Basic IS-IS Configuration

Getting Started with IS-IS Configuration

IS-IS configuration involves enabling the routing process, configuring the NET address, and enabling IS-IS on interfaces. This section covers the fundamental configuration steps to get IS-IS operational.

Configuration Prerequisites

Before You Begin
  • Plan your area design
  • Choose NET addressing scheme
  • Determine router levels
  • Plan metric strategy
  • Consider authentication needs
Required Information
  • Area ID for each router
  • System ID for each router
  • NET address format
  • Interface IP addresses
  • Desired router levels

Basic Configuration Steps

1
Enable IS-IS Routing Process

Start the IS-IS routing process with an optional tag

router isis [tag]
2
Configure NET Address

Set the Network Entity Title (router identifier)

net 49.0001.1921.6800.0001.00
3
Enable on Interfaces

Enable IS-IS on each participating interface

ip router isis [tag]
4
Verify Configuration

Check IS-IS operation and adjacencies

show isis neighbors

Minimal Configuration Example

Basic IS-IS Setup

# Global configuration
router isis
 net 49.0001.1921.6800.0001.00
 is-type level-1-2
 metric-style wide

# Interface configuration
interface GigabitEthernet0/0
 ip address 192.168.1.1 255.255.255.0
 ip router isis
 no shutdown

interface GigabitEthernet0/1
 ip address 10.1.1.1 255.255.255.252
 ip router isis
 no shutdown

NET Address Planning

NET Address Structure

AFI
49
Private use
Area
0001
Area ID
System ID
1921.6800.0001
Router identifier
NSEL
00
Always 00

System ID Assignment Strategies

Strategy Format Example Use Case
IP-based Convert IP to hex 192.168.1.1 → 1921.6800.0101 Easy correlation with IP
Sequential Sequential numbering 0000.0000.0001, 0000.0000.0002 Simple assignment
Geographic Location-based NYC1.0000.0001 Large organizations
MAC-based MAC address Use last 6 bytes of MAC Unique identification

Level Configuration

IS-IS Router Types

IS-IS routers can be configured to operate at different levels depending on their role in the network hierarchy. Understanding and properly configuring router levels is crucial for optimal IS-IS operation.

Level-1 Only

Configuration:

is-type level-1

Behavior:

  • Intra-area routing only
  • Forms L1 adjacencies
  • Uses default route for inter-area
  • Maintains L1 LSDB only
Level-2 Only

Configuration:

is-type level-2-only

Behavior:

  • Inter-area routing only
  • Forms L2 adjacencies
  • Backbone router
  • Maintains L2 LSDB only
Level-1-2

Configuration:

is-type level-1-2

Behavior:

  • Both intra and inter-area
  • Forms both L1 and L2 adjacencies
  • Area border router
  • Maintains both LSDBs

Level Configuration Examples

Router Level Configuration

# Level-1 only router (intra-area)
router isis
 net 49.0001.1921.6800.0001.00
 is-type level-1

# Level-2 only router (backbone)
router isis
 net 49.0001.1921.6800.0002.00
 is-type level-2-only

# Level-1-2 router (area border router)
router isis
 net 49.0001.1921.6800.0003.00
 is-type level-1-2

# Per-interface level override
interface GigabitEthernet0/0
 isis circuit-type level-1
 
interface GigabitEthernet0/1
 isis circuit-type level-2-only

Level Selection Guidelines

Level-1 Routers

Use for routers that only need to route within an area and don't need to participate in inter-area routing.

  • Access layer routers
  • Stub area routers
  • End-of-branch routers
Level-2 Only Routers

Use for pure backbone routers that only handle inter-area traffic.

  • Core backbone routers
  • Inter-area transit routers
  • WAN aggregation routers
Level-1-2 Routers

Use for routers that need to participate in both intra-area and inter-area routing.

  • Area border routers
  • Distribution layer routers
  • Multi-area connection points

Adjacency Formation Rules

Router A Router B L1 Adjacency L2 Adjacency Requirement
Level-1 Level-1 Yes No Same area
Level-2 Level-2 No Yes Any area
Level-1-2 Level-1 Yes No Same area
Level-1-2 Level-2 No Yes Any area
Level-1-2 Level-1-2 Yes Yes L1: Same area, L2: Any area

Interface Configuration

IS-IS Interface Parameters

IS-IS behavior can be fine-tuned on a per-interface basis. Understanding these parameters allows for optimal network design and troubleshooting.

