IS-IS
Encyclopedia
Intermediate System To Intermediate System (IS-IS), 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 datagrams through a packet-switched network. The protocol was defined in ISO/IEC 10589:2002 as an international standard within the Open Systems Interconnection
(OSI) reference design. Though originally an ISO standard, the IETF republished the protocol as an Internet Standard
in RFC 1142. IS-IS has been called "the de facto standard for large service provider network backbones."
, 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
(RFC 1930).
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.
as part of DECnet
Phase V. It was standardized by the ISO
in 1992 as ISO 10589 for communication between network devices which are termed Intermediate Systems (as opposed to end systems or hosts) by the ISO. The purpose of IS-IS was to make possible the routing of datagrams using the ISO-developed OSI
protocol stack
called 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 (RFC 1195).
for computing the best path through the network. As a result, they are conceptually similar. Both support variable length subnet masks, can use multicast
to discover neighboring routers using hello packets, and can support authentication of routing updates.
While OSPF is natively built to route IP and is itself a Layer 3
protocol that runs on top of IP, IS-IS is natively an OSI network layer protocol (it is at the same layer as CLNS
). The widespread adoption of IP worldwide may have contributed to OSPF's popularity. IS-IS does not use IP to carry routing information messages.
IS-IS is neutral regarding the type of network addresses for which it can route. OSPF, on the other hand, was designed for IPv4. This allowed IS-IS to be easily used to support IPv6. To operate with IPv6 networks, the OSPF protocol was rewritten in OSPF v3 (as specificed in RFC 2740).
IS-IS routers build a topological representation of the network. This map indicates the subnets which each IS-IS router can reach, and the lowest-cost (shortest) path to a subnet is used to forward traffic.
IS-IS differs from OSPF in the way that "areas" are defined and routed between. IS-IS routers are designated as being: Level 1 (intra-area); Level 2 (inter area); or Level 1-2 (both). Level 2 routers are inter area routers that can only form relationships with other Level 2 routers. Routing information is exchanged between Level 1 routers and other Level 1 routers, and Level 2 routers only exchange information with other Level 2 routers. Level 1-2 routers exchange information with both levels and are used to connect the inter area routers with the intra area routers. In OSPF, areas are delineated on the interface such that an area border router (ABR) is actually in two or more areas at once, effectively creating the borders between areas inside the ABR, whereas in IS-IS area borders are in between routers, designated as Level 2 or Level 1-2. The result is that an IS-IS router is only ever a part of a single area. IS-IS also does not require Area 0 (Area Zero) to be the backbone area through which all inter-area traffic must pass. The logical view is that OSPF creates something of a spider web or star topology of many areas all attached directly to Area Zero and IS-IS by contrast creates a logical topology of a backbone of Level 2 routers with branches of Level 1-2 and Level 1 routers forming the individual areas.
IS-IS also differs from OSPF in the methods by which it reliably floods topology and topology change information through the network. However, the basic concepts are similar.
OSPF has a larger set of extensions and optional features. However IS-IS is less "chatty" and can scale to support larger networks. Given the same set of resources, IS-IS can support more routers in an area than OSPF. This has contributed to IS-IS as an ISP-scale protocol.
The TCP/IP implementation, known as "Integrated IS-IS" or "Dual IS-IS", is described in RFC 1195.
Routing protocol
A routing protocol is a protocol that specifies how routers communicate with each other, disseminating information that enables them to select routes between any two nodes on a computer network, the choice of the route being done by routing algorithms. Each router has a priori knowledge only of...
designed to move information efficiently within a computer network
Computer network
A computer network, often simply referred to as a network, is a collection of hardware components and computers interconnected by communication channels that allow sharing of resources and information....
, a group of physically connected computers or similar devices.
It accomplishes this by determining the best route
Routing
Routing is the process of selecting paths in a network along which to send network traffic. Routing is performed for many kinds of networks, including the telephone network , electronic data networks , and transportation networks...
for datagrams through a packet-switched network. The protocol was defined in ISO/IEC 10589:2002 as an international standard within the Open Systems Interconnection
Open Systems Interconnection
Open Systems Interconnection is an effort to standardize networking that was started in 1977 by the International Organization for Standardization , along with the ITU-T.-History:...
