Configuring IS-IS Protocol

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Configuring IS
-
IS Protocol


MAR 21, 2003

b
y
Catherine Paquet
,
Diane Teare

Article is provided courtesy of
Cisco Press


Overview of
IS
-
IS Routing

IS
-
IS

IS
-
IS is a dynamic link
-
state routing protocol used in an ISO CLNS environment
for routing CLNP. Routers
usually operate as ISs and can exchange reachability
information with other ISs using the IS
-
IS protocol. As an IS, a Cisco router can be a
Level 1 router, a Level 2 router, or a Level 1

2 router. In the latter case, the router can
advertise itself at Leve
l 1 as an exit point from the area. Integrated IS
-
IS allows the IS
-
IS
protocol to propagate routing information for other protocols as well as

or instead of

CLNS. Specifically, IS
-
IS can route CLNS, IP, or both (this latter is called
dual

mode).


Who Uses
IS
-
IS?

IS
-
IS is popular among telcos and large ISPs. This popularity finds its roots with ISPs
that were around at the beginning of the Internet and chose IS
-
IS over OSPF for their
IGP. It is believe that at that time, IS
-
IS had fewer technical limitations

than OSPF as an
IGP. Those ISPs have since become today's tier 1 carriers, so any appliance targeting tier
1 carriers must offer IS
-
IS.

The OSI protocol suite supports numerous standard protocols at the physical, data link,
network, transport, session, pr
esentation, and application layers.

OSI network layer addressing is implemented by using two types of hierarchical
addresses: network service access point (NSAP) addresses and a specific subset of
NSAPs cal
-

led network
-
entity titles (NETs). An NSAP is a c
onceptual point on the
boundary between the network and transport layers. The NSAP is the location at which
OSI network services are provided to the transport layer. Each transport layer entity is
assigned a single NSAP, which is individually addressed in
an OSI internetwork using
NSAP addresses.

The OSI protocol suite specifies two routing protocols at the network layer: End System
-
to
-
Intermediate System (ES
-
IS) and IS
-
IS. In addition, the OSI suite implements two
types of network services: connectionless

service and connection
-
oriented service.


Integrated IS
-
IS

As previously mentioned, IS
-
IS is the dynamic link
-
state routing protocol for the OSI
protocol stack. As such, it distributes routing information for routing CLNP data for the
ISO CLNS environment
.

Integrated IS
-
IS is an implementation of the IS
-
IS protocol for routing multiple network
protocols; it is an extended version of IS
-
IS for mixed ISO CLNS and IP environments.
Integrated IS
-
IS tags CLNP routes with information regarding IP networks and su
bnets. It
can be used purely for IP routing, purely for ISO routing, or for a combination of the two.

Integrated IS
-
IS provides an alternative to OSPF in an IP environment, mixing ISO
CLNS and IP routing in one protocol.

IS
-
IS has
level 1, level 2, and le
vel 1

2 r
outers
.