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What CNI are you using? Flannel? Calico?

@mammoth-memory-36508 I’m using calico (rke2 default)

To change the CIDR you'll need to make a new IPPool, turn off the old one, and restart all your pods.

to be concrete, i’m using canal

Ah, then it's Flannel really handling this portion. I unfortunately have not used Flannel.

okay but currently i’ve an existing rke2 cluster, created with rke2 defaults and only one thing i changed for canal (to change the networking to use my private vlan and custom MTU):

apiVersion: <http://helm.cattle.io/v1|helm.cattle.io/v1>
kind: HelmChartConfig
metadata:
  name: rke2-canal
  namespace: kube-system
spec:
  valuesContent: |-
    flannel:
      iface: "vlan0"
    calico:
      vethuMTU: 1350

In Calico, it uses a CRD so you modify the IPPool CRD. It also as a HelmChart, you can modify that helm chart with values pre-provisonoing and it'll apply it. If Flannel is Helm installed (looks like it is), maybe a helm list -Aa and find the Flannel installation and update the values there.

thanks @mammoth-memory-36508 I think i might have found the root cause for this problem. Check it here: https://github.com/longhorn/longhorn/discussions/6605