(OSI) reference design. Though originally an ISO standard, the IETF republished the protocol as an Internet Standard
Internet standard
In computer network engineering, an Internet Standard is a normative specification of a technology or methodology applicable to the Internet. Internet Standards are created and published by the Internet Engineering Task Force .-Overview:...
in RFC 1142. IS-IS has been called "the de facto standard for large service provider network backbones."
Description
IS-IS is an interior gateway protocolInterior gateway protocol
An interior gateway protocol is a routing protocol that is used to exchange routing information within an autonomous system ....
, designed for use within an administrative domain or network. This is in contrast to Exterior Gateway Protocols, primarily Border Gateway Protocol
Border Gateway Protocol
The Border Gateway Protocol is the protocol backing the core routing decisions on the Internet. It maintains a table of IP networks or 'prefixes' which designate network reachability among autonomous systems . It is described as a path vector protocol...
(BGP), which is used for routing between autonomous systems
Autonomous system (Internet)
Within the Internet, an Autonomous System is a collection of connected Internet Protocol routing prefixes under the control of one or more network operators that presents a common, clearly defined routing policy to the Internet....
(RFC 1930).
IS-IS is a link-state routing protocol
Link-state routing protocol
A link-state routing protocol is one of the two main classes of routing protocols used in packet switching networks for computer communications . Examples of link-state routing protocols include OSPF and IS-IS....
, 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
Open Shortest Path First
Open Shortest Path First is an adaptive routing protocol for Internet Protocol networks. It uses a link state routing algorithm and falls into the group of interior routing protocols, operating within a single autonomous system . It is defined as OSPF Version 2 in RFC 2328 for IPv4...
protocol, IS-IS uses Dijkstra's algorithm
Dijkstra's algorithm
Dijkstra's algorithm, conceived by Dutch computer scientist Edsger Dijkstra in 1956 and published in 1959, is a graph search algorithm that solves the single-source shortest path problem for a graph with nonnegative edge path costs, producing a shortest path tree...
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 Digital Equipment CorporationDigital Equipment Corporation
Digital Equipment Corporation was a major American company in the computer industry and a leading vendor of computer systems, software and peripherals from the 1960s to the 1990s...
as part of DECnet
DECnet
DECnet is a suite of network protocols created by Digital Equipment Corporation, originally released in 1975 in order to connect two PDP-11 minicomputers. It evolved into one of the first peer-to-peer network architectures, thus transforming DEC into a networking powerhouse in the 1980s...
Phase V. It was standardized by the ISO
International Organization for Standardization
The International Organization for Standardization , widely known as ISO, is an international standard-setting body composed of representatives from various national standards organizations. Founded on February 23, 1947, the organization promulgates worldwide proprietary, industrial and commercial...
in 1992 as ISO 10589 for communication between network devices which are termed Intermediate Systems (as opposed to end systems or hosts) by the ISO. The purpose of IS-IS was to make possible the routing of datagrams using the ISO-developed OSI
Open Systems Interconnection
Open Systems Interconnection is an effort to standardize networking that was started in 1977 by the International Organization for Standardization , along with the ITU-T.-History:...
protocol stack
Protocol stack
The protocol stack is an implementation of a computer networking protocol suite. The terms are often used interchangeably. Strictly speaking, the suite is the definition of the protocols, and the stack is the software implementation of them....
called CLNS
CLNS
CLNS is an abbreviation of Connectionless Network Service.It is an OSI Network Layer service that does not require a circuit to be established before data is transmitted...
.
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
Internet Protocol
The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
(IP), the Network Layer
Network Layer
The network layer is layer 3 of the seven-layer OSI model of computer networking.The network layer is responsible for packet forwarding including routing through intermediate routers, whereas the data link layer is responsible for media access control, flow control and error checking.The network...
protocol of the global Internet. This version of the IS-IS routing protocol was then called Integrated IS-IS (RFC 1195).
Comparison with OSPF
Both IS-IS and OSPF are link state protocols, and both use the same Dijkstra algorithmDijkstra's algorithm
Dijkstra's algorithm, conceived by Dutch computer scientist Edsger Dijkstra in 1956 and published in 1959, is a graph search algorithm that solves the single-source shortest path problem for a graph with nonnegative edge path costs, producing a shortest path tree...
for computing the best path through the network. As a result, they are conceptually similar. Both support variable length subnet masks, can use multicast
Multicast
In computer networking, multicast is the delivery of a message or information to a group of destination computers simultaneously in a single transmission from the source creating copies automatically in other network elements, such as routers, only when the topology of the network requires...
to discover neighboring routers using hello packets, and can support authentication of routing updates.
