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Copyright © 2019 Huawei Technologies Co., Ltd. All rights reserved.
IS-IS Principles and Configurations

Foreword
⚫ Intermediate System-to-Intermediate System (IS-IS) is a link-state based
IGP. It uses the Shortest Path First (SPF) algorithm to calculate routes. IS-IS
is a dynamic routing protocol initially designed by the International
Organization for Standardization (ISO) for its Connectionless Network
Protocol (CLNP).
⚫ To support IP routing, the Internet Engineering Task Force (IETF) extends
and modifies IS-IS in RFC 1195. This enables IS-IS to be applied to TCP/IP
and OSI environments. This type of IS-IS is called Integrated IS-IS. IS-IS has
been widely used in large ISP networks because of its simplicity and high
scalability.

Objectives
⚫ Upon completion of this section, you will be able to:
 Master IS-IS principles and configurations
 Be familiar with differences between IS-IS and OSPF

Contents
1. IS-IS Principles
2. Differences Between IS-IS and OSPF
3. IS-IS Configurations

⚫ Backbone network:
Flat area, fast convergence, and
high transmission volume
⚫ Campus network:
A variety of areas, changeable policies,
and fine-grained scheduling
Application
ER
CS
SW
ER
CS
SW SW
OSPF
CE
PE
CE
PE
PE
PE
PE
CE
CE
IS-IS+BGP

Origin
Application
layer
Presentation
layer
Session
layer
Transport
layer
Network
layer
Data link
layer
Physical
layer
OSI
Application
layer
Transport
layer
Network
layer
Data link
layer
Physical
layer
TCP/IP
ARP
TCP
IP
ES-IS
TP0-TP4
CLNP IS-IS
OSPF
Integrated
IS-IS
⚫ Integrated IS-IS characteristics:
 Supports CLNP and IP networks.
 Works at the data link layer.
⚫ OSPF characteristics:
 Supports only IP networks.
 Works at the IP layer.

⚫ Establish a neighbor relationship
Route Calculation
⚫ Perform SPF route calculation
Shortest path tree
This process is
similar to that in OSPF
H e l l o
⚫ Synchronize the LSDB
LSDB
LSP1
LSP2
-------
LSDB
LSP3
LSP4
-------

Address Structure
High Order DSP System ID SEL
IDI
AFI
Area ID (1-13B) 6B 1B
NSAP
IDP DSP
49.0 0 0 1 . 0000 . 0 0 0 0 . 0 0 0 1 . 0 0
A r e a I D Sy s t e m I D N - S E L
NET is a special type of NSAP address (SEL=00). When configuring
IS-IS on a router, you only need to consider the NET. For example:
TCP/IP
protocol stack
IP protocol IP address OSPF Area ID+Router
ID
OSI CLNP
protocol
NSAP
address
IS-IS NET identifier

⚫ IS-IS routers are classified into three types:
 Level-1 router: supports only Level-1 LSDB.
 Level-2 router: supports only Level-2 LSDB.
 Level-1-2 router: is the default router type and supports both Level-1 and Level-2 LSDBs.
Router Types
To establish a
Level-1 adjacency,
ensure that two
routers use the
same area ID.
Same area
Area 49.0001
Different
areas
Area 49.0001 Area 49.0002
×

⚫ IS-IS currently supports only P2P and broadcast network types.
Hello Packet
Broadcast multiple
access network type
P2P
Broadcast
Point-to-Point IIH
L1 LAN IIH
Mac:0180-c200-0014
L2 LAN IIH
Mac:0180-c200-0015
OR
Point-to-point network type