Essential Interface Commands

Basic Interface Configuration

# Enable IS-IS on interface
interface GigabitEthernet0/0
 ip router isis [tag]
 
# Set interface metric
 isis metric 100
 isis metric 150 level-2
 
# Set circuit type (level)
 isis circuit-type level-1
 isis circuit-type level-2-only
 isis circuit-type level-1-2
 
# Configure Hello parameters
 isis hello-interval 10
 isis hello-interval 15 level-2
 isis hello-multiplier 3
 
# Set DIS priority
 isis priority 64
 isis priority 100 level-2

Interface Parameter Details

Parameter Command Default Range Description
Metric isis metric 10 1-16777215 Link cost for routing calculations
Hello Interval isis hello-interval 10 seconds 1-65535 Frequency of Hello packets
Hello Multiplier isis hello-multiplier 3 3-1000 Number of missed Hellos before neighbor down
DIS Priority isis priority 64 0-127 Priority for DIS election (LAN only)
CSNP Interval isis csnp-interval 10 seconds 1-65535 CSNP transmission interval (DIS only)

Network Type Considerations

Broadcast Networks
Configuration:
interface GigabitEthernet0/0
 ip router isis
 isis priority 100
 isis hello-interval 10
 isis csnp-interval 10
Characteristics:
  • DIS election required
  • Pseudonode creation
  • CSNP synchronization
  • Priority-based DIS selection
Point-to-Point
Configuration:
interface Serial0/0
 ip router isis
 isis network point-to-point
 isis hello-interval 10
 isis metric 100
Characteristics:
  • No DIS election
  • Direct adjacency
  • PSNP acknowledgment
  • Simpler operation

Advanced Interface Features

Advanced Interface Configuration

# Passive interface (advertise but don't send Hellos)
interface Loopback0
 ip router isis
 isis passive

# Authentication on interface
interface GigabitEthernet0/0
 isis password cisco123
 isis password cisco456 level-2

# Mesh group configuration (reduces LSP flooding)
interface Serial0/0
 isis mesh-group 100

# BFD for fast failure detection
interface GigabitEthernet0/0
 isis bfd
 bfd interval 50 min_rx 50 multiplier 3

# Wide metrics only
interface GigabitEthernet0/0
 isis metric 1000 level-1
 isis metric 1500 level-2

Interface Troubleshooting Commands

Interface Verification Commands

# Show IS-IS interface details
show isis interface
show isis interface GigabitEthernet0/0
show isis interface detail

# Show interface-specific neighbors
show isis neighbors GigabitEthernet0/0

# Show interface database
show isis database detail

# Debug interface events
debug isis adj-packets
debug isis hello-packets
debug isis spf-events

Verification Commands

Essential Verification Commands

Proper verification is crucial for ensuring IS-IS is operating correctly. This section covers the most important show commands and their interpretation.

Core Verification Commands

Primary IS-IS Show Commands

# Show IS-IS process and configuration
show isis
show isis topology
show clns neighbors

# Show neighbors and adjacencies
show isis neighbors
show isis neighbors detail

# Show interfaces running IS-IS
show isis interface
show isis interface brief

# Show link state database
show isis database
show isis database detail
show isis database level-1
show isis database level-2

# Show routing table
show ip route isis
show clns route

Command Output Analysis

show isis neighbors

Router# show isis neighbors

System Id      Type Interface   IP Address      State Holdtime Circuit Id
Router2        L1   Gi0/0       192.168.1.2     UP    28       Router2.01
Router3        L2   Gi0/1       10.1.1.2        UP    26       Router3.02
Router4        L1L2 Se0/0       172.16.1.2      INIT  25       00
  • System Id: Neighbor's system identifier
  • Type: Adjacency level (L1, L2, L1L2)
  • Interface: Local interface
  • State: Adjacency state (UP, INIT, DOWN)
  • Holdtime: Time until neighbor considered down
  • Circuit Id: Circuit identifier for the adjacency

show isis database

Router# show isis database

IS-IS Level-1 Link State Database:
LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime  ATT/P/OL
Router1.00-00       * 0x00000003   0x8A52        1194          0/0/0
Router2.00-00         0x00000004   0x7B41        1156          0/0/0
Router2.01-00         0x00000001   0x6C30        1167          0/0/0

IS-IS Level-2 Link State Database:
LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime  ATT/P/OL
Router1.00-00       * 0x00000005   0x9B63        1198          0/0/0
Router3.00-00         0x00000002   0x8C52        1178          0/0/0
  • LSPID: LSP identifier (System ID + Pseudonode + LSP number)
  • * (asterisk): Indicates locally generated LSP
  • LSP Seq Num: Sequence number for LSP versioning
  • LSP Holdtime: Time until LSP expires
  • ATT/P/OL: Attached/Partition/Overload bits

Troubleshooting Checklist

Adjacency Issues
Verify interfaces are up and configured for IS-IS
Check area addresses match (for Level-1 adjacencies)
Verify compatible IS types and circuit types
Check authentication configuration matches
Verify MTU compatibility
Routing Issues
Check LSP database synchronization
Verify SPF calculation completed
Check metric configuration
Verify route leaking configuration
Check for overload bit setting

Debug Commands

Debug Command Warning

Debug commands can generate high CPU usage. Use with caution in production environments and disable when troubleshooting is complete.

Common Debug Commands

# Debug adjacency formation
debug isis adj-packets
debug isis hello-packets

# Debug LSP processing
debug isis lsp-gen
debug isis update-packets

# Debug SPF calculation
debug isis spf-events
debug isis spf-statistics

# Debug routing table updates
debug isis rib

# Turn off all ISIS debugging
undebug all isis
no debug isis
Previous: Chapter 4 - Routing and LSPs Next: Chapter 6 - Advanced IS-IS