isisEXEC

show isis database

Displays the contents of the IS-IS link-state database (LSDB) on Cisco IOS-XR, showing all LSPs and their details.

Overview

The 'show isis database' command is a fundamental diagnostic tool for IS-IS networks on Cisco IOS-XR. It displays the contents of the IS-IS Link State Database (LSDB), which is a collection of Link State Packets (LSPs) that each router in the IS-IS domain generates. The LSDB is the core of the IS-IS link-state routing protocol; each router uses the LSDB to compute the shortest path tree and populate the routing table. This command is essential for verifying network convergence, troubleshooting routing issues, and monitoring the health of the IS-IS domain.

On Cisco IOS-XR, the command provides output separated by IS-IS level (Level-1 and Level-2). Each LSP is identified by its LSPID, which consists of the system ID (typically the router ID) and a pseudonode identifier (00 for non-pseudonode LSPs, or a non-zero value for pseudonode LSPs on broadcast networks). The output includes the sequence number, checksum, holdtime (remaining lifetime), and flags (ATT, P, OL). The detail option reveals additional information such as neighbors (IS neighbors and IP extended neighbors) and advertised IP prefixes with metrics.

This command is used in various troubleshooting workflows. For example, if a router is missing routes, you can check if its LSP is present in the LSDB of other routers. If the LSP is missing, it may indicate a connectivity issue or that the router is not generating LSPs. If the LSP is present but with an overload bit set, the router is signaling that it should not be used for transit traffic. The command also helps in verifying that the LSDB is synchronized across all routers; mismatched sequence numbers or checksums can indicate corruption or a database inconsistency. In summary, 'show isis database' is a powerful command for deep-dive analysis of IS-IS operations.

Syntax·EXEC

show isis database [level-1 | level-2] [detail] [lsp-id] [verbose] [summary] [nsf] [local] [systemid <system-id>]

When to Use This Command

Verify that all routers in the IS-IS domain have consistent LSDBs to ensure proper route propagation.
Troubleshoot missing or incomplete routing information by checking for missing LSPs.
Monitor the LSDB size and LSP aging to detect potential instability or flapping.
Inspect specific LSP details to verify advertised prefixes, neighbors, and metrics.

Parameters

Parameter	Syntax	Required	Description
level-1 \| level-2	level-1 \| level-2	Optional	Filters the output to show only Level-1 or Level-2 LSPs. If omitted, both levels are displayed.
detail	detail	Optional	Displays detailed information for each LSP, including neighbors and advertised prefixes.
lsp-id	lsp-id	Optional	Specifies a particular LSP to display, identified by its LSPID (e.g., 0000.0000.0001.00-00).
verbose	verbose	Optional	Provides even more detailed output, including sub-TLV information.
summary	summary	Optional	Displays a summary of the LSDB, including count of LSPs per level.
nsf	nsf	Optional	Displays Non-Stop Forwarding (NSF) related information in the LSDB.
local	local	Optional	Displays only the locally generated LSPs.
systemid	systemid <system-id>	Optional	Filters LSPs by the specified system ID (e.g., 0000.0000.0001).

Command Examples

Basic LSDB Summary

show isis database

IS-IS Level-1 Link State Database:
LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime  ATT/P/OL
0000.0000.0001.00-00  0x0000000e   0x1234        1042          0/0/0
0000.0000.0002.00-00  0x0000001a   0x5678        987           0/0/0
0000.0000.0003.00-00  0x00000022   0x9abc        1123          1/0/0

IS-IS Level-2 Link State Database:
LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime  ATT/P/OL
0000.0000.0001.00-00  0x0000000f   0xdef0        1050          0/0/0
0000.0000.0002.00-00  0x0000001b   0x1111        1001          0/0/0
0000.0000.0004.00-00  0x00000005   0x2222        1190          0/0/0

Each line shows an LSP: LSPID (system ID + pseudonode), sequence number, checksum, holdtime (remaining lifetime in seconds), and ATT/P/OL flags (Attached, Partition, Overload). Healthy values: increasing sequence numbers, checksums matching across routers, holdtime near max (1200s). Problem: stale LSPs with low holdtime or overload bit set.