Neighbor Relationship Establishment
⚫ ISO10589 defines two-way handshake, while RFC3373 defines P2P three-way handshake.
point-to-point IIH
RTA RTB
SYS ID 0000.0000.0001 SYS ID 0000.0000.0002
point-to-point IIH
State: up
State: up
Two-way handshake
⚫ Establishing neighbor relationship of the multicast access (MA) network type
requires three-way handshake.
RTA RTB
SYS ID 0000.0000.0001 SYS ID 0000.0000.0002
LAN IIH (system ID: 0000.0000.0001 neighbor: null)
LAN IIH (system ID: 0000.0000.0002 neighbor: RTA)
LAN IIH (system ID: 0000.0000.0001 neighbor: RTB)
State: initialized
State: up
State: up
Three-way handshake
DIS election

Comparisons Between DIS and DR
Virtual router
(pseudo node)
Leve-1
Leve-1 Leve-1-2
DIS
Item IS-IS DIS OSPF DR
Election priority Devices with all priorities participate
in DIS election
The device with priority 0 does not
participate in DR election
Election waiting time Two Hello intervals 40s
Backup No backup BDR for backup
Adjacency All routers establish adjacencies DR others establish 2-way neighbor
relationships
Preemption Yes No
Function
Send CSNPs periodically to guarantee
LSDB synchronization on MA
networks.
Reduce LSA flooding
Leve-1 Leve-1-2
Leve-1

TLV
TYPE LENGTH VALUE
IS-IS packet
structure
 LSP PDU: is used to exchange link state
information.
 Real node LSP
 Pseudo node LSP (exists only in broadcast links)
 SNP PDU: is used to maintain and synchronize
the LSDB and carries summary information.
 CSNP (used to synchronize LSPs)
 PSNP (used to request and acknowledge LSPs)
Link State Information Transmission
IS-IS packets fall into two types: Level-1 and Level-2 packets. The destination MAC
addresses of all IS-IS packets in MA networks are multicast MAC addresses:
Level-1 address: 0180-C200-0014
Level-2 address: 0180-C200-0015

Link State Information Exchange
CSNP
A.00-00
①
PSNP
A.00-00
②
LSP
A.00-00
③

PSNP
A.00-00
④
Retransmission expires
LSP
A.00-00
⑤
PSNP
A.00-00
Retransmit PSNP
⑥
⚫ CSNPs are sent only once on a P2P
network. That is, a CSNP is sent
immediately after a neighbor
relationship is established.
③
PSNP
A.00-00
B.00-00
B.01-00
LSP
C.00-00
C.00-01
①
CSNP
A.00-00
B.00-00
B.01-00
C.00-00
C.00-01
②
LSP
A.00-00
B.00-00
B.01-00
④
⚫ CSNPs are multicast only by the DIS
on an MA network at the default
interval of 10s.
Pseudo node LSP
Fragment LSP
RTA RTB
P2P
RTA
RTC
RTB
DIS MA

Routing Algorithms
RTA
RTE
RTA RTB
RTC
RTD
RTE
⚫ IS-IS route cost calculation:
 The default interface cost is 10.
⚫ SPF calculation process:
 Complete LSDB synchronization
within a single area.
 Generate the network topology.
 Generate the shortest path tree,
using the local node as the root.

⚫ The IS-IS area boundary is a router, while the OSPF area boundary is a router
interface.
Network Hierarchy and Routing Domain
L2
L2
L2
L1/2
L2
L1/2
L1
L1/2
L1
L1
L1
L1/2
L1
L1
Area 49.0001
Area 49.0002
Area 49.0003
Area 49.0004
Area 49.0005
Backbone

L1 L1
Maintain L1 LSDB
L1/2
L1 neighbor
Area 49.0001
L1 neighbor
Backbone area
Maintain L2 LSDB
L2
Area 49.0002
L2 neighbor
L2 neighbor
L2
⚫ Area 49.0001 accesses Area 49.0002
 The Level-1-2 router RTA generates an
LSP with the ATT bit set to 1.
 A Level-1 router generates a default
route with the next hop pointing to the
Level-1-2 router after receiving the LSP
with the ATT bit 1.
⚫ Area 49.0002 accesses Area 49.0001
 The Level-1-2 router, RTA, adds specific routes to Area
49.0001 in the Level-2 LSDB.
 A Level-2 router calculates specific routes to Area
49.0001 through SPF calculation.
RTA
Inter-Area Routing

