Intermediate System to Intermediate System

Intermediate System to Intermediate System
Name	IS-IS
Full name	Intermediate System to Intermediate System
Developed by	International Organization for Standardization (ISO)
Initial release	1980s
Latest version	ISO/IEC 10589
Category	Routing protocol
Platform	Open Systems Interconnection

Intermediate System to Intermediate System

Intermediate System to Intermediate System is a link-state routing protocol originating from the International Organization for Standardization standards work for the Open Systems Interconnection suite, later adapted for use in Internet Protocol networks and standardized in ISO/IEC 10589. It is implemented in network operating systems by vendors such as Cisco Systems, Juniper Networks, Huawei, Arista Networks and Ciena, and deployed by service providers including AT&T, Verizon Communications, Deutsche Telekom, NTT, and content networks like Akamai Technologies and Cloudflare. The protocol is used in large-scale backbone networks operated by operators such as Level 3 Communications, Telefonica, Orange S.A., CenturyLink and enterprise campus networks for scalability and fast convergence.

Overview

IS-IS was designed by the International Organization for Standardization as part of the OSI protocol family and is specified in ISO/IEC 10589. It was extended to support Internet Protocol by the Internet Engineering Task Force work implemented in routers from Cisco Systems and Juniper Networks, and compared in operational contexts with Open Shortest Path First and distance-vector protocols like Routing Information Protocol. Operators such as Sprint Corporation, Vodafone, SK Telecom, Telstra and BT Group have used the protocol in backbone and metro environments due to support for multi-area hierarchies, tagging via Multiprotocol Label Switching, and traffic engineering synergy with Generalized MPLS and Segment Routing.

Protocol Operation

IS-IS routers, called Intermediate Systems, establish adjacency via Hello exchanges modeled on ISO link-state design and maintain a Link State Database analogous to the OSPF database; implementations in Cisco IOS, Juniper JUNOS, Arista EOS, Huawei VRP and Cumulus Networks perform SPF calculations using Dijkstra’s algorithm similar to techniques used in Shortest Path First research. The protocol operates over data link layers such as Ethernet, PPP, SONET, and tactical links used by NATO and telecommunication backbones like Telia Company and Deutsche Telekom. IS-IS supports IPv4 and IPv6 by using TLV structures, adopted in vendor stacks and integrated with control plane functions in projects from IETF working groups and open-source platforms such as FRRouting, Quagga, Bird Internet Routing Daemon, and OpenBGPD.

Packet Types and Format

IS-IS packet types include Hello (IIH), Link State Packet (LSP), Complete Sequence Number PDU (CSNP), and Partial Sequence Number PDU (PSNP), carrying TLVs that encapsulate attributes in a way standardized by ISO/IEC 10589 and extended by IETF documents for IPng and IPv6. Packet formats are used by vendors like Juniper Networks and Cisco Systems and analyzed in academic work from institutions such as MIT, Stanford University, Carnegie Mellon University, ETH Zurich and University of California, Berkeley. The TLV mechanism enables integration of addresses, metrics, administrative tags, and extensions for applications developed by IETF groups and implemented in network OSes by Arista Networks and Huawei.

Routing Algorithms and Metrics

IS-IS uses a link-state database and runs a shortest-path-first computation, Dijkstra-style, to derive loop-free routes; metric considerations in operational practice are influenced by traffic engineering designs from AT&T, Verizon Communications, Telefonica, and research from Bell Labs and Bellcore. Network architects compare IS-IS metrics with alternatives used in OSPF and path-selection strategies in BGP to design inter-domain policies deployed by Level 3 Communications, NTT Communications, Time Warner Cable and content delivery networks like Akamai Technologies. Advanced metrics include administrative weights, TE metrics for MPLS networks, and scaling approaches used in large deployments by Google and Facebook for data center fabrics.

Implementation and Deployment

Major router vendors such as Cisco Systems, Juniper Networks, Huawei, Arista Networks and Ciena provide IS-IS in device software; open-source implementations include FRRouting, Quagga, Bird Internet Routing Daemon, and OpenBSD's OpenBGPD variants. IS-IS is deployed in backbone networks operated by AT&T, Verizon Communications, Deutsche Telekom, NTT, BT Group, Vodafone and cloud providers like Amazon Web Services, Google Cloud Platform, Microsoft Azure for interconnection and within campus fabrics at enterprises including Siemens, Siemens AG, General Electric, IBM and Oracle Corporation. Operational best practices have been documented in technical papers from IETF meetings, presentations at conferences hosted by RIPE NCC, ARIN, APNIC and standards harmonization forums such as ETSI.

Security Considerations

IS-IS lacks built-in strong cryptographic authentication in its original ISO specification; mechanisms used in real-world networks include MD5/MD5-like authentication, packet filtering, TTL security, and control-plane protections implemented by vendors like Cisco Systems, Juniper Networks and Arista Networks, with operational controls recommended by IETF and security analysis from US-CERT and academic groups at University of Cambridge and Imperial College London. Operators including Verizon Communications, AT&T, NTT, Deutsche Telekom and Vodafone deploy route filtering, distribution of prefix-lists, and infrastructure isolation strategies similar to those used for BGP to mitigate spoofing, topology poisoning, and denial-of-service threats explored in publications by IETF and security conferences such as Black Hat and DEF CON.

Category:Routing protocols