIS will send periodic CSNPs on the LAN segment and reply to PSNPs from other routers. If the DIS stops communicating, a new DIS will be elected in the segment.

Authentication

IS-IS supports authentication of protocol data units (PDUs), including IIH (IS-IS Hello), link-state PDUs (LSPs), and sequence-number PDUs (SNPs). Authentication may be configured on a per-interface and per-level (Level 1 or Level 2) basis. Authentication mechanisms include simple password authentication and cryptographic authentication using HMAC MD5 and SHA variants.

IPv6 support and multi-topology

Unlike OSPF, which operates at Layer 3, IS-IS encapsulates its PDUs into Layer 2 frames and does not depend on Layer 3 protocols such as IPv4 or IPv6. To support IPv6 routing information, TLV 232 for IPv6 interface address and TLV 236 for IPv6 reachability were added. To display supported Layer 3 protocols (also known as NLPID, or Network Layer Protocol ID), TLV 129 is used. IPv4 has a code here of 0xCC, and IPv6 has a code of 0x8E. The IPv4 and IPv6 topologies may not overlap due to misconfiguration or lack of support for IPv6 by routers in the network. In that situation, multi-topology support is added to IS-IS. TLV 229 was added to indicate support for multi-topologies such as IPv4 and IPv6 unicast. If multi-topology is enabled, IS-IS will calculate separate SPF trees for IPv4 and IPv6. This uses twice the resources, but prevents traffic black holes. When multi-topology is enabled, IS-IS will use TLV 222 for IS reachability, TLV 235 for IP reachability and TLV 236 for IPv6 reachability.

Path selection

IS-IS computes the shortest paths by running Dijkstra's algorithm independently on each link-state database (LSDB): once for Level 1 and once for Level 2. Each router builds its own shortest-path tree from its position in the network. Path cost is the sum of the individual link metrics along the path; the lowest cost wins. When multiple paths to the same destination have the same cost, IS-IS can install all of them in the routing table for load balancing. This capability is known as equal-cost multipath (ECMP). When a router learns the same destination prefix from multiple sources (from Level 1 and Level 2 or internal and external routes, for example), IS-IS applies a fixed preference order. With narrow metrics (TLVs 128 and 130), the preference order is:

PreferenceRoute types
1L1 intra-area (internal metric); L1 external (internal metric)
2L2 intra-area (internal metric); L2 external (internal metric); L1→L2 inter-area (internal metric); L1→L2 inter-area external (internal metric)
3L2→L1 inter-area (internal metric); L2→L1 inter-area external (internal metric)
4L1 external (external metric)
5L2 external (external metric); L1→L2 inter-area external (external metric)
6L2→L1 inter-area external (external metric)

Wide metrics (TLV 135 for IPv4, TLV 236 for IPv6) do not support the internal-external metric distinction. RFC 7775 defines a separate preference order for these TLVs:

PreferenceRoute types
1L1 intra-area; L1 external
2L2 intra-area; L2 external; L1→L2 inter-area; L2→L2 inter-area
3L2→L1 inter-area; L1→L1 inter-area

BFD support

IS-IS uses Hello packets (IIH) to share information about routers and establish adjacencies. Hello packets also help detect faults between neighboring routers. Fault detection can be sped up by lowering the hello packet transmission intervals, but this increases CPU load. As an alternative, Bidirectional Forwarding Detection (BFD) can be used. BFD is a low-overhead fault-detection protocol that operates independently of the routing protocol and can provide sub-second detection with minimal impact on CPU.

Other uses

IS-IS is the base for the control plane in Shortest Path Bridging (SPB). SPB enables equal-cost multipath routing among Ethernet switches in a mesh topology; Ethernet frames are forwarded along multiple load-balanced, service-specific paths, which are all equally short. To support this, SPB extends IS-IS with new TLVs.

References

  1. ↑ "X.225 : Information technology – Open Systems Interconnection – Connection-oriented Session protocol: Protocol specification". Archived from the original on 1 February 2021. Retrieved 10 March 2023.
  2. ↑ "ISO/IEC 10589:2002 – Information technology – Telecommunications and information exchange between systems – Intermediate System to Intermediate System intra-domain routing information exchange protocol for use in conjunction with the protocol for providing the connectionless-mode network service (ISO 8473)". ISO website. International Organization for Standardization (ISO). November 2002. Retrieved May 24, 2017.
  3. 1 2 "Free-of-charge PDF copy of ISO/IEC 10589:2002". ISO website. International Organization for Standardization. Retrieved May 24, 2017.
  4. 1 2 "Cisco Learning Network". learningnetwork.cisco.com. Retrieved 2026-07-06.
  5. ↑ D. Oran, ed. (February 1990). OSI IS-IS Intra-domain Routing Protocol. Network Working Group. doi:10.17487/RFC1142. RFC 1142. Historic. Obsoleted by RFC 7142.
  6. ↑ M. Shand; L. Ginsberg (February 2014). Reclassification of RFC 1142 to Historic. Internet Engineering Task Force. doi:10.17487/RFC7142. ISSN 2070-1721. RFC 7142. Informational. Obsoletes RFC 1142.
  7. ↑ R. Callon (December 1990). Use of OSI IS-IS for Routing in TCP/IP and Dual Environments. Network Working Group. doi:10.17487/RFC1195. RFC 1195. Proposed Standard. Updated by RFC 1349, 5304 and 5302.
  8. ↑ Gredler, Hannes; Goraiski, Walter (2005). The complete IS-IS routing protocol. Springer. p. 1. ISBN 1-85233-822-9.
  9. 1 2 M. Bhatia; V. Manral; Y. Ohara (July 2005). IS-IS and OSPF Difference Discussions (Internet-Draft). Internet Engineering Task Force (IETF). draft-bhatia-manral-diff-isis-ospf-01. Retrieved June 3, 2026.
  10. ↑ Bhatia, Manav (22 July 2005). "IS-IS and OSPF Difference Discussions". Internet Engineering Task Force. Retrieved 24 August 2026.
  11. ↑ "Publicly Available Standards". standards.iso.org. Retrieved 24 August 2026.
  12. ↑ "Intermediate System-to-Intermediate System (IS-IS) TLVs". Cisco. Retrieved 2025-06-23.
  13. ↑ "Understanding Wide IS-IS Metrics for Traffic Engineering | Junos OS | Juniper Networks". www.juniper.net. Retrieved 2025-06-23.
  14. ↑ Li, T.; Smit, H.; Przygienda, T. (October 2008). "Domain-Wide Prefix Distribution with Two-Level IS-IS". Internet Engineering Task Force. Retrieved 10 August 2026.
  15. ↑ Ginsberg, L.; Litkowski, S.; Previdi, S. (February 2016). "IS-IS Route Preference for Extended IP and IPv6 Reachability". Internet Engineering Task Force. Retrieved 10 August 2026.
  16. ↑ D. Allan; N. Bragg; P. Unbehagen (April 2011). D. Fedyk; P. Ashwood-Smith (eds.). IS-IS Extensions Supporting IEEE 802.1aq Shortest Path Bridging. Internet Engineering Task Force. doi:10.17487/RFC6329. ISSN 2070-1721. RFC 6329. Proposed Standard.