Differences Between IS-IS and OSPF
Use IS-IS or
OSPF based on
your service
requirements
Item IS-IS OSPF
Network types Less More
Cost style
Complicate
d
Simple
Area types Less More
Packet types Simple Various
Route
convergence
Faster Fast
Scalability High Medium
Routing load
capacity
Higher High

Terms
Abbreviation OSI Term IETF Term
IS Intermediate System Router
ES End System Host
DIS
Designated Intermediate
System
Designated Router in
OSPF
SysID System ID Router ID in OSPF
LSP Link State PDU LSA in OSPF
IIH IS-IS Hello PDU Hello packet in OSPF
PSNP
Partial Sequence Number
PDU
LSR or LSAck in OSPF
CSNP
Complete Sequence
Number PDU
DD packet in OSPF

⚫ In the figure, all routers in the customer network need to run IS-IS to ensure reachable routes on the network. All IS-IS
processes use the process ID 100. RTA is the DIS in Area 49.0001. The network between RTD and RTE must be P2P
network. RTE imports the direct route 192.168.X.X and requires RTA to access Area 49.0002 through the optimal path.
⚫ Perform the configuration correctly to meet the preceding customer requirements.
IS-IS Route Configuration Requirements
import1
RTB
RTC
RTD
Area 49.0001
Area 49.0002
Level-1 Level-2
RTA
192.168.1.0/24;
192.168.2.0/24;
192.168.3.0/24;
G0/0/0
G0/0/0
G0/0/1
G0/0/1
G0/0/0
G0/0/0
G0/0/0
S1/0/0
S1/0/0
RTE

IS-IS Route Configuration (1)
isis 100
network-entity
49.0001.0000.0000.0001.00
is-level level-1
#
interface GigabitEthernet0/0/0
isis enable 100
isis dis-priority 120 level-1
isis 100
network-entity
49.0001.0000.0000.0002.00
#
isis enable 100
isis 100
network-entity
49.0001.0000.0000.0003.00
#
isis enable 100
import1
RTB
RTC
RTD
Area 49.0001
Area 49.0002
Level-1 Level-2
RTA
192.168.1.0/24;
192.168.2.0/24;
192.168.3.0/24;
G0/0/0
G0/0/0
G0/0/1
G0/0/1
G0/0/0
G0/0/0
G0/0/0
S1/0/0
S1/0/0
RTE

import1
RTB
RTC
RTD
Area 49.0001
Area 49.0002
Level-1 Level-2
RTA
192.168.1.0/24;
192.168.2.0/24;
192.168.3.0/24;
G0/0/0
G0/0/0
G0/0/1
G0/0/1
G0/0/0
G0/0/0
G0/0/0
S1/0/0
S1/0/0
RTE
isis 100
network-entity
49.0002.0000.0000.0005.00
is-level level-2
#
isis enable 100
isis circuit-type P2P
isis 100
network-entity
49.0002.0000.0000.0004.00
is-level level-2
#
isis enable 100
isis circuit-type P2P

import1
RTB
RTC
RTD
Area 49.0001
Area 49.0002
Level-1 Level-2
RTA
192.168.1.0/24;
192.168.2.0/24;
192.168.3.0/24;
G0/0/0
G0/0/0
G0/0/1
G0/0/1
G0/0/0
G0/0/0
G0/0/0
S1/0/0
S1/0/0
RTE
isis 100
network-entity 49.0001.0000.0000.0002.00
import-route isis level-2 into level-1
#
isis enable 100
isis 100
network-entity 49.0001.0000.0000.0003.00
import-route isis level-2 into level-1
#
interface Serial 1/0/0
isis enable 100
isis 100
import-route direct

Quiz
1. How many IS-IS router types exist?
2. What is the function of PSNPs in neighbor interaction?
3. What are the advantages of IS-IS compared with OSPF?

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