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Nexus 5K Config-Sync : Things to look out for

I've looked through the Cisco config guides for setting up Config sync and have successfully used it to configure dual-homed FEX's ports.

Is there anything I should be aware of that can cause issues?

I enabled config sync on an existing pair of 5Ks.  In other words, the 5Ks already have existing configurations on them before I enabled config sync.

Richard Michael
Cisco Employee


Just make sure that whenever you do the changes make it identical and dont change it manually. The configuration is kept identical by making changes to the switch  profile on one switch (either side is fine) and then using the verify  and commit options to start the sync to the other switch. The person  doing the configuration does

not manually enter the changes in both sides (and in fact doing so will cause an error).

By chance if you enter the error state or switch by itself enters into the error state, then we follow certain procedures to come out of the problem,

The below document will give you a good understanding about the troubleshooting,



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Hi Richard. Thank you for your response. Are you saying that once config sync is enabled, I should no longer make changes on the individual switch in conf terminal mode?  What if I was just connecting a server to one 5K and do not need to configure a vpc?


According to the this link, you still have to configure the single port in switch profile. Config sync is used when your devices are connected to both 5ks, so you don't have configure the same thing twice.  If your device is going to be singly attached, there is no need for config sync.

Guidelines and Limitations

The guidelines for configuration synchronization are as follows:

You must configure the following interfaces in a switch profile:

Port-channel interfaces

Ports that are not channel-group members

You must configure all port-channel members outside the switch profile in configuration terminal mode.

You must follow configurations in a specified order.

Depending on the type of vPC topology (active/active or straight-through) and the type of configuration that is needed (port channel, nonport channel, FEX, QoS, and so on), you must use the switch profile mode or the configuration terminal mode. See the "At-A-Glance Configuration Modes" section to identify what mode is used for different types of configurations.

Configuration synchronization has the following configuration limitations:

FCoE in vPC Topologies—FCoE configurations are not supported in switch profiles because configurations are typically different on peer switches. If you enable FCoE on a vPC peer switch, you must not configure the port channel in the switch profile.

Feature Commands—The feature feature name commands that enable a conditional feature are not supported in switch profiles. You should independently configure these commands on each peer switch in configuration terminal mode.

Configuration Rollback and Conditional Features—With configuration synchronization, when a conditional feature is present in a checkpoint and not in the running configuration, a configuration rollback to that checkpoint fails. The workaround is to reconfigure the conditional feature ("feature xyz") before the configuration rollback is executed. This workaround also applies to the vpc domain command and the peer-keepalive command in vpc-domain mode.



Thank you Reza,

To confirm what you just said.

I am no longer allowed to make individual changes to individual ports?

I want to use config-sync to make changes to my FEX ports since the FEX are dual homed.  But, there are instances where we might one to connect a server to just one port on one of the Nexus 5Ks.  Are you saying that by enabling config-sync, I am no longer allowed to do that?



I have never deployed config sync, but if I am interpreting the document correctly, it says that if you are configuring an interface that is not going to be in a portchannel, as in case of a single attached server, you would still need to configure it within the config sync profile.  I think, if you have a hybrid environment of single and dual attached servers, you would need to do some testing with config sync to make sure it is actually working properly.


Just to add to Reza's comment, you can deploy the config the way you want whether its a single attached or dual attached server.You have to make sure that for any dual attached server/Fex ports config, you configure everything under the config-sync feature.

Hope this helps.


-amit singh

Okay thank you Amit.  That's what I thought.  I couldn't find that information in the Cisco docs so I just wanted to see what was everyone else's experience.  In the past I configured config-sync on brand new pairs of nexus 5Ks but never really on an existing pair.  Also, my previous clients always had 100% identical port configuration on both switches.

In my current environment, we have single attached servers directly into the nexus 5Ks as well as dual attached servers.  I just wanted to make sure that we can use config-sync for the dual attached servers, but configure the single attached servers the traditional way.