OVN VM guest NIC MTU is set correctly, but host-side tap interface stays at default 1500

[toaksoy]

Please confirm

• I have searched existing issues to check if an issue already exists for the bug I encountered.

Distribution

Ubuntu

Distribution version

24.04.4

Output of "snap list --all lxd core20 core22 core24 snapd"

$ snap list --all lxd core20 core22 core24 snapd
Name    Version         Rev    Tracking       Publisher   Notes
core22  20260225        2411   latest/stable  canonical✓  base
core24  20260211        1499   latest/stable  canonical✓  base,disabled
core24  20260317        1587   latest/stable  canonical✓  base
lxd     5.21.4-1374f39  38368  5.21/stable    canonical✓  in-cohort,held
snapd   2.74.1          26382  latest/stable  canonical✓  snapd

Output of "lxc info" or system info if it fails

$ lxc info
config:
  cluster.https_address: 10.XX.YY.11:8443
  core.https_address: '[::]:8443'
  instances.migration.stateful: "true"
  network.ovn.northbound_connection: ssl:10.XX.YY.11:6641,ssl:10.XX.YY.12:6641,ssl:10.XX.YY.13:6641
  user.microcloud: 2.1.2
api_extensions:
- storage_zfs_remove_snapshots
- container_host_shutdown_timeout
- container_stop_priority
- container_syscall_filtering
- auth_pki
- container_last_used_at
- etag
- patch
- usb_devices
- https_allowed_credentials
- image_compression_algorithm
- directory_manipulation
- container_cpu_time
- storage_zfs_use_refquota
- storage_lvm_mount_options
- network
- profile_usedby
- container_push
- container_exec_recording
- certificate_update
- container_exec_signal_handling
- gpu_devices
- container_image_properties
- migration_progress
- id_map
- network_firewall_filtering
- network_routes
- storage
- file_delete
- file_append
- network_dhcp_expiry
- storage_lvm_vg_rename
- storage_lvm_thinpool_rename
- network_vlan
- image_create_aliases
- container_stateless_copy
- container_only_migration
- storage_zfs_clone_copy
- unix_device_rename
- storage_lvm_use_thinpool
- storage_rsync_bwlimit
- network_vxlan_interface
- storage_btrfs_mount_options
- entity_description
- image_force_refresh
- storage_lvm_lv_resizing
- id_map_base
- file_symlinks
- container_push_target
- network_vlan_physical
- storage_images_delete
- container_edit_metadata
- container_snapshot_stateful_migration
- storage_driver_ceph
- storage_ceph_user_name
- resource_limits
- storage_volatile_initial_source
- storage_ceph_force_osd_reuse
- storage_block_filesystem_btrfs
- resources
- kernel_limits
- storage_api_volume_rename
- network_sriov
- console
- restrict_devlxd
- migration_pre_copy
- infiniband
- maas_network
- devlxd_events
- proxy
- network_dhcp_gateway
- file_get_symlink
- network_leases
- unix_device_hotplug
- storage_api_local_volume_handling
- operation_description
- clustering
- event_lifecycle
- storage_api_remote_volume_handling
- nvidia_runtime
- container_mount_propagation
- container_backup
- devlxd_images
- container_local_cross_pool_handling
- proxy_unix
- proxy_udp
- clustering_join
- proxy_tcp_udp_multi_port_handling
- network_state
- proxy_unix_dac_properties
- container_protection_delete
- unix_priv_drop
- pprof_http
- proxy_haproxy_protocol
- network_hwaddr
- proxy_nat
- network_nat_order
- container_full
- backup_compression
- nvidia_runtime_config
- storage_api_volume_snapshots
- storage_unmapped
- projects
- network_vxlan_ttl
- container_incremental_copy
- usb_optional_vendorid
- snapshot_scheduling
- snapshot_schedule_aliases
- container_copy_project
- clustering_server_address
- clustering_image_replication
- container_protection_shift
- snapshot_expiry
- container_backup_override_pool
- snapshot_expiry_creation
- network_leases_location
- resources_cpu_socket
- resources_gpu
- resources_numa
- kernel_features
- id_map_current
- event_location
- storage_api_remote_volume_snapshots
- network_nat_address
- container_nic_routes
- cluster_internal_copy
- seccomp_notify
- lxc_features
- container_nic_ipvlan
- network_vlan_sriov
- storage_cephfs
- container_nic_ipfilter
- resources_v2
- container_exec_user_group_cwd
- container_syscall_intercept
- container_disk_shift
- storage_shifted
- resources_infiniband
- daemon_storage
- instances
- image_types
- resources_disk_sata
- clustering_roles
- images_expiry
- resources_network_firmware
- backup_compression_algorithm
- ceph_data_pool_name
- container_syscall_intercept_mount
- compression_squashfs
- container_raw_mount
- container_nic_routed
- container_syscall_intercept_mount_fuse
