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OCPBUGS-87249: on AWS, select associated IPv6 CIDR block for egress IP subnet #228

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OCPBUGS-87249: on AWS, select associated IPv6 CIDR block for egress IP subnet #228

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27 changes: 18 additions & 9 deletions pkg/cloudprovider/aws.go
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@pperiyasamy pperiyasamy Jul 16, 2026

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so previously does it took 'disassociated' subnet entry at [0] which is different from host subnet, what was that subnet value ? just curious.

Original file line number Diff line number Diff line change
Expand Up @@ -276,23 +276,32 @@ func (a *AWS) getSubnet(networkInterface *ec2.InstanceNetworkInterface) (*net.IP
var v4Subnet, v6Subnet *net.IPNet
subnet := describeOutput.Subnets[0]
if subnet.CidrBlock != nil && *subnet.CidrBlock != "" {
_, subnet, err := net.ParseCIDR(*subnet.CidrBlock)
_, v4Net, err := net.ParseCIDR(*subnet.CidrBlock)

@arghosh93 arghosh93 Jul 14, 2026

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Why do you have to change the variable name here?

@jechen0648 jechen0648 Jul 14, 2026

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this subnet variable is used for v4 and v6 below, it's like one variable used for two different ipstack types, it is easy to get confused , that is why I changed the variable name. But I can revert if you prefer minimal change

if err != nil {
return nil, nil, fmt.Errorf("error: unable to parse IPv4 subnet, err: %v", err)
}
v4Subnet = subnet
v4Subnet = v4Net
}

// I don't know what it means to have several IPv6 CIDR blocks defined for
// one subnet, specially given that you can only have one IPv4 CIDR block
// defined...¯\_(ツ)_/¯
// Let's just pick the first.
if len(subnet.Ipv6CidrBlockAssociationSet) > 0 && subnet.Ipv6CidrBlockAssociationSet[0].Ipv6CidrBlock != nil && *subnet.Ipv6CidrBlockAssociationSet[0].Ipv6CidrBlock != "" {
_, subnet, err := net.ParseCIDR(*subnet.Ipv6CidrBlockAssociationSet[0].Ipv6CidrBlock)
// A subnet can have multiple IPv6 CIDR block associations (e.g., a previous
// "disassociated" one and a current "associated" one). On dualstack AWS clusters
// after subnet CIDR changes, picking the wrong (stale) CIDR causes egress IP
// assignment to fail because OVN-Kubernetes selects IPs from the wrong range.
// Always select the first association in "associated" state.
for _, assoc := range subnet.Ipv6CidrBlockAssociationSet {
if assoc.Ipv6CidrBlock == nil || *assoc.Ipv6CidrBlock == "" {
continue
}
if assoc.Ipv6CidrBlockState == nil || assoc.Ipv6CidrBlockState.State == nil ||
*assoc.Ipv6CidrBlockState.State != ec2.SubnetCidrBlockStateCodeAssociated {

@pperiyasamy pperiyasamy Jul 16, 2026

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is there a possibility of more than one associated subnet ? hope you have checked it.

@jechen0648 jechen0648 Jul 17, 2026

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Yes, technically AWS allows multiple IPv6 CIDR blocks in associated state simultaneously (e.g., when additional IPv6 prefixes are added to a subnet). The current code takes the first associated entry, silently ignoring any others. This means EgressIPs can only be assigned from the first CIDR. This is consistent with how we handle the node's subnet today. If multiple associated entries exist, should I add a klog.Warningf when a second associated entry is found to make this situation visible in logs? Is that what you expect?

continue
}
_, v6Net, err := net.ParseCIDR(*assoc.Ipv6CidrBlock)
if err != nil {
return nil, nil, fmt.Errorf("error: unable to parse IPv6 subnet, err: %v", err)
}
v6Subnet = subnet
v6Subnet = v6Net
break
}

return v4Subnet, v6Subnet, nil
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