- IS-IS is an intermediate system to intermediate system (IS-IS) routing protocol that uses link state packets (LSPs) to advertise routing information within an autonomous system. - It runs the shortest path first (SPF) algorithm on the link state database to determine the best paths to destinations. Areas and levels are used to structure large IS-IS networks hierarchically. - IS-IS is similar to OSPF in using areas, SPF algorithm, and link state flooding but differs in packet formats, database structures, backbone definitions, and neighbor establishment.

1. ISSUE
Fixed Network Curriculum
Development Section
ODA000009 IS-IS Routing
Protocol
1.0

2. Objectives
 IS-IS basic principle
 Upon completion of this course,you will be able to:

3. IS-IS Overview
 IS means intermediate system, and is the OSI language of the
routers.
 IS-IS is a kind of link status protocol
 IS-IS network can be divided into multiple sub-domains
 IS-IS can be expanded easily
 IS-IS is defined in 1992 ISO/IEC10589 Recommendation

4. Brief Introduction to the Link Status Protocol
RTB
RTA
RTD
RTC
RTE
LSP:RTB.00-00
LSP:RTA.00-00
LSP:RTC.00-00
LSP:RTD.00-00 LSP:RTE.00-00

5. SPF Algorithm Example
1
2
3
5
RTA RTB
RTD
RTC
RTD LSP
RTC LSP
RTB LSP
RTA LSP
LSDB
Topology structure
of the network
1
2
3
5
D
A
C
B
Link status
database of each
router
Weighted directed
graph obtained
from the link status
database

6. SPF Algorithm Example
1
2
3
D
A
C
B
1
2
3
D
A
C
B
1
2
3
D
A
C
B
1
2
3
D
A
C
B
Taking itself as the root node, each router calculates the
smallest generation tree

7. Basic Concepts of L1 and L2
Area 1
Level1 Area
Area 2
Level1 Area
Level2 Backbone
L2 L2
L1
L1
L1
L1
L1/L2
L1/L2

8. Introduction to ISIS Basic Work
Level 1
area
Area
49.0001
Level 1
area
Area
49.0002
Level 1
area
Area
49.0003
Level 2
Backbone
L1
L1/L2
L1 L1
L1/L2
L1/L2

9. Packet Classification
 Link Status Packet (LSP)
 IS-IS Hello Packet
 Complete Sequence Number Packet (CSNP)
 Part Sequence Number Packet (PSNP)

10. Process of Neighbor Relationship Establishment
between Two Routers
Router A Router B
Down
Init
Up
Up
Up
Up
Hello (DR=0.0.0.0)
Hello (DR=0.0.0.0)
Hello (DR is selected)
LSP
CSNP
PSNP
Down
Init
Up
Up
Up
Up

11. DIS and Pseudo node
Physical view of the
broadcast sub-network
Logical view of the
broadcast sub-network
The broadcast sub-
network is
externally
considered as a
pseudo node

12. The broadcast sub-network is externally considered
as a pseudonode
AFI Variable Length Area Address System ID NSEL
1 byte 1~12 bytes 6 bytes 1 byte
Area ID Sys ID NESL

13. Node Identification
 In IS-IS, each IS is identified by its system ID with a length of
six bytes. However, as the concept of pseudo node is
introduced in the broadcast sub-network and NBMA, to
identify the pseudo node, one byte is extended behind the
system ID:
 For a normal node (non-pseudo node), one byte of 0 is
extended behind the system ID
 For a pseudo node, one byte of interface ID on the DIS where
the pseudo node lies is added behind the DIS system ID.

14. Flood Algorithm
 Roles of the flood algorithm:
 To ensure the consistence of the link status databases between
respective ISs
 Flood principles:
 If the received information is newer than its own, update its own
database, spread it over the network and check if it is necessary
to run the SPF algorithm;
 If the received information is older than its own, update the
sender of the information;
 If the received information is the same as its own, send the
acknowledgement message.

15. IS-IS vs. OSPF
 Common Points:
 Both are internal gateway protocols (IGP) and used in the same
autonomous system (AS);
 Both are routing protocols based on the link status database,
adopting SPF algorithm to calculate the route;
 Both features fast convergence and no loop, applicable to large-
scale networks;
 Both adopt Hello protocol to form and maintain the neighbor
relationship

16. IS-IS vs. OSPF
 Common Points:
 Both use areas to form the two-layer hierarchical topology, both
with the concept of Area
 Both can aggregate the route between areas and support VLSM
and CIDR, suitable for large-scale networks
 Both elect DR on the broadcast network, and generate LSA
(LSP) to describe the whole network segment
 Both have the ability to verify the protocol packet.

17. IS-IS vs. OSPF
 Common Points:
 Both adopt interface cost to describe and calculate the route
overhead;
 The route information is updated in the trigger mode, instead of
periodically sending the whole route information;
 For the link status databases, there is an aging update
mechanism to ensure their correctness and completeness;
 Routers at the border areas of the network with poor processing
ability can perfectly run the protocols through reasonable
planning.

18. IS-IS vs. OSPF
 Differences:
 The origin and design purpose of the protocol standard are
different;
 The enveloping modes of the protocol packet and the supported
network protocols are different;
 The relationship between the routers and areas and the
organization modes of the link status database are different;
 Definitions of the backbone areas are different;

19. IS-IS vs. OSPF
 Differences:
 The complexity of Hello protocol and requirement for the
neighbor relationship establishment are different;
 Aging modes of LSP (LSA) in the database are different;
 DR election modes on the broadcast network are different;
 Support capabilities to different models of the network types are
different;
 Distinguishing abilities to the metric of the link layer and the
supported maximum metric are different.