IS-IS Part 1: Single-Area Fundamentals, Adjacencies, and LSDB Synchronization

This guide covers Single-Area IS-IS (Area 49.0001) configuration across a 6-node Nokia SR OS topology in Containerlab, ISO Network Entity Title (NET) addressing, point-to-point adjacency formation, and Link-State Database (LSDB) synchronization.

SR OS Version: 25.7.R1
CLI Mode: MD-CLI (Model-Driven CLI)

[!NOTE] Base Interface Prerequisites: This lab builds upon the physical foundation established in OSPF Part 1: Topology Setup and Interface Addressing. Ensure that cards (iom-1), MDAs (me6-100gb-qsfp28), port breakouts (port 1/1/c1 connector breakout c1-100g), system loopbacks, and /30 point-to-point IP interfaces are operational prior to activating IS-IS.

Architecture and NET Addressing

Unlike OSPF where routing processes and interfaces are identified by 32-bit IPv4 Router IDs, IS-IS runs directly on the Data Link Layer (Layer 2) using ISO/OSI encapsulation. In IS-IS, the router itself is addressed using a Network Entity Title (NET).

A NET address is a specialized variation of an ISO NSAP (Network Service Access Point) address. It ranges from 8 to 20 bytes long (typically configured as 10 bytes) and is formatted as a series of hexadecimal characters separated by dots. In a NET, the trailing N-Selector (N-SEL) byte is always set to 00, designating the network layer process of the router itself rather than an upper-layer transport service.

NET Address Format

A Network Entity Title comprises three structured components:

Area ID (1-13 Bytes)      System ID (6 Bytes)       N-SEL (1 Byte)
+-----------------------+-------------------------+----------------+
|        49.0001        |   0101 . 0000 . 1001    |       00       |
+-----------------------+-------------------------+----------------+

Area ID (49.0001): Variable length from 1 to 13 bytes (typically 3 bytes in IP networks). AFI 49 defines an ISO locally assigned private address space, followed by Area 0001.
System ID (0101.0000.1001): A fixed 6-byte identifier uniquely identifying the router within the area. In IP networks, this is derived from the system loopback IP:
- IP 10.100.1.1 → pad each octet to 3 digits: 010.100.001.001 → group into 4-character blocks: 0101.0000.1001.
N-Selector (00): Identifies the network service entity (always 00 for a router node, completing the 10-byte total NET length: 3 bytes Area + 6 bytes System ID + 1 byte N-SEL).

In Nokia SR OS MD-CLI, the area address is configured under /configure router isis 0 area-address 49.0001. SR OS pairs the configured area with the router’s system interface IP to form the complete NET.

ISO NET Addressing Plan

R1 (CE1) --- R2 (PE1) === R3 (P1) / R4 (P2) === R5 (PE2) --- R6 (CE2)

The table below outlines the IS-IS ISO NET addressing plan derived from each node’s loopback interface:

Node	Role	System IPv4	System ID	ISO NET Address
R1	CE1	10.100.1.1/32	`0101.0000.1001`	`49.0001.0101.0000.1001.00`
R2	PE1	10.100.1.2/32	`0101.0000.1002`	`49.0001.0101.0000.1002.00`
R3	P1	10.100.1.3/32	`0101.0000.1003`	`49.0001.0101.0000.1003.00`
R4	P2	10.100.1.4/32	`0101.0000.1004`	`49.0001.0101.0000.1004.00`
R5	PE2	10.100.1.5/32	`0101.0000.1005`	`49.0001.0101.0000.1005.00`
R6	CE2	10.100.1.6/32	`0101.0000.1006`	`49.0001.0101.0000.1006.00`

Configuration

Enable IS-IS instance 0, set the area address to 49.0001, limit adjacencies to Level 1 (level-capability 1), set reference-bandwidth to 1000 gigabps, configure physical interfaces as point-to-point, and secure links with MD5 authentication.

Router R1 (CE1)

/configure router isis 0 admin-state enable
/configure router isis 0 area-address 49.0001
/configure router isis 0 level-capability 1
/configure router isis 0 reference-bandwidth 1000 gigabps
/configure router isis 0 authentication-type message-digest
/configure router isis 0 authentication-key "NokiaISIS2026"
/configure router isis 0 interface "system" passive true
/configure router isis 0 interface "system" interface-type point-to-point
/configure router isis 0 interface "toR2" interface-type point-to-point

commit

Router R2 (PE1)

/configure router isis 0 admin-state enable
/configure router isis 0 area-address 49.0001
/configure router isis 0 level-capability 1
/configure router isis 0 reference-bandwidth 1000 gigabps
/configure router isis 0 authentication-type message-digest
/configure router isis 0 authentication-key "NokiaISIS2026"
/configure router isis 0 interface "system" passive true
/configure router isis 0 interface "system" interface-type point-to-point
/configure router isis 0 interface "toR1" interface-type point-to-point
/configure router isis 0 interface "toR3" interface-type point-to-point
/configure router isis 0 interface "toR4" interface-type point-to-point

commit

Router R3 (P1)