- container_disk_ceph
- virtual-machines
- image_profiles
- clustering_architecture
- resources_disk_id
- storage_lvm_stripes
- vm_boot_priority
- unix_hotplug_devices
- api_filtering
- instance_nic_network
- clustering_sizing
- firewall_driver
- projects_limits
- container_syscall_intercept_hugetlbfs
- limits_hugepages
- container_nic_routed_gateway
- projects_restrictions
- custom_volume_snapshot_expiry
- volume_snapshot_scheduling
- trust_ca_certificates
- snapshot_disk_usage
- clustering_edit_roles
- container_nic_routed_host_address
- container_nic_ipvlan_gateway
- resources_usb_pci
- resources_cpu_threads_numa
- resources_cpu_core_die
- api_os
- container_nic_routed_host_table
- container_nic_ipvlan_host_table
- container_nic_ipvlan_mode
- resources_system
- images_push_relay
- network_dns_search
- container_nic_routed_limits
- instance_nic_bridged_vlan
- network_state_bond_bridge
- usedby_consistency
- custom_block_volumes
- clustering_failure_domains
- resources_gpu_mdev
- console_vga_type
- projects_limits_disk
- network_type_macvlan
- network_type_sriov
- container_syscall_intercept_bpf_devices
- network_type_ovn
- projects_networks
- projects_networks_restricted_uplinks
- custom_volume_backup
- backup_override_name
- storage_rsync_compression
- network_type_physical
- network_ovn_external_subnets
- network_ovn_nat
- network_ovn_external_routes_remove
- tpm_device_type
- storage_zfs_clone_copy_rebase
- gpu_mdev
- resources_pci_iommu
- resources_network_usb
- resources_disk_address
- network_physical_ovn_ingress_mode
- network_ovn_dhcp
- network_physical_routes_anycast
- projects_limits_instances
- network_state_vlan
- instance_nic_bridged_port_isolation
- instance_bulk_state_change
- network_gvrp
- instance_pool_move
- gpu_sriov
- pci_device_type
- storage_volume_state
- network_acl
- migration_stateful
- disk_state_quota
- storage_ceph_features
- projects_compression
- projects_images_remote_cache_expiry
- certificate_project
- network_ovn_acl
- projects_images_auto_update
- projects_restricted_cluster_target
- images_default_architecture
- network_ovn_acl_defaults
- gpu_mig
- project_usage
- network_bridge_acl
- warnings
- projects_restricted_backups_and_snapshots
- clustering_join_token
- clustering_description
- server_trusted_proxy
- clustering_update_cert
- storage_api_project
- server_instance_driver_operational
- server_supported_storage_drivers
- event_lifecycle_requestor_address
- resources_gpu_usb
- clustering_evacuation
- network_ovn_nat_address
- network_bgp
- network_forward
- custom_volume_refresh
- network_counters_errors_dropped
- metrics
- image_source_project
- clustering_config
- network_peer
- linux_sysctl
- network_dns
- ovn_nic_acceleration
- certificate_self_renewal
- instance_project_move
- storage_volume_project_move
- cloud_init
- network_dns_nat
- database_leader
- instance_all_projects
- clustering_groups
- ceph_rbd_du
- instance_get_full
- qemu_metrics
- gpu_mig_uuid
- event_project
- clustering_evacuation_live
- instance_allow_inconsistent_copy
- network_state_ovn
- storage_volume_api_filtering
- image_restrictions
- storage_zfs_export
- network_dns_records
- storage_zfs_reserve_space
- network_acl_log
- storage_zfs_blocksize
- metrics_cpu_seconds
- instance_snapshot_never
- certificate_token
- instance_nic_routed_neighbor_probe
- event_hub
- agent_nic_config
- projects_restricted_intercept
- metrics_authentication
- images_target_project
- cluster_migration_inconsistent_copy
- cluster_ovn_chassis
- container_syscall_intercept_sched_setscheduler
- storage_lvm_thinpool_metadata_size
- storage_volume_state_total
- instance_file_head
- instances_nic_host_name
- image_copy_profile
- container_syscall_intercept_sysinfo
- clustering_evacuation_mode
- resources_pci_vpd
- qemu_raw_conf
- storage_cephfs_fscache
- network_load_balancer
- vsock_api
- instance_ready_state
- network_bgp_holdtime
- storage_volumes_all_projects
- metrics_memory_oom_total
- storage_buckets
- storage_buckets_create_credentials
- metrics_cpu_effective_total
- projects_networks_restricted_access
- storage_buckets_local
- loki
- acme
- internal_metrics
- cluster_join_token_expiry
- remote_token_expiry
- init_preseed
- storage_volumes_created_at
- cpu_hotplug
- projects_networks_zones
- network_txqueuelen
- cluster_member_state
- instances_placement_scriptlet
- storage_pool_source_wipe
- zfs_block_mode
- instance_generation_id
- disk_io_cache
- amd_sev
- storage_pool_loop_resize