Detailed LSP Inspection

show isis database 0000.0000.0001.00-00 detail

IS-IS Level-1 LSP detail:
  LSPID                 : 0000.0000.0001.00-00
  Sequence number       : 0x0000000e
  Checksum              : 0x1234
  Lifetime              : 1042 seconds
  Attributes            : L1
  LSP MTU               : 1492
  Attached bits         : 0
  Partition repair      : 0
  Overload bit          : 0
  IS Neighbor(s):
    * 0000.0000.0002.00  Metric: 10
  IP Extended Neighbor(s):
    * 0000.0000.0003.00  Metric: 20
  IP Prefix(es):
    * 10.1.1.0/24        Metric: 10
    * 192.168.1.0/24     Metric: 20

Detail shows LSP attributes, neighbors (IS and IP extended), and advertised prefixes with metrics. Healthy: consistent metrics, correct neighbors. Problem: missing prefixes, high metrics, or overload bit set.

Understanding the Output

The 'show isis database' output is organized by IS-IS level (Level-1 and Level-2). Each LSP is identified by its LSPID, which includes the system ID (router ID) and a pseudonode number (00 for non-pseudonode). The sequence number increments with each LSP update; a rapidly increasing sequence number may indicate flapping. The checksum should match across all routers for the same LSP; mismatches indicate corruption. The holdtime is the remaining lifetime in seconds; values near 0 indicate the LSP is about to expire. The ATT/P/OL flags: ATT (Attached) indicates the router is connected to another area, P (Partition) indicates partition repair, OL (Overload) indicates the router is overloaded and should not be used for transit. In detail output, you see neighbors and prefixes with metrics. Healthy LSDBs have all routers present, consistent sequence numbers, and no overload bits set. Problems include missing LSPs, low holdtimes, or overload bits indicating issues.

Configuration Scenarios

Verifying LSDB Consistency After a Network Change

After adding a new router to an IS-IS area, you need to verify that its LSP is propagated to all other routers.

Topology

R1 --- R2 --- R3 (new router)

Steps

1.On R2, run 'show isis database' to see if R3's LSP appears.
2.Check the sequence number and holdtime to ensure it's being refreshed.
3.On R1, run the same command to confirm R3's LSP is present.

Configuration

! No configuration change needed; verification only.

Verify: R3's LSP should appear in the LSDB of R1 and R2 with a reasonable holdtime (e.g., > 1000 seconds).

Watch out: If R3's LSP does not appear, check IS-IS adjacency between R2 and R3 using 'show isis neighbors'.

Troubleshooting with This Command

When troubleshooting IS-IS routing issues on Cisco IOS-XR, 'show isis database' is a critical command. Start by checking the LSDB summary to see if all expected LSPs are present. If a router's LSP is missing, verify that the router is generating LSPs (check its IS-IS process) and that adjacencies are up. If the LSP is present but with a low holdtime, the router may be experiencing issues refreshing its LSP; check for CPU or memory problems. If the overload bit is set, the router is intentionally not forwarding transit traffic; this could be due to a manual configuration or a condition like high CPU. Compare sequence numbers across routers for the same LSP; if they differ, there may be a database corruption or a partitioning issue. Use the detail option to inspect advertised prefixes; missing prefixes could indicate a redistribution issue or a configuration error. Also, check the ATT bit on Level-1-2 routers; if it is not set when expected, default routes may not be injected into Level-1 areas. In summary, systematic examination of the LSDB helps isolate problems to specific routers or links.

CCNA Exam Tips

Remember that the overload bit (OL) is set during graceful shutdown or when the router is overloaded; it prevents transit traffic.

The ATT bit is set by Level-1-2 routers to indicate they can reach other areas; it is used for default route injection.

Sequence numbers should be monotonically increasing; a reset to 0x00000001 indicates a router reboot or LSP purge.

Common Mistakes

Confusing LSPID with system ID: LSPID includes a pseudonode number (e.g., .00-00).

Assuming all LSPs are in Level-2; many networks use Level-1 only or both levels.

Ignoring the holdtime: a low holdtime may indicate a network issue or that the LSP is about to expire.

Platform Notes

On Cisco IOS-XR, the 'show isis database' command is similar to Cisco IOS but with some differences. IOS-XR uses a more structured output and supports additional parameters like 'nsf' and 'local'. The output format is also slightly different; for example, IOS-XR displays the ATT/P/OL flags as a single field (e.g., 0/0/0) whereas IOS may show them separately. The holdtime in IOS-XR is displayed in seconds, while IOS may show it in minutes. Additionally, IOS-XR supports the 'verbose' option for even more detail. On other platforms like Juniper Junos, the equivalent command is 'show isis database' with similar output but different formatting. In terms of version differences, newer IOS-XR releases may include additional TLV support and enhanced detail. Always refer to the specific release documentation for exact syntax and output.

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