/configure router isis 0 admin-state enable
/configure router isis 0 area-address 49.0001
/configure router isis 0 level-capability 1
/configure router isis 0 reference-bandwidth 1000 gigabps
/configure router isis 0 authentication-type message-digest
/configure router isis 0 authentication-key "NokiaISIS2026"
/configure router isis 0 interface "system" passive true
/configure router isis 0 interface "system" interface-type point-to-point
/configure router isis 0 interface "toR2" interface-type point-to-point
/configure router isis 0 interface "toR5" interface-type point-to-point

commit

Router R4 (P2)

/configure router isis 0 admin-state enable
/configure router isis 0 area-address 49.0001
/configure router isis 0 level-capability 1
/configure router isis 0 reference-bandwidth 1000 gigabps
/configure router isis 0 authentication-type message-digest
/configure router isis 0 authentication-key "NokiaISIS2026"
/configure router isis 0 interface "system" passive true
/configure router isis 0 interface "system" interface-type point-to-point
/configure router isis 0 interface "toR2" interface-type point-to-point
/configure router isis 0 interface "toR5" interface-type point-to-point

commit

Router R5 (PE2)

/configure router isis 0 admin-state enable
/configure router isis 0 area-address 49.0001
/configure router isis 0 level-capability 1
/configure router isis 0 reference-bandwidth 1000 gigabps
/configure router isis 0 authentication-type message-digest
/configure router isis 0 authentication-key "NokiaISIS2026"
/configure router isis 0 interface "system" passive true
/configure router isis 0 interface "system" interface-type point-to-point
/configure router isis 0 interface "toR3" interface-type point-to-point
/configure router isis 0 interface "toR4" interface-type point-to-point
/configure router isis 0 interface "toR6" interface-type point-to-point

commit

Router R6 (CE2)

/configure router isis 0 admin-state enable
/configure router isis 0 area-address 49.0001
/configure router isis 0 level-capability 1
/configure router isis 0 reference-bandwidth 1000 gigabps
/configure router isis 0 authentication-type message-digest
/configure router isis 0 authentication-key "NokiaISIS2026"
/configure router isis 0 interface "system" passive true
/configure router isis 0 interface "system" interface-type point-to-point
/configure router isis 0 interface "toR5" interface-type point-to-point

commit

Verification

1. Verifying IS-IS Interfaces

Verify that all interfaces show operational state Up and network type p2p:

A:admin@R2 (PE1)# /show router isis 0 interface

===============================================================================
Rtr Base ISIS Instance 0 Interfaces 
===============================================================================
Interface                        Level CircID  Oper      L1/L2 Metric     Type
                                               State                      
-------------------------------------------------------------------------------
system                           L1L2  1       Up        0/0              p2p
toR1                             L1L2  2       Up        10/10            p2p
toR3                             L1L2  3       Up        10/10            p2p
toR4                             L1L2  4       Up        10/10            p2p
-------------------------------------------------------------------------------
Interfaces : 4
===============================================================================

2. Checking Adjacency States

Verify that neighboring routers have formed bidirectional Level 1 adjacencies:

A:admin@R2 (PE1)# /show router isis 0 adjacency

===============================================================================
Rtr Base ISIS Instance 0 Adjacency 
===============================================================================
System ID                Usage State Hold Interface                     MT-ID
-------------------------------------------------------------------------------
R1 (CE1)                 L1    Up    19   toR1                          0
R3 (P1)                  L1    Up    24   toR3                          0
R4 (P2)                  L1    Up    24   toR4                          0
-------------------------------------------------------------------------------
Adjacencies : 3
===============================================================================

3. Point-to-Point Three-Way Adjacency (RFC 5303)

Standard ISO 10589 uses a 2-way handshake on point-to-point links, which cannot detect unidirectional link failures. RFC 5303 defines the Point-to-Point Three-Way Adjacency Option (TLV 240), transitioning through DOWN → INIT → UP.

