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NX-OS Troubleshooting Tools

Chapter Description

In this sample chapter from Troubleshooting Cisco Nexus Switches and NX-OS, you will review the various tools available on the Nexus platform that can help in troubleshooting and day-to-day operation.


Network issues are hard to troubleshoot and investigate if the device contains no information. For instance, if an OSPF adjacency goes down and no correlating alert exists, determining when the problem happened and what caused the problem is difficult. For these reasons, logging is important. All Cisco routers and switches support logging functionality. Logging capabilities are also available for specific features and protocols. For example, logging can be enabled for BGP session state changes or OSPF adjacency state changes.

Table 2-3 lists the various logging levels that can be configured.

Table 2-3 Logging Levels

Level Number

Level Name

















When the higher value is set, all the lower logging levels are enabled by default. If the logging level is set to 5 (Notifications), for example, all events falling under the category from 0 to 5 (Emergency to Notifications) are logged. For troubleshooting purpose, setting the logging level to 7 (Debugging) is good practice.

Multiple logging options are available on Cisco devices:

  • Console logging

  • Buffered logging

  • Logging to syslog server

Console logging is important when the device is experiencing crashes or a high CPU condition and access to the terminal session via Telnet or Secure Shell (SSH) is not available. However, having console logging enabled when running debugs is not a good practice because some debug outputs are chatty and can flood the device console. As a best practice, console logging should always be disabled when running debugs. Example 2-28 illustrates how to enable console logging on Nexus platforms.

Example 2-28 Configuring Console Logging

NX-1(config)# logging console ?
   <0-7>  0-emerg;1-alert;2-crit;3-err;4-warn;5-notif;6-inform;7-debug
NX-1(config)# logging console 6

NX-OS not only provides robust logging, but it also is persistent across reloads. All the buffered logging is present in the /var/log/external/ directory. To view the internal directories, use the command show system internal flash. This command lists all the internal directories that are part of the flash along with their utilization. The buffered log messages are viewed using the command show logging log.

Example 2-29 displays the directories present in the flash and the contents of the /var/log/external/ directory. If the show logging log command does not display output or the logging gets stopped, check the /var/log/ directory to ensure that space is available for that directory.

Example 2-29 Internal Flash Directories

NX-1# show system internal flash
Mount-on                  1K-blocks      Used   Available   Use%  Filesystem
/                            409600     69476      340124     17   /dev/root
/proc                             0         0           0      0   proc
/sys                              0         0           0      0   none
/debugfs                          0         0           0      0   nodev
/cgroup                           0         0           0      0   vdccontrol
/isan                        716800    519548      197252     73   none
/etc                           5120      1632        3488     32   none
/nxos/tmp                     20480      1536       18944      8   none
/var/log                      51200       108       51092      1   none
/var/home                      5120         0        5120      0   none
/var/tmp                     307200       460      306740      1   none
/var/sysmgr                 1048576       144     1048432      1   none
/var/sysmgr/ftp              409600        80      409520      1   none
/dev/shm                    1048576    353832      694744     34   none
/volatile                    204800         0      204800      0   none
/debug                         2048        28        2020      2   none
/dev/mqueue                       0         0           0      0   none
/mnt/cfg/0                   325029     12351      295897      5   /dev/sda5
/mnt/cfg/1                   325029     12349      295899      5   /dev/sda6
/mnt/cdrom                      350       350           0    100   /dev/scd0
/var/sysmgr/startup-cfg       40960      4192       36768     11   none
/dev/pts                          0         0           0      0   devpts
/mnt/pss                     325061      8898      299380      3   /dev/sda3
/bootflash                  3134728    202048     2773444      7   /dev/sda4
/smack                            0         0           0      0   smackfs
NX-1# show system internal dir /var/log/external/
                                                           ./         240
                                                          ../         300
                                                libfipf.5834            0
                                                 l2fm_ut.txt          774
                                                  plcmgr.dbg           21
                                                    snmp_log          180
                                                libfipf.3884            0
                                                libfipf.3855            0
                                            syslogd_ha_debug        11221
                                                    messages        25153
                                                startupdebug         3710
                                                       dmesg@          31

The logging level is also defined for various NX-OS components so that the user can control logging for chatty components or disable certain logging messages for less chatty or less important components. This is achieved by setting the logging level of the component using the command logging level component-name level. Example 2-30 demonstrates setting the logging level of the ARP and Ethpm components to 3 to reduce unwanted log messages.

Example 2-30 NX-OS Component Logging Level

NX-1(config)# logging level arp 3
NX-1(config)# logging level ethpm 3

The most persistent form of logging is to use a syslog server to log all the device logs. A syslog server is anything from a text file to a custom application that actively stores device logging information in a database.

Example 2-31 illustrates the syslog logging configuration. Before configuring syslog-based logging on NX-OS, the command logging timestamp [microseconds | milliseconds | seconds] must be enabled for the logging messages so that all log messages have time stamps. This helps when investigating the log messages. Generally, management interfaces are configured with a management VRF. In such cases, the syslog host must be specified using the logging server ip-address use-vrf vrf-name command on NX-OS so that the router knows from which VRF routing table the server is reachable. If the VRF option is not specified, the system does a lookup in default VRF (the global routing table).

