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Chapter 3

ECE495 Chapter Notes - Chapter 3: Connectionless-Mode Network Service, Private Network, Routing Protocol


Department
Electrical and Computer Engineering
Course Code
ECE495
Professor
mint
Chapter
3

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IS-IS v1.32 – Aaron Balchunas
* * *
All original material copyright © 2007 by
Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
1
- IS-IS -
IS-IS Fundamentals
IS-IS (Intermediate System -to- Intermediate System) is a standardized
link-state protocol that was developed to be the definitive routing protocol
for the OSI (Open Systems Interconnect) Model, which was developed by
ISO (International Standards Organization). IS-IS shares many
similarities to OSPF. Though it was designed as an interior gateway protocol
(IGP), IS-IS is predominantly used by ISPs, due to its scalability.
IS-IS adheres to the following Link State characteristics:
IS-IS allows for a hierarchical network design using Areas.
IS-IS will form neighbor relationships with adjacent routers of the
same IS-IS type.
Instead of advertising the distance to connected networks, IS-IS
advertises the status of directly connected “links” in the form of
Link-State Packets (LSPs). IS-IS will only send out updates when
there is a change to one of its links, and will only send the change in
the update.
IS-IS uses the Dijkstra Shortest Path First algorithm to determine
the shortest path.
IS-IS is a classless protocol, and thus supports VLSMs.
Other characteristics of IS-IS include:
IS-IS was originally developed to route the ISO address space, and
thus is not limited to IP routing.
IS-IS routes have an administrative distance is 115.
IS-IS uses an arbitrary cost for its metric. IS-IS additionally has three
optional metrics: delay, expense, and error. Cisco does not support
these optional metrics.
IS-IS has no hop-count limit.
The IS-IS process builds and maintains three separate tables:
A neighbor table – contains a list of all neighboring routers.
A topology table – contains a list of all possible routes to all known
networks within an area.
A routing table – contains the best route for each known network.
IS-IS is only available on enterprise versions of the Cisco IOS.

Only pages 1-3 are available for preview. Some parts have been intentionally blurred.

IS-IS v1.32 – Aaron Balchunas
* * *
All original material copyright © 2007 by
Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
2
IS-IS Protocols and Addressing
IS-IS consists of three sub-protocols that work in tandem to achieve end-to-
end routing which ISO defined as Connectionless Network Service
(CLNS):
CLNP (Connectionless Network Protocol) serves as the Layer-3
protocol for IS-IS (and was developed by ISO).
ES-IS (End System -to- Intermediate System) – used to route
between hosts and routers.
IS-IS (Intermediate System -to- Intermediate System) – used to
route between routers.
IS-IS was originally developed to route ISO CLNP addresses (outlined in
RFC 1142). However, CLNP addressing never became prominently used.
Thus, IS-IS was modified to additionally support IP routing, and became
Integrated (or Dual) IS-IS (outlined in RFC 1195).
The IS-IS CLNP address is hexadecimal and of variable length, and can
range from 64 to 160 bits in length. The CLNP address contains three
“sections,” including:
Area field – (variable length)
ID field – (from 8 to 64 bits, though usually 48 bits)
Selector (SEL) field - (8 bits)
Thus, the CLNP address identifies the “Area” in which a device is located,
the actual host “ID,” and the destination application on that host, in the form
of the “SEL” field. The CNLP address is logically segmented even further,
as demonstrated by the following table:
IDP
DSP
AFI IDI HO-DSP System-ID NSEL
Area Field ID Field
SEL Field
Observe the top row of the above figure. The ISO CLNP address provides
granular control by separating internal and external routing information:
The IDP (Initial Domain Part) portion of the address identifies
the Autonomous System of the device (and is used to route to or
between Autonomous Systems)
The DSP (Domain Specific Part) portion of the address is used to
route within the autonomous system.

Only pages 1-3 are available for preview. Some parts have been intentionally blurred.

IS-IS v1.32 – Aaron Balchunas
* * *
All original material copyright © 2007 by
Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
3
IS-IS Protocols and Addressing (continued)
IDP
DSP
AFI IDI HO-DSP System-ID NSEL
Area Field ID Field
SEL Field
The IDP portion of the address is separated into two “sections,” including:
AFI (Authority and Format Identifier) – specifies the
organization authorized to assign addresses, and the format and
length of the rest of the CLNP address. The AFI is always 8 bits.
IDI (Initial Domain Identifier) – identifies the “sub-
organization” under the parent AFI organization. The length of the
IDI is dependent on the chosen AFI.
An AFI of 0x49 indicates a private CLNP address, which cannot be routed
globally (the equivalent of an IPv4 private address). An AFI of 0x47 is
commonly used for global IS-IS networks, with the IDI section identifying
specific organizations.
The AFI plus the IDI essentially identify the autonomous system of the
address. However, this is not the equivalent of a BGP AS number, nor is it
compatible with BGP as an exterior routing protocol.
The DSP portion of the address is separated into three “sections,” including:
HO-DSP (High Order DSP) – identifies the area within an
autonomous system
System ID – identifies the specific host. Usually 48 bits (or 6 octets)
in length, to accommodate MAC addresses
NSEL – identifies the destination upper layer protocol of the host
(always 8 bits)
Two “types” of CLNP addresses are defined:
NET address – does not contain upper-layer information (in other
words, the SEL field is always set to 0x00)
NSAP address – the “full” CLNP address, with populated Area, ID,
and SEL fields.
Please note: A NET address is simply an NSAP address with a zero value in
the SEL field.
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