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2 FN410T in FX2 in VLT-Mode


we have a FX2 with 2 FN410T as IOMs. I changed the mode from Standalone to VLT and I'm not sure if this works correctly. I did it strongly like advised in the Deployment-Guide found here:


But the Backup-Link in the 2 modules are a 169-IP-address wich is a little suspicous for me. Also I tried to reboot one of the 2 IOM. I first set the 2 external ports to down wich sets also the internal ports down. That makes me a bit wondering. But the connection was still okay and the other IOM did all the work.

Than I reloaded the first IOM wich was offline and after it was back the connection to the servers was lost for appr. a minute. And this is totaly weird in my opinion.

The Dashboard of the 2 IOMs say it is all okay and VLT is up and running with all necessary links and heartbeats and so on. But why is after a reload the connection lost for a while?

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RE: 2 FN410T in FX2 in VLT-Mode

According to the documentation, the VLTi link is going to use the management IP.

Page 36:

# vlt domain 1

# peer-link port-channel 127

# back-up destination

# unit-id 1

What do you have the backup destination set to? The management IP set to? What show command are you running to see the 169 command?


Here is another VLT document with some good best practices to follow.



I would have a look at adjusting the delay timer.

"The delay-restore feature waits for all saved Configurations to be applied, then starts a Configurable timer. After the timer expires, the VLT ports are enabled one-by-one in a controlled manner. The delay between bringing up each VLT port-channel is proportional to the number of physical members in the port-channel. The default is 90 seconds. To change the duration of the Configurable timer, use the delay-restore command."

Daniel Covey
Dell EMC| Enterprise Support Services
Get support on Twitter:@DellCaresPRO
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RE: 2 FN410T in FX2 in VLT-Mode

Yes, I expected the backup-link to be the Management-IP. And I configured one on each of the IOM. And they are both pingable by each other. Than I used the VLT-Mode wich is kinda autoconfiguration. The PO 127 as VLT-Peer-Link is configured and working. But this mode seems not allowing me to configure the VLT-Domain. 

I got the address by a standard "sh run". Here's the vlt-domain part:

vlt domain 1
 peer-link port-channel 127
 back-up destination
 system-mac mac-address 00:01:06:01:02:09
 unit-id 0

I'll have a look at the document you mentioned. Thx. But I suppose I'm not allowed to change this parameter in this mode.

And I thought that VLT is the possibility to have maintenance (incl. a reload) on one of the IOMs without service interruption? And now a reload without any maintenance gives me the interruption!?

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RE: 2 FN410T in FX2 in VLT-Mode

You should be able to use the command # back-up distention, to make the change to the management IP.

The service should not be interrupted when just one switch is down. When you said the connection to the server was lost for a minute, I thought you meant the connection to the switch that was reloaded. But I guess you meant ALL connections to the servers were lost, including the switch that stayed up? If that is the case, then that is not normal operation. Did the logs record anything during this time?

Daniel Covey
Dell EMC| Enterprise Support Services
Get support on Twitter:@DellCaresPRO
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RE: 2 FN410T in FX2 in VLT-Mode

No, sorry. I do not have this command. In EXEC and CONF I have the following:

asset-tag                       Write AssetTag information
bmp                             BMP commands
cd                              Change current directory
clear                           Reset functions
clock                           Manage the system clock
clone-config                    Configure cloning related commands
configure                       Configuring from terminal
copy                            Copy from one file to another
debug                           Debug functions
delete                          Delete a file
diag                            Run diagnosis
dir                             List files on a filesystem
disable                         Turn off privileged commands
enable                          Turn on privileged commands
exit                            Exit from the EXEC
format                          Format a filesystem
monitor                         Monitoring feature
no                              Negate a command
offline                         Take stack-unit offline
online                          Bring stack-unit online
ping                            Send echo messages
power-cycle                     Power-cycle the unit(s)
pwd                             Display current working directory
redundancy                      stack-unit failover
release                         Release the ip address back to dhcp server
reload                          Halt and perform a cold restart
rename                          Rename a file
renew                           Renew the lease of IP address obtained through dhcp
reset                           Reset selected card
restore                         Restore sub commands
show                            Show running system information
ssh-peer-stack-unit             Open a SSH connection to the peer stack-unit
stack-unit                      Stack-Unit
start                           Start shell
stop                            Stop the BMP functionality
tcpdump                         Capture the TCP traffic at CPU
tdr-cable-test                  Start diagnostics (TDR test) for the cable
telnet                          Open a telnet connection
telnet-peer-stack-unit          Open a telnet connection to the peer stack-unit
terminal                        Set terminal line parameters
traceroute                      Trace route to destination
undebug                         Disable debugging
upgrade                         Upgrade subcommands
upload                          Upload file
write                           Write running configuration to memory or terminal
aaa                             Authentication, Authorization and Accounting
banner                          Define a login banner
boot                            Modify system boot parameters
clock                           Configure time-of-day clock
dcb                             Data Center Bridging
default                         Set a command to its default
enable                          Modify enable parameters
end                             Exit from configuration mode
exit                            Exit from configuration mode
feature                         Feature enable options
ftp-server                      FTP configuration subcommands
hostname                        Set system's network name
interface                       Select an interface to configure
io-aggregator                   I/O Aggregator Blade Switch configuration
ip                              Global IP configuration subcommands
lacp                            Configure LACP
line                            Configure a terminal line
logging                         Modify message logging facilities
management                      Create a management crypto or route, etc
monitor                         Monitor monitored ports
ntp                             Configure NTP
protocol                        Select a protocol to configure
radius-server                   Set up RADIUS server
redundancy                      Set up stack-unit redundancy configuration
reload-type                     Enter reload-type submode
script                          Start or stop a script
service                         Service selected component
snmp-server                     Modify SNMP parameters
stack-unit                      Configure stack-unit
tacacs-server                   Set up TACACS+ server
uplink-state-group              Uplink state group creation and configurations
username                        Establish user name authentication

