Introduction to Integrated IS-IS 255
Foundation Topics
Introduction to Integrated IS-IS
IS-IS is an Interior Gateway Protocol (IGP) developed in the 1980s by DEC and submitted to the
International Organization for Standardization (ISO) as the routing protocol for Open System
Interconnection (OSI). The creation of IS-IS was part of an attempt to produce an international
standard protocol suite that could compete with TCP/IP.
IS-IS was developed to provide
A nonproprietary protocol
A large and hierarchical addressing scheme
An efficient protocol, allowing for fast, accurate convergence and low network overhead
The United States mandated that every system operated by the government be capable of
running OSI; IS-IS was extended to pass IP routes to aid in this transition to OSI. In the end,
however, the Internet, built on TCP/IP, prevailed as the de facto alternative to an international
When IS-IS is used to support IP, it is properly referred to as Integrated IS-IS. This book
simplifies that to “IS-IS” in many cases because any mention of IS-IS herein refers to its use as
an IP routing protocol.
In recent years there has been renewed interest in IS-IS. This new interest is because IS-IS is
protocol independent, scales well, and has the capacity to define type of service (ToS) routing
(but ToS routing is not supported by IOS). IS-IS has been dusted off as a routing protocol for IPv6
or for use with MPLS, but this interest has yet to extend to widespread adoption.
Understanding the Lingo
In OSI-speak, a router is referred to as an intermediate-system (IS) and a PC is called an
end-system (ES). Thus, IS-IS is a router-to-router protocol.
The network layer protocol in OSI is called the Connectionless Network Protocol (CLNP) and is
used for the Connectionless Network Service (CLNS). IS-IS implementers need to understand
only one detail of OSI: CLNS addressing. Because IS-IS started as an OSI routing protocol it uses
a CLNS address as a router ID and to group the routers into areas. No actual CLNS traffic needs
to be passed; the address is only used administratively.
256 Chapter 9: Fundamentals of the Integrated IS-IS Protocol
OSI supports four routing levels, with IS-IS used for the middle two:
Level 0 routing is used to find end-systems and uses end system-to-intermediate system (ES-IS).
Level 1 routing is used to exchange routes within an area.
Level 2 is the backbone between areas.
Level 3 routing is used between autonomous systems and is the province of the Interdomain
Routing Protocol (IDRP).
IS-IS is responsible for Levels 1 and 2. Routers may be in a Level 1 area, in the Level 2 backbone, or
both. Level 1-2 routers connect areas to the backbone. Each level uses Dijkstra’s SPF algorithm to
select paths and each level converges quickly.
Network Layer Protocols Used in Integrated IS-IS
The IS-IS protocol data unit (PDU) is encapsulated directly into the data-link frame. All IS-IS
packets share the same eight-octet header. After the fixed header, there are a number of optional
variable-length fields that contain specific routing-related information. These variable-length
fields are called TLV.
Each IS-IS PDU begins with a standard header. Next are the specific fields and the variable-length
fields. The following sections describe the three IS-IS packet types: Hellos, LSPs, and SNPs.
Adjacencies are formed by exchanging hellos—there are three different types of hellos.
End system hellos (ESH)ISO end systems use ESHs to attach to routers. IP end systems
do not speak ESH, so IS-IS just attaches the local subnet.
Intermediate system hellos (ISH)Routers use ISHs to announce themselves back to
the ES.
Intermediate-to-intermediate hellos (IIH)Routers use IIHs to meet IS neighbors. IIH is
transmitted separately at Level 1 and Level 2.
Because the point-to-point and broadcast media work differently, adjacencies are formed differently.
A point-to-point network has only one other router with which to communicate. Broadcast networks
are multiaccess networks and can have a mixture of both Level 1 and Level 2 routers. For this reason,
the broadcast or LAN network has two Hello formats: the Level 1 format and the Level 2 format.
Hellos for broadcast media are referred to as LAN Hellos. Point-to-point Hello packets are used over
point-to-point links. LAN Hello packets are used over broadcast links.

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