We configure a dedicated memory log (log-id 50) to trace IS-IS packet exchanges:

edit-config global

/configure log log-id 50 description "IS-IS Packet Trace"
/configure log log-id 50 source debug
/configure log log-id 50 destination memory max-entries 200

commit

//debug router isis packet detail

Inspecting the debug log displays the received Point-to-Point Hello PDU carrying the RFC 5303 Three-Way Adjacency TLV:

RFC 5303 Three-Way Adjacency Handshake Debug Output

4. Link-State Database (LSDB) Inspection

Verify that all routers hold consistent copies of the Level 1 LSDB:

A:admin@R1 (CE1)# /show router isis 0 database

===============================================================================
Rtr Base ISIS Instance 0 Database 
===============================================================================
LSP ID                                  Sequence  Checksum Lifetime Attributes
-------------------------------------------------------------------------------

Displaying Level 1 database
-------------------------------------------------------------------------------
R1 (CE1).00-00                          0x3       0x33f    1153     L1
R2 (PE1).00-00                          0x5       0xc80f   1155     L1
R3 (P1).00-00                           0x4       0xd2fb   1155     L1
R4 (P2).00-00                           0x3       0x764f   1152     L1
R5 (PE2).00-00                          0x6       0xf14c   1154     L1
R6 (CE2).00-00                          0x5       0xe180   1149     L1
Level (1) LSP Count : 6

Displaying Level 2 database
-------------------------------------------------------------------------------
Level (2) LSP Count : 0
===============================================================================

LSP ID Breakdown: `R1 (CE1).00-00`

R1 (CE1): System name associated with System ID 0101.0000.1001.
.00: Pseudonode Identifier. Zero (00) indicates a real router LSP. Non-zero values indicate a broadcast LAN pseudonode.
-00: LSP Fragment Number. Used when an LSP exceeds MTU.

Inspecting the detailed LSP output displays TLVs carried inside:

A:admin@R1 (CE1)# /show router isis 0 database "R1 (CE1).00-00" detail

===============================================================================
Rtr Base ISIS Instance 0 Database (detail)
===============================================================================

Displaying Level 1 database
-------------------------------------------------------------------------------
LSP ID    : R1 (CE1).00-00                              Level     : L1 
Sequence  : 0x4                    Checksum  : 0xb070   Lifetime  : 1080
Version   : 1                      Pkt Type  : 18       Pkt Ver   : 1
Attributes: L1                     Max Area  : 3        Alloc Len : 1492
SYS ID    : 0101.0000.1001         SysID Len : 6        Used Len  : 166
 
TLVs : 
  Area Addresses:
    Area Address : (3) 49.0001
  Supp Protocols:
    Protocols     : IPv4
  IS-Hostname   : R1 (CE1)
  Router ID   :
    Router ID   : 10.100.1.1
  IS Neighbors  :
    Virtual Flag  : 0
    Default Metric: (I) 10
    Delay Metric  : (I) 0
    Expense Metric: (I) 0
    Error Metric  : (I) 0
    Neighbor      : R2 (PE1).00                         
  Internal Reach:
    Default Metric: (I) 10
    Delay Metric  : (I) 0
    Expense Metric: (I) 0
    Error Metric  : (I) 0
    IP Address    : 192.168.12.0
    IP Mask       : 255.255.255.252
    Default Metric: (I) 0
    Delay Metric  : (I) 0
    Expense Metric: (I) 0
    Error Metric  : (I) 0
    IP Address    : 10.100.1.1
    IP Mask       : 255.255.255.255
  I/F Addresses :
    I/F Address   : 10.100.1.1
    I/F Address   : 192.168.12.1
  TE IS Nbrs   :
    Nbr   : R2 (PE1).00                         
    Default Metric  : 10
    Sub TLV Len     : 12
    IF Addr   : 192.168.12.1
    Nbr IP    : 192.168.12.2
  TE IP Reach   :
    Default Metric  : 10
    Control Info:    , prefLen 30
    Prefix   : 192.168.12.0
    Default Metric  : 0
    Control Info:    , prefLen 32
    Prefix   : 10.100.1.1
  Authentication:
    Auth Type     : MD5(54) (16 bytes)
    Auth Info     : ed3c5bf81674ca6f87c2420b7ad66b34

Level (1) LSP Count : 1

Displaying Level 2 database
-------------------------------------------------------------------------------
Level (2) LSP Count : 0
===============================================================================

5. Point-to-Point vs. Broadcast (DIS Mechanics)

On broadcast interfaces, IS-IS elects a Designated Intermediate System (DIS) to represent the multi-access segment as a pseudonode.

To observe this, change the interface type on both ends of the link between R1 (CE1) and R2 (PE1) to broadcast (IS-IS requires matching interface types on both endpoints to establish an adjacency):

# On Router R1 (CE1)
/configure router isis 0 interface "toR2" interface-type broadcast
commit

# On Router R2 (PE1)
/configure router isis 0 interface "toR1" interface-type broadcast
commit

Inspecting the interface detail on R1 (CE1) and R2 (PE1) reveals the elected DIS:

A:admin@R1 (CE1)# /show router isis 0 interface "toR2" detail | match "Desg. IS"
  Desg. IS      : R1 (CE1)
  Desg. IS      : R1 (CE1)

A:admin@R2 (PE1)# /show router isis 0 interface "toR1" detail | match "Desg. IS"
  Desg. IS      : R1 (CE1)
  Desg. IS      : R2 (PE1)

The command yields two entries per interface because level-capability defaults to L1L2. In Level 1, both routers agree that R1 (CE1) is the DIS. With both routers configured with the default DIS priority of 64, the tie-breaker is decided by the highest Subnetwork Point of Attachment (SNPA/MAC address) on the link. In Level 2, no adjacency exists (Adjacencies : 0), causing each router to locally designate itself.