While OSPF is natively built to route IP and is itself a Layer 3
Network Layer
The network layer is layer 3 of the seven-layer OSI model of computer networking.The network layer is responsible for packet forwarding including routing through intermediate routers, whereas the data link layer is responsible for media access control, flow control and error checking.The network...
protocol that runs on top of IP, IS-IS is natively an OSI network layer protocol (it is at the same layer as CLNS
CLNS
CLNS is an abbreviation of Connectionless Network Service.It is an OSI Network Layer service that does not require a circuit to be established before data is transmitted...
). The widespread adoption of IP worldwide may have contributed to OSPF's popularity. IS-IS does not use IP to carry routing information messages.
IS-IS is neutral regarding the type of network addresses for which it can route. OSPF, on the other hand, was designed for IPv4. This allowed IS-IS to be easily used to support IPv6. To operate with IPv6 networks, the OSPF protocol was rewritten in OSPF v3 (as specificed in RFC 2740).
IS-IS routers build a topological representation of the network. This map indicates the subnets which each IS-IS router can reach, and the lowest-cost (shortest) path to a subnet is used to forward traffic.
IS-IS differs from OSPF in the way that "areas" are defined and routed between. IS-IS routers are designated as being: Level 1 (intra-area); Level 2 (inter area); or Level 1-2 (both). Level 2 routers are inter area routers that can only form relationships with other Level 2 routers. Routing information is exchanged between Level 1 routers and other Level 1 routers, and Level 2 routers only exchange information with other Level 2 routers. Level 1-2 routers exchange information with both levels and are used to connect the inter area routers with the intra area routers. In OSPF, areas are delineated on the interface such that an area border router (ABR) is actually in two or more areas at once, effectively creating the borders between areas inside the ABR, whereas in IS-IS area borders are in between routers, designated as Level 2 or Level 1-2. The result is that an IS-IS router is only ever a part of a single area. IS-IS also does not require Area 0 (Area Zero) to be the backbone area through which all inter-area traffic must pass. The logical view is that OSPF creates something of a spider web or star topology of many areas all attached directly to Area Zero and IS-IS by contrast creates a logical topology of a backbone of Level 2 routers with branches of Level 1-2 and Level 1 routers forming the individual areas.
IS-IS also differs from OSPF in the methods by which it reliably floods topology and topology change information through the network. However, the basic concepts are similar.
OSPF has a larger set of extensions and optional features. However IS-IS is less "chatty" and can scale to support larger networks. Given the same set of resources, IS-IS can support more routers in an area than OSPF. This has contributed to IS-IS as an ISP-scale protocol.
The TCP/IP implementation, known as "Integrated IS-IS" or "Dual IS-IS", is described in RFC 1195.
Related protocols
- Fabric Shortest Path First (FSPF)FSPFFabric Shortest Path First is a routing protocol used in Fibre Channel networks. It calculates the best path between switches, establishes routes across the fabric and calculates alternate routes in event of a failure or topology change...
- IEEE 802.1aqIEEE 802.1aq802.1aq Shortest Path Bridging or SPB in computer networking is a technology that greatly simplifies the creation and configuration of carrier, enterprise, and cloud networks which virtually eliminates human error, while enabling multipath routing...
- Shortest Path Bridging (SPB) - Transparent Interconnect of Lots of Links (TRILL)TrillTrill may refer to:* Trill , a type of musical ornament* Trill consonant, a type of sound used in some languages*Trill, a type of bird food-Fiction:* Trill , two symbiotic races of aliens in the fictional Star Trek universe...
External links
- ISO/IEC 10589:2002 Second Edition
- OSPF and IS-IS: A Comparative Anatomy by Dave Katz, Juniper
- Collection of RFCs pertaining to IS-IS
- Configuring integrated IS-IS on Cisco Routers
- IS-IS and OSPF difference discussion (Vishwas Manral, Manav Bhatia and Yasuhiro Ohara)
- Sample isisd.conf file: used with Quagga