isisEXEC

show isis neighbor

Displays IS-IS neighbor adjacencies and their state on Cisco IOS-XR.

Overview

The 'show isis neighbor' command is a fundamental troubleshooting tool for IS-IS on Cisco IOS-XR. It displays the current state of IS-IS adjacencies formed with neighboring routers. IS-IS (Intermediate System to Intermediate System) is a link-state routing protocol that uses a two-level hierarchy (Level-1 and Level-2) to scale within large networks. Adjacencies are formed when routers exchange Hello packets on an interface. This command shows each neighbor's system ID, interface, state (Up, Down, Init), holdtime, and type (L1, L2, L1L2). On IOS-XR, the output also includes IETF-NSF capability, which indicates support for Non-Stop Forwarding during process restarts. Use this command to verify that adjacencies are established after configuration, to monitor stability, and to diagnose issues like mismatched areas, authentication failures, or MTU problems. It is typically the first step in IS-IS troubleshooting, followed by 'show isis database' and 'show route isis'.

Syntax·EXEC

show isis neighbor [interface-type interface-instance] [detail] [system-id <system-id>] [level-1 | level-2] [private] [location <node-id>]

When to Use This Command

Verify IS-IS adjacency formation after configuring interfaces.
Troubleshoot missing or flapping IS-IS neighbors.
Check neighbor state and hold timer for stability.
Monitor IS-IS adjacency details for specific interfaces or levels.

Parameters

Parameter	Syntax	Required	Description
interface-type interface-instance	GigabitEthernet0/0/0/0	Optional	Filters output to neighbors on a specific interface. Useful for focusing on a single link.
detail	detail	Optional	Provides extended information including area addresses, IP addresses, uptime, and topology.
system-id	system-id <system-id>	Optional	Filters by neighbor system ID (e.g., 'R2'). Useful for checking a specific neighbor.
level-1 \| level-2	level-1	Optional	Displays only Level-1 or Level-2 adjacencies. Helpful in multi-level networks.
private	private	Optional	Shows private information for internal use.
location	location <node-id>	Optional	Specifies a specific line card or node in distributed systems.

Command Examples

Basic IS-IS neighbor display

show isis neighbor

IS-IS process: core
Neighbors for process core, router-id 10.1.1.1

System Id      Interface   SNPA           State Holdtime Type IETF-NSF
R2             GigabitEthernet0/0/0/0   *PtoP*         Up    27      L1   Capable
R3             GigabitEthernet0/0/0/1   *PtoP*         Up    25      L2   Capable

Shows two neighbors: R2 on Gi0/0/0/0 (L1 adjacency, holdtime 27s) and R3 on Gi0/0/0/1 (L2 adjacency, holdtime 25s). State is Up, IETF-NSF Capable indicates non-stop forwarding support.

Detailed IS-IS neighbor information

show isis neighbor detail

IS-IS process: core
Neighbors for process core, router-id 10.1.1.1

System Id: R2
  Interface: GigabitEthernet0/0/0/0, SNPA: *PtoP*
  State: Up, Holdtime: 27, Type: L1
  IETF-NSF: Capable
  Area Address(es): 49.0001
  IP Address(es): 10.1.12.2*
  Uptime: 00:12:34
  Adjacency Formed: 00:12:34
  N-flag: On
  Topology: IPv4 Unicast

Detailed output includes area address, IP address of neighbor, uptime, and topology. The asterisk (*) next to IP indicates the address used for adjacency.

Understanding the Output

The output shows IS-IS neighbors with key fields: System Id (hostname or system ID), Interface (local interface), SNPA (Subnetwork Point of Attachment, *PtoP* for point-to-point), State (Up, Down, Init), Holdtime (remaining seconds before adjacency times out), Type (L1, L2, L1L2), and IETF-NSF (Non-Stop Forwarding capability). A healthy adjacency shows State Up and Holdtime decreasing from the configured hold interval (typically 30s) then resetting. A Down state or holdtime of 0 indicates a problem. The detail view adds area addresses, IP addresses, uptime, and topology information, useful for verifying correct configuration and identifying mismatched areas or addresses.

