NX-OS N7K VPC consistency check

Unanswered Question
Mar 10th, 2011

When changing MTU, for example, on a VPC port on one side, VPC consistency check runs right away and disables access on the VPC:

2011 Mar 10 10:08:40 KKA7K-A %VSHD-5-VSHD_SYSLOG_CONFIG_I: Configured from vty by rrodichev on [email protected]/7

2011 Mar 10 10:08:55 KKA7K-A %ASCII-CFG-6-INFORMATION: Reading ACFG Runtime information

2011 Mar 10 10:08:55 KKA7K-A %VPC-3-INTF_CONSISTENCY_FAILED: In domain 1, VPC 20 configuration is not consistent (Compatibility check failed for MTU)

2011 Mar 10 10:08:55 KKA7K-A %ETHPORT-3-IF_ERROR_VLANS_SUSPENDED: VLANs 64,80 on Interface port-channel20 are being suspended. (Reason: Compatibility check failed for MTU)

2011 Mar 10 10:08:55 KKA7K-A %STP-6-PORT_DELETED: Interface port-channel20, removed from VLAN0064

2011 Mar 10 10:08:55 KKA7K-A %STP-6-PORT_DELETED: Interface port-channel20, removed from VLAN0080

2011 Mar 10 10:08:57 KKA7K-A %ASCII-CFG-6-INFORMATION: Reading ACFG Runtime information

How do you make sure that production traffic is not affected when making changes to the VPC ports? Changing MTU is not supposed to affect traffic.

I have this problem too.
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Roman Rodichev Wed, 03/16/2011 - 20:56

According to TAC this is by design. So even if you want to change such trivial setting as MTU on a VPC you better do this off hours. I feel this is a limitation that should not exist in NX-OS. There's got to be a way to make trivial VPC changes without dropping traffic. There should be like a 1 minute window or something for you to make change on the second switch, or maybe some kind of automatic configuration replication to second switch (similar to VSS).

Sometimes I wish Nexus had VSS.

Zizhen Gao Fri, 03/18/2011 - 11:26

One clarification on changing paramaters (e.g. MTU) on a vPC on N7K:

During a configuration change on a vPC, one vPC peer conveys configuration information to the other vPC peer in order to verify that vPC member ports can actually form a PortChannel, which is the consistency check. As an example, if two ports that are going to join the channel has different MTU settings, even for a short duration, this is a misconfiguration.

Depending on the severity of the misconfiguration, vPC may either warn the user (Type 2 misconfiguration) or suspend the PortChannel (Type 1 misconfiguration). MTU is Type 1 parameter therefore changing its value would cause the vPC to be suspended, therefore service disruption. One can use "show vpc consistency" global or interface to check the type of parameters to be configured. The link below gives more details on the consistency check:


That said, the concern is valid. There's a graceful consistency check feature coming out which avoid the service disruption during type-1 configuration change.


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