Early research and development: Merging the networks and creating the Internet: Commercialization, privatization, broader access leads to the modern Internet: Examples of Internet services: 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.
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] IS-IS is an interior gateway protocol, designed for use within an administrative domain or network.
[5] IS-IS is a link-state routing protocol, operating by reliably flooding link state information throughout a network of routers.
Packets (datagrams) are then forwarded, based on the computed ideal path, through the network to the destination.
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 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).
Similar problem is observed in 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.
IS-IS solves this problem in a very elegant manner - in each LSP there is TLV 137, which displays hostname of the originating router.
On the other hand, if needed, hostnames and their matching System IDs can be easily seen from IS-IS which keeps their list.
This is different from OSPF, where ABR generates default route to stub area routers and sends it via LSA 3.
IS-IS LSPs contain specific information, encoded to Attribute block in LSP header, which is 8 bits long.
Obviously, nowadays with higher link speeds and more hops in the path it would be challenging to stay within these limits.
Compared to OSPF, in IS-IS rules and conditions of adjacency formation are much simpler and mainly depend on the router level.
In order to overcome this issue, on each LAN segment a designated intermediate system (DIS) is elected.
The function of DIS is to send periodic CSNPs on the LAN segment and reply to PSNPs from other routers.
In addition, to protect from replay attack, IS-IS uses increasing Sequence number in IIH.
But there can be situations, when IS-IS router has exactly the same prefix in different level databases, or external and internal.
Technically, it is possible to lower hello and hold time intervals to detect failure faster, but this can put unnecessary stress to router.
SPB allows for shortest-path forwarding in an Ethernet mesh network context utilizing multiple equal cost paths.