Configuration Scenarios

Verifying IS-IS adjacency after configuration

Two routers (R1 and R2) connected via GigabitEthernet0/0/0/0, both configured for IS-IS Level-1.

Topology

R1 --- Gi0/0/0/0 --- R2

Steps

1.Configure IS-IS on both routers with matching network entity titles (NET) and area.
2.Enable IS-IS on the interface with 'router isis' and 'interface GigabitEthernet0/0/0/0'.
3.Use 'show isis neighbor' to verify adjacency state.

Configuration

! R1
router isis core
 net 49.0001.0100.0000.0001.00
 is-type level-1
!
interface GigabitEthernet0/0/0/0
 ip router isis core
!
! R2
router isis core
 net 49.0001.0100.0000.0002.00
 is-type level-1
!
interface GigabitEthernet0/0/0/0
 ip router isis core

Verify: Run 'show isis neighbor' on R1; expect R2 in Up state with holdtime around 27-30s.

Watch out: If state remains Init, check that both interfaces are not passive and that hello/padding is enabled by default.

Troubleshooting with This Command

When an IS-IS adjacency fails to come up, start with 'show isis neighbor'. If the neighbor is not listed, check interface status with 'show ip interface brief' and ensure IS-IS is enabled. If the neighbor appears in 'Init' state, it means Hello packets are received but not all parameters match. Common causes: mismatched area addresses (check with 'show isis neighbor detail'), authentication mismatch, or MTU issues (IS-IS Hellos are padded to full MTU by default). Use 'debug isis adj' to see Hello exchange details. If the neighbor flaps (Up/Down), check holdtime: if it counts down to 0 and resets, the adjacency is unstable. Also verify that the interface is not flapping and that the circuit type (L1/L2) matches. On IOS-XR, the 'show isis neighbor' command also shows IETF-NSF capability; if one side is NSF-capable and the other is not, it may cause issues during process restart. For persistent problems, compare 'show isis protocol' on both routers to ensure consistent configuration.

CCNA Exam Tips

Remember that 'show isis neighbor' is used to verify adjacency state; a neighbor in 'Init' state often indicates a hello/passive interface issue.

In CCNP exams, know that the holdtime should be consistent with the configured hello multiplier; a decreasing holdtime that never resets means adjacency is failing.

The 'detail' keyword reveals area addresses and IP addresses, which are critical for troubleshooting Level-1/Level-2 mismatches.

Common Mistakes

Confusing 'show isis neighbor' with 'show isis adjacency' (the latter is not a valid IOS-XR command).

Assuming 'Up' state means routing is working; verify that the neighbor's IP is reachable and LSDBs are synchronized.

Forgetting that on point-to-point links, SNPA shows '*PtoP*' instead of a MAC address; this is normal.

Platform Notes

On Cisco IOS-XR, 'show isis neighbor' is the primary command for adjacency verification. Unlike classic IOS, IOS-XR does not have 'show isis adjacency' – the neighbor command serves that purpose. The output format is similar but includes the IETF-NSF field. In IOS-XR, the 'detail' option provides area addresses and IP addresses, which are not shown in the basic output. For distributed systems (e.g., CRS), the 'location' parameter allows viewing neighbors on specific line cards. The command also supports XML output for automation. Compared to Juniper Junos, the equivalent is 'show isis adjacency', which provides similar information but with different formatting. In IOS-XR, the holdtime is displayed in seconds and decrements from the configured hello multiplier (default 3 x hello interval). The SNPA field shows '*PtoP*' for point-to-point links, while on broadcast links it shows the MAC address. Note that IOS-XR uses a process name (e.g., 'core') which appears in the output header.

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