IS-IS

This article is about a computer network protocol. For topics with similar names, see Isis (disambiguation).

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Early research and development: 1960–4 (1960–4): RAND networking concepts developed 1962–4 (1962–4): ARPA networking ideas 1965 (1965): NPL network concepts conceived 1966 (1966): Merit Network founded 1967 (1967): ARPANET planning begins 1967 (1967): Symposium on Operating Systems Principles 1969 (1969): NPL followed by the ARPANET carry their first packets 1970 (1970): Network Information Center (NIC) 1971 (1971): Tymnet switched-circuit network 1972 (1972): Merit Network's packet-switched network operational 1972 (1972): Internet Assigned Numbers Authority (IANA) established 1973 (1973): CYCLADES network demonstrated 1973 (1973): PARC Universal Packet development begins 1974 (1974): Transmission Control Program specification published 1975 (1975): Telenet commercial packet-switched network 1976 (1976): X.25 protocol approved and deployed on public data networks 1978 (1978): Minitel introduced 1979 (1979): Internet Activities Board (IAB) 1980 (1980): USENET news using UUCP 1980 (1980): Ethernet standard introduced 1981 (1981): BITNET established Merging the networks and creating the Internet: 1981 (1981): Computer Science Network (CSNET) 1982 (1982): TCP/IP protocol suite formalized 1982 (1982): Simple Mail Transfer Protocol (SMTP) 1983 (1983): Domain Name System (DNS) 1983 (1983): MILNET split off from ARPANET 1984 (1984): OSI Reference Model released 1985 (1985): First .COM domain name registered 1986 (1986): NSFNET with 56 kbit/s links 1986 (1986): Internet Engineering Task Force (IETF) 1987 (1987): UUNET founded 1988 (1988): NSFNET upgraded to 1.5 Mbit/s (T1) 1988 (1988): Morris worm 1988 (1988): Complete Internet protocol suite 1989 (1989): Border Gateway Protocol (BGP) 1989 (1989): PSINet founded, allows commercial traffic 1989 (1989): Federal Internet Exchanges (FIX East|FIXes) 1990 (1990): GOSIP (without TCP/IP) 1990 (1990): ARPANET decommissioned 1990 (1990): Advanced Network and Services (ANS) 1990 (1990): UUNET/Alternet allows commercial traffic 1990 (1990): Archie search engine 1991 (1991): Wide area information server (WAIS) 1991 (1991): Gopher 1991 (1991): Commercial Internet eXchange (CIX) 1991 (1991): ANS CO+RE allows commercial traffic 1991 (1991): World Wide Web (WWW) 1992 (1992): NSFNET upgraded to 45 Mbit/s (T3) 1992 (1992): Internet Society (ISOC) established 1993 (1993): Classless Inter-Domain Routing (CIDR) 1993 (1993): InterNIC established 1993 (1993): AOL added USENET access 1993 (1993): Mosaic web browser released 1994 (1994): Full text web search engines 1994 (1994): North American Network Operators' Group (NANOG) established Commercialization, privatization, broader access leads to the modern Internet: 1995 (1995): New Internet architecture with commercial ISPs connected at NAPs 1995 (1995): NSFNET decommissioned 1995 (1995): GOSIP updated to allow TCP/IP 1995 (1995): very high-speed Backbone Network Service (vBNS) 1995 (1995): IPv6 proposed 1996 (1996): AOL changes pricing model from hourly to monthly 1998 (1998): Internet Corporation for Assigned Names and Numbers (ICANN) 1999 (1999): IEEE 802.11b wireless networking 1999 (1999): Internet2/Abilene Network 1999 (1999): vBNS+ allows broader access 2000 (2000): Dot-com bubble bursts 2001 (2001): New top-level domain names activated 2001 (2001): Code Red I, Code Red II, and Nimda worms 2003 (2003): UN World Summit on the Information Society (WSIS) phase I 2003 (2003): National LambdaRail founded 2004 (2004): UN Working Group on Internet Governance (WGIG) 2005 (2005): UN WSIS phase II 2006 (2006): First meeting of the Internet Governance Forum 2010 (2010): First internationalized country code top-level domains registered 2012 (2012): ICANN begins accepting applications for new generic top-level domain names 2013 (2013): Montevideo Statement on the Future of Internet Cooperation 2014 (2014): NetMundial international Internet governance proposal 2016 (2016): ICANN contract with U.S. Dept. of Commerce ends, IANA oversight passes to the global Internet community on October 1st Examples of Internet services: 1989 (1989): AOL dial-up service provider, email, instant messaging, and web browser 1990 (1990): IMDb Internet movie database 1994 (1994): Yahoo! web directory 1995 (1995): Amazon online retailer 1995 (1995): eBay online auction and shopping 1995 (1995): Craigslist classified advertisements 1995 (1995): AltaVista search engine 1996 (1996): Outlook (formerly Hotmail) free web-based e-mail 1996 (1996): RankDex search engine 1997 (1997): Google Search 1997 (1997): Babel Fish automatic translation 1998 (1998): Yahoo Groups (formerly Yahoo! 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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 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".

