Content Overview In recent years, the
Intermediate System-to-Intermediate System (IS-IS) routing
protocol has become increasingly popular, with widespread usage
among service providers. IS-IS enables very fast convergence
and is very scalable. It is also a very flexible protocol and
has been extended to incorporate leading edge features, such as
Multiprotocol Label Switching Traffic Engineering (MPLS/TE).The
features of IS-IS include the following: - Hierarchical
routing
- Classless behavior
- Rapid flooding of
new information
- Fast convergence
- Very
scalable
- Flexible timer tuning
- Protocol
independent
IS-IS is a dynamic, link-state,
intradomain, interior gateway protocol (IGP). It is part of the
Open Systems Interconnection (OSI) suite of protocols, and an
open-standard protocol based primarily on RFC 2328. The OSI
suite uses Connectionless Network Service (CLNS) to provide
connectionless delivery of data, and the Layer 3 protocol of
Connectionless Network Protocol (CLNP). CLNP is the solution
for “unreliable” (connectionless) delivery of data, similar to
IP. IS-IS uses CLNS addresses to identify the routers and to
build the link-state database (LSDB). The original IS-IS
operates in CLNS only. However, Integrated IS-IS supports IP
routing as well as CLNS. CLNS addresses are required for
configuring and troubleshooting IS-IS, even when it is used
only for IP. IS-IS supports different data-link environments,
such as Ethernet and Frame Relay. ISIS supports variable-length
subnet masking (VLSM) and rapid convergence as does Open
Shortest Path First (OSPF) and Enhanced Interior Gateway
Routing Protocol (EIGRP). Each protocol has advantages and
disadvantages, but this commonality makes all of them scalable
and appropriate for supporting today’s large-scale networks.
This module provides an overview of the IS-IS protocol and
basic configuration examples. Each of the important IS-IS
commands is explained and described in an example.
Content 4.1 IS-IS Fundamentals 4.1.1
Uses for IS-IS Routing The simplicity and stability of
IS-IS makes it robust in large internetworks. It is a proven
and extensible IP routing protocol that converges quickly and
supports VLSM. Originally developed by the Digital Equipment
Corporation in the late 1980s, IS-IS was later handed over to
the ISO for standardization. IS-IS was published as a public
standard in ISO 9542 and republished as RFC 995. Integrated
IS-IS (or dual IS-IS), which provides support for IP routing,
is specified in RFC 1195 and offers support for IP and OSI
protocols. Large ISPs chose IS-IS because of its scalability,
convergence, and stability. One reason IS-IS is popular with
ISP networks is because in 1990 the U.S. government required
ISPs to support the OSI protocols, although this requirement
was later dropped. IS-IS is comparable to, and in some cases
preferable to, OSPF. However, it is harder to find information
and expertise on IS-IS. Nevertheless, some of the largest
networks in the world continue to use IS-IS, which is a tribute
to its capabilities. Web Links Introduction to
Intermediate System-to-Intermediate System Protocol
http://www.cisco.com/warp/public/cc/pd/iosw/prodlit/
insys_wp.pdf#search=%22AFI%20IS-IS%22 IS-IS Support
Page
http://www.cisco.com/en/US/tech/tk365/tk381/
tsd_technology_support_sub-protocol_home.html
Content
4.1 IS-IS Fundamentals 4.1.2
IS-IS and OSPF IS-IS and OSPF are more similar than
dissimilar. Both routing protocols have the following
characteristics: - Open-standard link-state routing
protocols
- Support VLSM
- Support route
summarization between areas
- Similar mechanisms to
maintain the health of the LSDB (link-state advertisements
[LSAs], link-state aging timers, and LSDB
synchronization)
- Use the shortest path first (SPF)
algorithm, with similar update, decision, and flooding
processes
- Successful in the largest and most
demanding ISP networks
- Converge quickly after network
changes
The ISO developed IS-IS, and the Internet
Engineering Task Force (IETF) developed OSPF. Because most of
the development of these two routing protocols was done
concurrently, the development groups produced two protocols
that are very similar and each protocol benefited because of
the other. The practical differences between the two protocols
deal with perceived issues of resource usage and the ability to
customize their behavior. Most debates of the merits of these
protocols are colored by the fact that different groups with
different cultures developed the protocols. Digital Equipment
Corporation originally developed IS-IS for DECnet Phase V. In
1987, it was selected by ANSI to be the OSI IGP. At that time,
it was capable of routing CLNP only. ISO is an international
standards development process. According to Christian Huitema
in his book Routing in the Internet, groups within the
ISO and outside the U.S. did not approve of TCP/IP because of
its origin (it was also called the U.S. Department of Defense
protocol). From the perspective of ISO, IP development was
chaotic and imprecise, based on the famous maxim of “loose
consensus and running code.” From the perspective of the early
Internet engineers, the ISO process was slow, irritating, and
disenfranchising. In 1988, the U.S. National Science Foundation
Network (NSFnet) was created. The IGP used was based on an
early draft of IS-IS. The extensions to IS-IS for handling IP
were developed in 1988. OSPF development began during this time
and was loosely based on IS-IS. In 1989, OSPF version 1 (OSPF
v1) was published, and conflict ensued between the proponents
of IS-IS and OSPF. The IETF eventually supported both, although
it continued to favor OSPF. With the unofficial endorsement of
the IETF, OSPF eventually became the more popular protocol. By
the mid-1990s, large ISPs in need of an IGP selected IS-IS for
two reasons. First, IS-IS supported both CLNS and IP, which
solved two problems at once. Second, OSPF was seen as immature
at the time.
Content 4.1 IS-IS
Fundamentals 4.1.3 IS-IS Routing ISO
specifications refer to routers as intermediate systems (ISs).
Therefore, IS-IS is a protocol that allows routers to
communicate with other routers. The original OSI protocol suite
supported numerous standard protocols at the physical, data
link, network, transport, session, presentation, and
application layers. Figure displays the structure of the OSI
protocol suite and the OSI reference model. The OSI protocol
suite uses CLNS to provide connectionless delivery of data, and
the actual Layer 3 protocol is the CLNP. CLNP is the solution
for "unreliable" (connectionless) delivery of data,
similar to IP. IS-IS requires CLNS addresses to identify the
routers and to build the LSDB. IS-IS serves as an IGP for the
CLNS. Note
The original IS-IS implementation only
supported CNLS. A newer version called Integrated IS-IS
supports IP routing, CLNS routing, or a combination of the two.
This module focuses on Integrated IS-IS with support for IP.
Figure compares IP services and the equivalent OSI service.
Web Links OSI Protocols
http://www.cisco.com/univercd/cc/td/doc/cisintwk/
ito_doc/osi_prot.htm#xtocid12
Content 4.1 IS-IS
Fundamentals 4.1.4 IS-IS Features IS-IS is
the dynamic link-state routing protocol for the OSI protocol
stack. It distributes routing information for routing CLNP data
for the ISO CLNS environment. The operation of IS-IS is similar
to that of OSPF. IS-IS allows the routing domain to be
partitioned into areas. IS-IS routers establish adjacencies
using a hello protocol, and exchange link-state information
using link-state packets (LSPs) throughout an area to build the
LSDB. Each router then runs Dijkstra’s SPF algorithm against
its LSDB to select the best paths. A minimal amount of
information is communicated between areas, which reduces the