Once elected on a broadcast segment, the DIS generates a Pseudonode LSP representing the multi-access LAN. Verifying the Level 1 LSDB on R1 (CE1) confirms the database count increased from 6 to 7:

A:admin@R1 (CE1)# /show router isis 0 database

===============================================================================
Rtr Base ISIS Instance 0 Database 
===============================================================================
LSP ID                                  Sequence  Checksum Lifetime Attributes
-------------------------------------------------------------------------------

Displaying Level 1 database
-------------------------------------------------------------------------------
R1 (CE1).00-00                          0x7       0x1539   953      L1
R1 (CE1).02-00                          0x1       0x2bda   953      L1
R2 (PE1).00-00                          0xf       0xde52   952      L1
R3 (P1).00-00                           0xa       0x56bb   867      L1
R4 (P2).00-00                           0x8       0xb63e   871      L1
R5 (PE2).00-00                          0xc       0x560    866      L1
R6 (CE2).00-00                          0x9       0x806f   855      L1
Level (1) LSP Count : 7

Displaying Level 2 database
-------------------------------------------------------------------------------
Level (2) LSP Count : 0
===============================================================================

The new LSP R1 (CE1).02-00 is the Pseudonode LSP:

R1 (CE1): The DIS router that originated the pseudonode.
.02: The local Circuit ID of interface toR2 on R1 (CE1).
-00: The LSP fragment number.

Differences between IS-IS DIS and OSPF DR:

Attribute	IS-IS DIS	OSPF DR
Backup Router	None (no BDR)	BDR elected
Preemption	Preemptive	Non-preemptive
Default Priority	64 (0–127)	1 (0–255)
Pseudonode LSP	`R1 (CE1).02-00`	Network LSA (Type 2)

6. End-to-End Route Validation

Verify the route table on R1 (CE1) for all remote loopback and transit subnets:

A:admin@R1 (CE1)# /show router route-table protocol isis

===============================================================================
Route Table (Router: Base)
===============================================================================
Dest Prefix[Flags]                            Type    Proto     Age        Pref
      Next Hop[Interface Name]                                    Metric   
-------------------------------------------------------------------------------
10.100.1.2/32                                 Remote  ISIS      00h00m54s  15
       192.168.12.2                                                 10
10.100.1.3/32                                 Remote  ISIS      00h00m54s  15
       192.168.12.2                                                 20
10.100.1.4/32                                 Remote  ISIS      00h00m54s  15
       192.168.12.2                                                 20
10.100.1.5/32                                 Remote  ISIS      00h00m27s  15
       192.168.12.2                                                 30
10.100.1.6/32                                 Remote  ISIS      00h00m22s  15
       192.168.12.2                                                 40
192.168.23.0/30                               Remote  ISIS      00h00m54s  15
       192.168.12.2                                                 20
192.168.24.0/30                               Remote  ISIS      00h00m54s  15
       192.168.12.2                                                 20
192.168.35.0/30                               Remote  ISIS      00h00m54s  15
       192.168.12.2                                                 30
192.168.45.0/30                               Remote  ISIS      00h00m54s  15
       192.168.12.2                                                 30
192.168.56.0/30                               Remote  ISIS      00h00m27s  15
       192.168.12.2                                                 40
-------------------------------------------------------------------------------
No. of Routes: 10
===============================================================================

7. End-to-End Data Plane Verification

Execute an end-to-end ping from R1 (CE1) across the network topology to R6 (CE2)’s system loopback (10.100.1.6):

End-to-End IS-IS Data Plane Reachability

Architecture and NET Addressing#

NET Address Format#

ISO NET Addressing Plan#

Configuration#

Router R1 (CE1)#

Router R2 (PE1)#

Router R3 (P1)#

Router R4 (P2)#

Router R5 (PE2)#

Router R6 (CE2)#

Verification#

1. Verifying IS-IS Interfaces#

2. Checking Adjacency States#

3. Point-to-Point Three-Way Adjacency (RFC 5303)#

4. Link-State Database (LSDB) Inspection#

LSP ID Breakdown: R1 (CE1).00-00#

5. Point-to-Point vs. Broadcast (DIS Mechanics)#

6. End-to-End Route Validation#

7. End-to-End Data Plane Verification#