IS-IS

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Intermediate System to Intermediate System (IS-IS, also written ISIS) is a routing protocol designed to move information efficiently within a computer network, a group of physically connected computers or similar devices. It accomplishes this by determining the best route for data through a packet switching network.

The IS-IS protocol is defined in ISO/IEC 10589:2002 [2] [3] as an international standard within the Open Systems Interconnection (OSI) reference design.

In 2005, IS-IS was called "the de facto standard for large service provider network backbones". [4]

Description

IS-IS is an interior gateway protocol, designed for use within an administrative domain or network. This is in contrast to exterior gateway protocols, primarily Border Gateway Protocol (BGP), which is used for routing between autonomous systems. [5]

IS-IS is a link-state routing protocol, operating by reliably flooding link state information throughout a network of routers. Each IS-IS router independently builds a database of the network's topology, aggregating the flooded network information. Like the OSPF protocol, IS-IS uses Dijkstra's algorithm for computing the best path through the network. Packets (datagrams) are then forwarded, based on the computed ideal path, through the network to the destination.

History

The IS-IS protocol was developed by a team of people working at Digital Equipment Corporation as part of DECnet Phase V.

The Internet Engineering Task Force (IETF) published IS-IS in 1990 [6] , but that RFC was later retracted and marked as historic [7] because it republished a draft rather than a final version of the International Organization for Standardization (ISO) standard, causing confusion.

The protocol was standardized by ISO in 1992 as ISO 10589, for communication between network devices that are termed Intermediate Systems (as opposed to end systems or hosts) by the ISO. The purpose of IS-IS was to make the routing of datagrams possible using the ISO-developed OSI protocol stack called Connectionless-mode Network Service (CLNS). IS-IS was developed at roughly the same time that the Internet Engineering Task Force IETF was developing a similar protocol called OSPF. IS-IS was later extended to support routing of datagrams in the Internet Protocol (IP), the network-layer protocol of the global Internet. This version of the IS-IS routing protocol was then called Integrated IS-IS. [8]

Packet types

IS-IS adjacency can be either broadcast or point-to-point.

IS-IS Hello PDU (IIH)
The IS-IS hello packets needs to be exchanged periodically between 2 routers to establish adjacency. Based on the negotiation, one of them will be selected as DIS (Designated IS). This hello packet will be sent separately for Level-1 or Level-2.
Link State PDU (LSP)
This contains the actual route information. This LSP can contain many type–length–values (TLVs).
Complete Sequence Number PDU (CSNP)
This packet will be sent only by the DIS. By default for every 10 seconds, CSNP packet will be transmitted by DIS. This will contain the list of LSP IDs along with sequence number and checksum.
Partial Sequence Number PDU (PSNP)
If the router which receives CSNP packet finds some discrepancy in its own database, it will send an PSNP request asking the DIS to send specific LSP back to it.

Other uses

IS-IS is also used as the control plane for IEEE 802.1aq Shortest Path Bridging (SPB). SPB allows for shortest-path forwarding in an Ethernet mesh network context utilizing multiple equal cost paths. This permits SPB to support large Layer 2 topologies, with fast convergence, and improved use of the mesh topology. [9] Combined with this is single point provisioning for logical connectivity membership. IS-IS is therefore augmented with a small number of TLVs and sub-TLVs, and supports two Ethernet encapsulating data paths, 802.1ad Provider Bridges and 802.1ah Provider Backbone Bridges. SPB requires no state machine or other substantive changes to IS-IS, and simply requires a new Network Layer Protocol Identifier (NLPID) and set of TLVs. This extension to IS-IS is defined in the IETF proposed standard RFC 6329.

Related Research Articles

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<span class="mw-page-title-main">OSI model</span> Model of communication of seven abstraction layers

The Open Systems Interconnection (OSI) model is a reference model from the International Organization for Standardization (ISO) that "provides a common basis for the coordination of standards development for the purpose of systems interconnection." In the OSI reference model, the communications between systems are split into seven different abstraction layers: Physical, Data Link, Network, Transport, Session, Presentation, and Application.

In the seven-layer OSI model of computer networking, the network layer is layer 3. The network layer is responsible for packet forwarding including routing through intermediate routers.

Link-state routing protocols are one of the two main classes of routing protocols used in packet switching networks for computer communications, the others being distance-vector routing protocols. Examples of link-state routing protocols include Open Shortest Path First (OSPF) and Intermediate System to Intermediate System (IS-IS).

Connectionless-mode Network Service (CLNS) or simply Connectionless Network Service is an OSI network layer datagram service that does not require a circuit to be established before data is transmitted, and routes messages to their destinations independently of any other messages. As such it is a "best-effort" rather than a "reliable" delivery service. CLNS is not an Internet service, but provides capabilities in an OSI network environment similar to those provided by the Internet protocol suite. The service is specified in ISO/IEC 8348, the OSI Network Service Definition

<span class="mw-page-title-main">Optimized Link State Routing Protocol</span> IP routing protocol optimized for mobile ad hoc networks

The Optimized Link State Routing Protocol (OLSR) is an IP routing protocol optimized for mobile ad hoc networks, which can also be used on other wireless ad hoc networks. OLSR is a proactive link-state routing protocol, which uses hello and topology control (TC) messages to discover and then disseminate link state information throughout the mobile ad hoc network. Individual nodes use this topology information to compute next hop destinations for all nodes in the network using shortest hop forwarding paths.

The Open Systems Interconnection protocols are a family of information exchange standards developed jointly by the ISO and the ITU-T. The standardization process began in 1977.

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An IPv6 packet is the smallest message entity exchanged using Internet Protocol version 6 (IPv6). Packets consist of control information for addressing and routing and a payload of user data. The control information in IPv6 packets is subdivided into a mandatory fixed header and optional extension headers. The payload of an IPv6 packet is typically a datagram or segment of the higher-level transport layer protocol, but may be data for an internet layer or link layer instead.

In computer networking, the link layer is the lowest layer in the Internet protocol suite, the networking architecture of the Internet. The link layer is the group of methods and communications protocols confined to the link that a host is physically connected to. The link is the physical and logical network component used to interconnect hosts or nodes in the network and a link protocol is a suite of methods and standards that operate only between adjacent network nodes of a network segment.

Fabric Connect.

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 routeing 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. ↑ "Free-of-charge PDF copy of ISO/IEC 10589:2002". ISO website. International Organization for Standardization . Retrieved May 24, 2017.
  4. ↑ Gredler, Hannes; Goraiski, Walter (2005). The complete IS-IS routing protocol. Springer. p. 1. ISBN   1-85233-822-9.
  5. ↑ J. Hawkinson; T. Bates (March 1996). Guidelines for creation, selection, and registration of an Autonomous System (AS). Network Working Group. doi: 10.17487/RFC1930 . BCP 6. RFC 1930.Best Current Practice 6. Updated by RFC  6996 and 7300.
  6. ↑ 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.
  7. ↑ M. Shand; L. Ginsberg (February 2014). Reclassification of RFC 1142 to Historic. Internet Engineering Task Force (IETF). doi: 10.17487/RFC7142 . ISSN   2070-1721. RFC 7142.Informational. Obsoletes RFC  1142.
  8. ↑ 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.
  9. ↑ Unbehagen, Paul; Bragg, Nigel; Allan, David; Fedyk, Don; Ashwood-Smith, Peter J. (April 2012). Fedyk, D.; Ashwood-Smith, P. (eds.). "IS-IS Extensions Supporting IEEE 802.1aq Shortest Path Bridging". IETF. doi: 10.17487/RFC6329 .
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