Example 2-31 Syslog Logging Configuration

NX-1(config)# logging timestamp milliseconds
NX-1(config)# logging server 7 use-vrf management

Debug Logfiles

NX-OS provides the user with an option to redirect debug output to a file. This is useful when running debugs and segregating debug outputs from regular log messages. Use the debug logfile file-name size size command. Example 2-32 demonstrates using the debug logfile command to capture debugs in a logfile. In this example, a debug logfile named bgp_dbg is created with a size of 10000 bytes. The size of the logfile ranges from 4096 bytes to 4194304 bytes. All the debugs that are enabled are logged under the logfile. To filter the debug output further to capture more precise debug output, use the debug-filter option. In the following example, a BGP update debug is enabled and the update debug logs are filtered for neighbor in a VRF context VPN_A.

Example 2-32 Capturing Debug in a Logfile on NX-OS

NX-1# debug logfile bgp_dbg size 100000
NX-1# debug ip bgp updates
NX-1# debug-filter bgp neighbor
NX-1# debug-filter bgp vrf VPN_A

The NX-OS software creates the logfile in the log: file system root directory, so all the created logfiles are viewed using dir log:. After the debug logfile is created, the respective debugs are enabled and all the debug outputs are redirected to the debug logfile. To view the contents of the logfile, use the show debug logfile file-name command.

Accounting Log

During troubleshooting, it is important to identify the trigger of the problem, which could be normal show command or a configuration change. For such issues, examining all the configuration and show commands during the time of the problem provides vital information.

NX-OS logs all this information into the accounting logfile, which is readily available to the users. Using the command show accounting log, users capture all the commands executed and configured on the system, along with the time stamp and user information. The accounting logs are persistent across reloads. By default, the accounting logs capture only the configuration commands. To allow the capture of show commands along with configuration commands, configure the command terminal log-all. Example 2-33 displays the output of the accounting log, highlighting the various configuration changes made on the device.

Example 2-33 Accounting Log

NX-1# show accounting log
Sun Apr  2 01:09:02 2017:type=update:id=vsh.12412:user=admin:cmd=configure terminal ;
  version 6.0(2)U6(9) (SUCCESS)
Sun Apr  2 01:09:03 2017:type=update:id=vsh.12412:user=admin:cmd=interface-vlan
Sun Apr  2 01:09:03 2017:type=update:id=vsh.12412:user=admin:cmd=configure terminal ;
  feature interface-vlan (SUCCESS)
Sun Apr  2 01:09:38 2017:type=update:id=vsh.12963:user=admin:cmd=configure terminal ;
  control-plane (SUCCESS)
Sun Apr  2 01:09:38 2017:type=update:id=vsh.12963:user=admin:cmd=configure terminal ;
  control-plane ; service-policy input copp-system-policy (SUCCESS
Sun Apr  2 01:09:38 2017:type=update:id=vsh.12963:user=admin:cmd=configure terminal ;
  hardware profile tcam region arpacl 128 (SUCCESS)
Sun Apr  2 01:09:38 2017:type=update:id=vsh.12963:user=admin:cmd=configure terminal ;
  hardware profile tcam region ifacl 256 (SUCCESS)
Sun Apr  2 01:09:38 2017:type=update:id=vsh.12963:user=admin:cmd=configure terminal ;
 ip ftp source-interface mgmt0 (SUCCESS)
! Output omitted for brevity


NX-OS provides continuous logging for all events that occur in the system for both hardware and software components as event-history logs. The event-history logs are VDC local and are maintained on a per-component basis. These logs reduce the need for running debugs in a live production environment and are useful for investigating a service outage even after the services are restored. The event-history logs are captured in the background for each component and do not have any impact on CPU utilization to perform this task.

The event-history log size is configurable to three sizes:

  • Large

  • Medium

  • Small

The event-history logs are viewed from the CLI of each component. For instance, the event-history is viewed for all ARP events using the command show ip arp internal event-history event. Example 2-34 displays the event-history logs for ARP and shows how to modify the event-history size. Disable the event-history logs by using the disabled keyword while defining the size of the event-history. Disabling event-history is not a recommended practice, however, because it reduces the chances of root causing a problem and understanding the sequence of events that occurred.

Example 2-34 ARP Event-History Logs and Buffer Size

NX-1# show ip arp internal event-history event
1) Event:E_DEBUG, length:143, at 449547 usecs after Mon May 29 11:11:38 2017
    [116] [4201]: Adj info: iod: 2, phy-iod: 2, ip:, mac: fa16.3ee2.
b6d3, type: 0, sync: FALSE, suppress-mode: ARP Suppression Disabled
2) Event:E_DEBUG, length:193, at 449514 usecs after Mon May 29 11:11:38 2017
    [116] [4201]: Entry added to ARP pt, added to AM for, fa16.3ee2.
b6d3, state 2 on interface mgmt0, physical interface mgmt0, ismct 0. R
earp (interval: 0, count: 0), TTL: 1500 seconds
3) Event:E_DEBUG, length:79, at 449432 usecs after Mon May 29 11:11:38 2017
    [116] [4201]: arp_add_adj: Updating MAC on interface mgmt0, phy-interface mgmt0
! Output omitted for brevity
NX-1(config)# ip arp event-history event size ?
  disabled  Disabled
            *Default value is small
  large     Large buffer
  medium    Medium buffer
  small     Small buffer
NX-1(config)# ip arp event-history event size large

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