Nothing to change to VLT or BACKUP. And as I said: the management-ip is set. Also Gaeway, routing, DNS, ...

The logs of the IOM wich should stayed up mention the following:

Aug 25 03:49:03: %STKUNIT0-M:CP %IFMGR-5-OSTATE_DN: Changed interface state to down: Te 0/9
Aug 25 03:49:03: %STKUNIT0-M:CP %IFMGR-5-OSTATE_DN: Changed interface state to down: Po 127
Aug 25 03:49:04: %STKUNIT0-M:CP %VLTMGR-6-VLT_ICL_DOWN: InterChassis Link is down (link down).
Aug 25 03:49:06: %STKUNIT0-M:CP %VLTMGR-6-VLT_HBEAT_DOWN: Heart beat link is down (down).
Aug 25 03:49:06: %STKUNIT0-M:CP %VLTMGR-6-VLT_ELECTION_ROLE: Chassis is transitioning to primary role.
Aug 25 03:49:06: %STKUNIT0-M:CP %VLTMGR-6-VLT_PEER_STATUS: Peer chassis is down.
Aug 25 03:50:49: %STKUNIT0-M:CP %LLDP-5-LLDP_PEER_AGE_OUT: DCBX operationally disabled due to LLDP peer timing out on interface Te 0/9
Aug 25 03:51:02: %STKUNIT0-M:CP %IFMGR-5-OSTATE_UP: Changed interface state to up: Te 0/9
Aug 25 03:51:02: %STKUNIT0-M:CP %IFMGR-5-OSTATE_UP: Changed interface state to up: Po 127
Aug 25 03:51:19: %STKUNIT0-M:CP %IFMGR-5-OSTATE_DN: Changed interface state to down: Te 0/9
Aug 25 03:51:20: %STKUNIT0-M:CP %IFMGR-5-OSTATE_DN: Changed interface state to down: Po 127
Aug 25 03:51:24: %STKUNIT0-M:CP %IFMGR-5-OSTATE_UP: Changed interface state to up: Te 0/9
Aug 25 03:51:24: %STKUNIT0-M:CP %IFMGR-5-OSTATE_UP: Changed interface state to up: Po 127
Aug 25 03:51:31: %STKUNIT0-M:CP %VLTMGR-6-VLT_ICL_UP: InterChassis Link is up.
Aug 25 03:51:32: %STKUNIT0-M:CP %VLTMGR-6-VLT_PEER_STATUS: Peer chassis is up.
Aug 25 03:51:32: %STKUNIT0-M:CP %VLTMGR-6-VLT_HBEAT_UP: Heart beat link is up.

And yes, I pinged a VM from the external side and have successful pings during the complete reload-process. when the IOM was up this ping was unseccessful for a while and came back than.

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RE: 2 FN410T in FX2 in VLT-Mode

The back-up command will be present under the vlt domain.

# vlt domain 1
# back-up destination x.x.x.x

I suggest getting the backup destination set correctly. Perhaps reduce the timer to 30 or 60 seconds. Look through that best practice list for any other tweaks that can be made. Then test again, hopefully the changes will get rid of the loss of connection that you saw.

Here is a KB article with some more information on testing.


In this forum post, a community member found that adjusting the rstp hello timers helped with switch facing interfaces. May be another area to fine tune.


Daniel Covey
Dell EMC| Enterprise Support Services
Get support on Twitter:@DellCaresPRO
Download our QRL app:iOS, Android, Windows
Dell Networking Resources

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RE: 2 FN410T in FX2 in VLT-Mode


thx for your suggestions. But I cannot Change to the vlt-mode. I showed you all comands I can issue in EXEC and conf-mode. But no VLT.

And RSPT is not enabled in the Moment.

Thx and regards

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RE: 2 FN410T in FX2 in VLT-Mode

suggest you change to full switch mode then manual configured VLT setting as you want.


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RE: 2 FN410T in FX2 in VLT-Mode


we are faceing the same problem with the VLT configured on the management interface IP. When the pair comes up we are experiencing a 30 sec timeout.

Did you find a solution for your case? Would you share it?



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