udn, nad template: Set explicit default MTU for locanet topology

For Localnet topology, the interface MTU should be default MTU (1500)
because the underlay is not part of the SDN and should not compensate
for the SDN overhead.

When MTU is unset, set the MTU explicitly in the NAD with default MTU (1500).

Signed-off-by: Or Mergi <[email protected]>

Author: ormergi

dist/templates/k8s.ovn.org_clusteruserdefinednetworks.yaml.j2

@@ -389,7 +389,8 @@ spec:
                       mtu:
                         description: |-
                           mtu is the maximum transmission unit for a network.
-                          mtu is optional, if not provided, the globally configured value in OVN-Kubernetes (defaults to 1400) is used for the network.
+                          mtu is optional. When omitted, the configured value in OVN-Kubernetes (defaults to 1500 for localnet topology)
+                          is used for the network.
                           Minimum value for IPv4 subnet is 576, and for IPv6 subnet is 1280.
                           Maximum value is 65536.
                           In a scenario `physicalNetworkName` points to OVS bridge mapping of a network configured with certain MTU settings,

go-controller/pkg/clustermanager/userdefinednetwork/template/net-attach-def-template.go

@@ -13,6 +13,7 @@ import (
 	"sigs.k8s.io/controller-runtime/pkg/client"
 
 	ovncnitypes "github.com/ovn-org/ovn-kubernetes/go-controller/pkg/cni/types"
+	"github.com/ovn-org/ovn-kubernetes/go-controller/pkg/config"
 	userdefinednetworkv1 "github.com/ovn-org/ovn-kubernetes/go-controller/pkg/crd/userdefinednetwork/v1"
 	"github.com/ovn-org/ovn-kubernetes/go-controller/pkg/types"
 	"github.com/ovn-org/ovn-kubernetes/go-controller/pkg/util"
@@ -155,7 +156,7 @@ func renderCNINetworkConfig(networkName, nadName string, spec SpecGetter) (map[s
 	case userdefinednetworkv1.NetworkTopologyLocalnet:
 		cfg := spec.GetLocalnet()
 		netConfSpec.Role = strings.ToLower(string(cfg.Role))
-		netConfSpec.MTU = int(cfg.MTU)
+		netConfSpec.MTU = localnetMTU(cfg.MTU)
 		netConfSpec.AllowPersistentIPs = cfg.IPAM != nil && cfg.IPAM.Lifecycle == userdefinednetworkv1.IPAMLifecyclePersistent
 		netConfSpec.Subnets = cidrString(cfg.Subnets)
 		netConfSpec.ExcludeSubnets = cidrString(cfg.ExcludeSubnets)
@@ -211,6 +212,17 @@ func renderCNINetworkConfig(networkName, nadName string, spec SpecGetter) (map[s
 	return cniNetConf, nil
 }
 
+func localnetMTU(desiredMTU int32) int {
+	// The MTU for localnet topology should be as the default MTU (1500) because the underlay
+	// is not part of the SDN and compensating for the SDN overhead (100) is not required.
+	mtu := config.Default.MTU + 100
+	if desiredMTU > 0 {
+		mtu = int(desiredMTU)
+	}
+
+	return mtu
+}
+
 func ipamEnabled(ipam *userdefinednetworkv1.IPAMConfig) bool {
 	return ipam == nil || ipam.Mode == "" || ipam.Mode == userdefinednetworkv1.IPAMEnabled
 }

go-controller/pkg/clustermanager/userdefinednetwork/template/net-attach-def-template_test.go

@@ -571,5 +571,34 @@ var _ = Describe("NetAttachDefTemplate", func() {
 			  "allowPersistentIPs": true
 			}`,
 		),
+		Entry("secondary network, localnet, when MTU is unset it should set default MTU",
+			udnv1.NetworkSpec{
+				Topology: udnv1.NetworkTopologyLocalnet,
+				Localnet: &udnv1.LocalnetConfig{
+					Role:                udnv1.NetworkRoleSecondary,
+					PhysicalNetworkName: "mylocalnet1",
+					VLAN:                &udnv1.VLANConfig{Mode: udnv1.VLANModeAccess, Access: &udnv1.AccessVLANConfig{ID: 200}},
+					Subnets:             udnv1.DualStackCIDRs{"192.168.100.0/24", "2001:dbb::/64"},
+					ExcludeSubnets:      []udnv1.CIDR{"192.168.100.1/32", "2001:dbb::0/128"},
+					IPAM: &udnv1.IPAMConfig{
+						Lifecycle: udnv1.IPAMLifecyclePersistent,
+					},
+				},
+			},
+			`{
+			  "cniVersion": "1.0.0",
+			  "type": "ovn-k8s-cni-overlay",
+			  "name": "cluster_udn_test-net",
+			  "netAttachDefName": "mynamespace/test-net",
+			  "role": "secondary",
+			  "topology": "localnet",
+		      "physicalNetworkName": "mylocalnet1",
+			  "subnets": "192.168.100.0/24,2001:dbb::/64",
+              "excludeSubnets": "192.168.100.1/32,2001:dbb::0/128",
+			  "mtu": 1500,
+              "vlanID": 200, 
+			  "allowPersistentIPs": true
+			}`,
+		),
 	)
 })

go-controller/pkg/crd/userdefinednetwork/v1/cudn.go

@@ -162,7 +162,8 @@ type LocalnetConfig struct {
 	IPAM *IPAMConfig `json:"ipam,omitempty"`
 
 	// mtu is the maximum transmission unit for a network.
-	// mtu is optional, if not provided, the globally configured value in OVN-Kubernetes (defaults to 1400) is used for the network.
+	// mtu is optional. When omitted, the configured value in OVN-Kubernetes (defaults to 1500 for localnet topology)
+	// is used for the network.
 	// Minimum value for IPv4 subnet is 576, and for IPv6 subnet is 1280.
 	// Maximum value is 65536.
 	// In a scenario `physicalNetworkName` points to OVS bridge mapping of a network configured with certain MTU settings,