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.

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, but that RFC was later retracted and marked as historic 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.

IS-IS terminology

In IS-IS world there is slightly different terminology which comes from ISO wording. Below is the ISO terminology and its counterpart which is widely used in standards and related documentation.

• Intermediate system - router
• Designated intermediate system - designated router
• End system - host
• Circuit - link
• Adjacency - neighborship

Circuit types

Compared to OSPF, IS-IS has only two circuit types - broadcast (LAN) and P2P. Therefore, designs such as P2MP are unavailable in IS-IS.

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.: There are 3 IS-IS hello packets depending on the circuit type -

• LAN L1 (PDU type 15)
• LAN L2 (PDU type 16)
• P2P (PDU type 17). As can be seen, on point-to-point links there are no separate hello packets per level, while on broadcast links - there are.

Link State PDU (LSP)

This contains the actual route information. This LSP can contain many type–length–values (TLVs). LSP header is called LSP ID and consists of System ID, Pseudonode ID and Fragment ID. : In example of LSP ID 1921.6820.0002.02-01

• 1921.6820.0002 is System ID (that generated this LSP),
• 02 is Pseudonode ID,
• 01 is Fragment ID.

If Pseudonode ID is equal to zero, then this is a real intermediate system. Any value different from zero means that this LSP is generated by DIS (Pseudonode).

If LSP is too big, then it gets fragmented. In order to indicate this, Fragment ID is used. If Fragment ID is equal to zero, then no fragmentation has occurred.

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.

IS-IS addressing and NET

From regular TCP/IP world we are used to know that each Layer 3 interface (including loopback) has its own IPv4 or IPv6 address. The most important point is that loopback interface always stays up (unless deleted) compared to physical or logical interfaces.

Therefore, ISO choose a different approach - instead of assigning layer 3 address to each interface, single address is assigned to loopback interface, while other interfaces are considered as unnumbered. This single address is called NET (Network Entity Title).

On a single intermediate system there can be up to 3 NET addresses. This is useful during migration from one area to another.

NET consists of Area, System ID and NSEL. Area itself consists of AFI (Address Family Identifier) and Area ID.

Area can have variable length of 1 - 13 bytes, System ID is 6 bytes and NSEL - 1 byte.

Let's check on an example NET of 49.0100.1921.6821.1138.00. Here,

• 49 is AFI, and in case of 49 it means "private address space", similar to RFC1918 for IPv4.,
• 0100 is Area ID,
• 49.0100 is Area,
• 1921.6821.1138 is System ID,
• 00 is NSEL, which must be zero. If not zero, then no IS-IS adjacency is formed.

Hostname resolution

Let's imagine, that engineer examines L2 or L1 database, or needs to view a specific LSP. Each LSP has LSP ID, consisting of System ID, Pseudonode ID and Fragment ID. Because generally System ID is router's loopback address, remembering which loopback address to which router is not always convenient.

Similar thing happens with OSPF, when LSDB or specific LSA is checked - they are listed by Advertising router, which is actually an IP. In case of OSPF, in order to overcome difficulty of remembering router IPs or consulting with list, local DNS resolution can be configured. But as it might be understood, this is not very convenient and fast way, especially during troubleshooting ongoing issues.

IS-IS solves this problem in a very elegant manner - in each LSP there is TLV 137, which displays hostname of the router. By this means, all routers know hostnames of other routers in the level by examining LSPs. That's why when viewing LSP in L2 or L1 database, they are displayed by hostname, not System ID.

From the other hand, if needed, hostnames and their matching System IDs can be easily seen from IS-IS.

Areas and levels

In IS-IS there is conception of areas, but here it works differently from OSPF. First of all, in contrary to OSPF, in IS-IS area is terminated on router, not link.

In IS-IS, backbone area consists of contiguous Level 2 routers. Level 1 areas can be thought of stub areas in OSPF, where very limited reachability information is available. L2/L1 routers act like area border routers (ABRs) between L1 routers and L2 routers by keeping two databases - L1 database and L2 database.

Here is very important role of L2/L1 router - if it is connected to L2 router in another area, then it sets ATT (ATTached bit) in its L1 LSP. L1 routers which receive this LSP (with ATT bit) add default route to originator of this LSP. This is different from OSPF, where ABR generates default route to stub area routers and send it via LSA 3.

Another difference of router in L1 area in IS-IS from router in stub area in OSPF is that L1 router can inject external routes into area, which travels up to L2/L1 router. With that, it resembles NSSA area in OSPF (where you cannot have external routes from backbone area, but you can inject external routes to NSSA area which are then translated to regular LSA 5 external routes by NSSA ABR).

However, by default, external L1 routes are not injected from L1 to L2. This can be changed by policy on L2/L1 router, which accepts L1 external routes and originates them into L2.

