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Juniper · Junos OS

Junos IS-IS Commands: Adjacencies, Levels and Metrics

Quick answer #

For an MX IS-IS fault, begin with show isis adjacency detail and show isis interface detail. Read the level as well as the state. An Up adjacency at the wrong level does not establish that the intended topology and route exchange are correct.

Scope: Junos OS IS-IS on MX routing platforms. EX/QFX IS-IS availability and options vary; do not infer platform support from MX examples. No SRX IS-IS support claim is made.

Commands #

All examples are operational read-only queries:

Commands
Junos OS · Operational CLI

Read-only
show isis adjacency
Read-only
show isis adjacency detail
Read-only
show isis adjacency extensive
Read-only
show isis interface
Read-only
show isis interface detail
Read-only
show isis database
Read-only
show route protocol isis

Use the adjacency instance selector when needed, for example show isis adjacency instance CORE. Start with summary output; collect extensive database output only for a question that needs its additional detail.

Read adjacency information #

The level field distinguishes Level 1, Level 2 and combined operation. Detail output also provides transition history and advertised capabilities. Compare the expected peer system identifier with the observed one before treating a connected circuit as the correct circuit.

If the session repeatedly changes state, preserve the last transition and hold-time observations. Extensive output can identify reasons such as a level mismatch, interface failure or a BFD-triggered loss. These are starting points for a joint investigation with the neighbor, not permission to change several settings at once.

A metric detail that matters #

Juniper documents an important difference between summary and detailed interface output: when a configured per-level metric exceeds 63 without wide-metrics-only, the detail and extensive views can show 63 while the plain interface view shows the configured value. Do not rely on a single displayed number when auditing a mixed narrow/wide-metric deployment.

Check both views and the effective protocol configuration before proposing a metric change. This reference deliberately provides no copy-and-paste metric conversion, because changing an IGP metric can move production traffic.

Workflow #

Identify the intended circuit, level and peer. Confirm the interface carries the expected protocol family and that the configured IS-IS interface is correct. Compare both ends, then inspect whether the relevant link-state information is present in the database. Finally check the required destination route and next hop in the appropriate routing table.

Pitfalls #

An IS-IS adjacency is not proof that every IPv4 or IPv6 topology has the required reachability. Distinguish an IS-IS link-state PDU from an MPLS label-switched path; both are commonly abbreviated LSP. Check interface faults before blaming the protocol, and avoid restarting routing processes just to obtain a fresh-looking table.

Continue with #

Junos Interface Errors and Optical Power Commands covers physical evidence, Junos Route Lookup and VRF Routing Table Commands covers route selection and Junos BGP Troubleshooting: Neighbors and Advertised Routes checks services that rely on IGP next-hop reachability. Subnet tools help verify the addressing recorded in the circuit plan.

Sources

Documentation reviewed: 8 October 2026