In case of OSPF, if one day it is needed to inject external routes into OSPF domain from stub area, this can be done only by changing area type from stub to NSSA, which causes tearing down OSPF neighborship. In IS-IS, this happens hit-less.

Wide metrics

When IS-IS was initially introduced, TLVs for IS reachability (TLV 2) and IP reachability (TLVs 128 and 130) could have interface metric no more than 63 (6 bits) and total accumulated path metric of no more than 1023 (10 bits).

Obviously, nowadays with higher link speeds and more hops it would be challenging to stay within these limits.

Therefore, 2 new TLVs - TLV 22 for Extended IS reachability and TLV 135 for Extended IP reachability - were introduced. With this, now link metric can be up to 16.7 million (24 bits) and total accumulated path metric can be up to 4 billion (32 bits).

Older style metric is therefore called narrow metrics, while new style metric - wide metrics.

Wide metrics or narrow metrics can be set on level base.

Adjacency formation

Compared to OSPF, in IS-IS rules of adjacency formation are much simpler.

• L1 router cannot form any adjacency with L2 router under any conditions.
• L1 router can form L1 adjacency with other L1 router if their areas match.
• L1 router can form L1 adjacency with L2/L1 router if their areas match.
• L2 router can form L2 adjacency with other L2 router regardless of their areas (they don't need to match).
• L2 router can form L2 adjacency with other L2/L1 router regardless of their areas (they don't need to match).
• L2/L1 router can form only L2 adjacency with other L2/L1 router if their areas don't match.
• L2/L1 router can form both L2 and L1 adjacency with other L2/L1 router if their areas match.

Broadcast segments and designated intermediate system

On broadcast networks IS-IS is prone to issue, similar to OSPF, when all routers on the broadcast segment need to form adjacency and exchange LSPs. Therefore, number of LSPs increase in square.

In order to overcome this issue, on each LAN segment a designated intermediate system (DIS) is elected. The router with the highest priority and System ID wins. But, if a new router shows up and has better priority or System ID, then it is elected as a new DIS.

Elected DIS router is a pseudonode, which uses resources (including System ID) of one real router. DIS describes adjacency between routers in the broadcast segment in hub-spoke manner, where DIS is the hub while other routers (including router, promoted to DIS) are spokes.

Pseudonode ID in LSPs, originated from DIS, always have Pseudonode ID field different from zero.

All routers on the LAN segment form adjacency with only DIS and exchanges LSPs with it.

The function of DIS is to send periodic CSNPs on the LAN segment and reply to PSNPs from other routers. In case of DIS failure a new DIS will be elected in the segment. The role of DIS is not as critical as of DR in OSPF. That's why there is no backup DIS (BDIS) elected in IS-IS compared to BDR in OSPF.

Authentication

IS-IS supports both simple password and MD5 authentication types. In IS-IS, per-level or per-interface authentication is possible.

In addition, to protect from replay attack, IS-IS uses increasing Sequence number in IIH.

IPv6 support and multi-topology

Because IS-IS encapsulates its PDUs into Layer 2 frame, it does not depend on Layer 3 protocols, such as IPv4 or IPv6. This is different from OSPF, which uses IPv4. Therefore, when IPv6 came up, adding IPv6 support to OSPF would require re-writing the protocol. That is how OSPFv3 was created.

In case of IS-IS, TLV 232 for IPv6 interface address and TLV 236 for IPv6 reachability were added to support IPv6. And of course, IPv6 needs to be enabled on the interface.

In order to display supported Layer 3 protocols, also called NLPID (Network Layer Protocol ID), TLV 129 is used. Here, IPv4 has code of 0xCC, while IPv6 - 0x8E.

There might be an issue, if IPv4 and IPv6 topologies do not overlap. This could happen due to misconfiguration or intentionally (if some routers between do not support IPv6). For this situations, multi-topology support is added to IS-IS.

TLV 229 was added to display supported multi-topologies, such as IPv4 unicast and IPv6 unicast.

If multi-topology is enabled, IS-IS will calculate separate SPF tree for IPv4 and IPv6. This means twice the resource usage, but from the other side, this prevents traffic blackholing.

When multi-topology is enabled, then IS-IS will use TLV 222 for Multi-topology IS reachability, TLV 235 for Multi-topology IP reachability and TLV 236 for Multi-topology IPv6 reachability.

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. 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 protocols

• Fabric Shortest Path First (FSPF)
• Transparent Interconnection of Lots of Links (TRILL)

External links

• IS-IS standard (ISO/IEC 10589:2002, Second Edition) – free-of-charge PDF version
• OSPF and IS-IS: A Comparative Anatomy by Dave Katz, Juniper
• Collection of RFCs pertaining to IS-IS Archived 2013-06-02 at the Wayback Machine
• IS-IS and OSPF difference discussion (Vishwas Manral, Manav Bhatia and Yasuhiro Ohara)
• Google Quagga IS-IS implementation
• Sample isisd.conf file: used with Quagga