- migration_vm_live
- ovn_nic_nesting
- oidc
- network_ovn_l3only
- ovn_nic_acceleration_vdpa
- cluster_healing
- instances_state_total
- auth_user
- security_csm
- instances_rebuild
- numa_cpu_placement
- custom_volume_iso
- network_allocations
- storage_api_remote_volume_snapshot_copy
- zfs_delegate
- operations_get_query_all_projects
- metadata_configuration
- syslog_socket
- event_lifecycle_name_and_project
- instances_nic_limits_priority
- disk_initial_volume_configuration
- operation_wait
- cluster_internal_custom_volume_copy
- disk_io_bus
- storage_cephfs_create_missing
- instance_move_config
- ovn_ssl_config
- init_preseed_storage_volumes
- metrics_instances_count
- server_instance_type_info
- resources_disk_mounted
- server_version_lts
- oidc_groups_claim
- loki_config_instance
- storage_volatile_uuid
- import_instance_devices
- instances_uefi_vars
- instances_migration_stateful
- container_syscall_filtering_allow_deny_syntax
- access_management
- vm_disk_io_limits
- storage_volumes_all
- instances_files_modify_permissions
- image_restriction_nesting
- container_syscall_intercept_finit_module
- device_usb_serial
- network_allocate_external_ips
- explicit_trust_token
- instance_import_conversion
- instance_create_start
- devlxd_images_vm
- instance_protection_start
- disk_io_bus_virtio_blk
- metadata_configuration_entity_types
- network_allocations_ovn_uplink
- network_ovn_uplink_vlan
- shared_custom_block_volumes
- metrics_api_requests
- projects_limits_disk_pool
- access_management_tls
- state_logical_cpus
- vm_limits_cpu_pin_strategy
- gpu_cdi
- metadata_configuration_scope
- unix_device_hotplug_ownership_inherit
- unix_device_hotplug_subsystem_device_option
- storage_ceph_osd_pool_size
- network_get_target
- network_zones_all_projects
- vm_root_volume_attachment
- projects_limits_uplink_ips
- entities_with_entitlements
- profiles_all_projects
- storage_driver_powerflex
- storage_driver_pure
- cloud_init_ssh_keys
- oidc_scopes
- project_default_network_and_storage
- ubuntu_pro_guest_attach
- images_all_projects
- client_cert_presence
- resources_device_fs_uuid
- clustering_groups_used_by
- container_bpf_delegation
- override_snapshot_profiles_on_copy
- backup_metadata_version
- storage_buckets_all_projects
- network_acls_all_projects
- networks_all_projects
- clustering_restore_skip_mode
- disk_io_threads_virtiofsd
- oidc_client_secret
- pci_hotplug
- device_patch_removal
api_status: stable
api_version: "1.0"
auth: trusted
public: false
auth_methods:
- tls
client_certificate: false
auth_user_name: ubuntu
auth_user_method: unix
environment:
  addresses:
  - 10.XX.YY.11:8443
  - 10.XX.YY.51:8443
  architectures:
  - x86_64
  - i686
  backup_metadata_version_range:
  - 1
  - 2
  certificate: |
    <TRUNCATED>
  certificate_fingerprint: b55c6edfd6263b3e518c331263cb5da6f0bfc3dd7fb2f37e51b8970aa155f4d3
  driver: lxc | qemu
  driver_version: 6.0.4 | 8.2.2
  instance_types:
  - container
  - virtual-machine
  firewall: nftables
  kernel: Linux
  kernel_architecture: x86_64
  kernel_features:
    bpf_token: "false"
    idmapped_mounts: "true"
    netnsid_getifaddrs: "true"
    seccomp_listener: "true"
    seccomp_listener_continue: "true"
    uevent_injection: "true"
    unpriv_binfmt: "true"
    unpriv_fscaps: "true"
  kernel_version: 6.8.0-106-generic
  lxc_features:
    cgroup2: "true"
    core_scheduling: "true"
    devpts_fd: "true"
    idmapped_mounts_v2: "true"
    mount_injection_file: "true"
    network_gateway_device_route: "true"
    network_ipvlan: "true"
    network_l2proxy: "true"
    network_phys_macvlan_mtu: "true"
    network_veth_router: "true"
    pidfd: "true"
    seccomp_allow_deny_syntax: "true"
    seccomp_notify: "true"
    seccomp_proxy_send_notify_fd: "true"
  os_name: Ubuntu
  os_version: "24.04"
  project: default
  server: lxd
  server_clustered: true
  server_event_mode: full-mesh
  server_name: XX-YY-Z1-IN01
  server_pid: 7395
  server_version: 5.21.4
  server_lts: true
  storage: ceph | cephfs
  storage_version: 17.2.9 | 17.2.9
  storage_supported_drivers:
  - name: btrfs
    version: 5.16.2
    remote: false
  - name: ceph
    version: 17.2.9
    remote: true
  - name: cephfs
    version: 17.2.9
    remote: true
  - name: dir
    version: "1"
    remote: false
  - name: lvm
    version: 2.03.11(2) (2021-01-08) / 1.02.175 (2021-01-08) / 4.48.0
    remote: false
  - name: pure
    version: 2.1.9 (iscsiadm) / 1.16 (nvme-cli)
    remote: true
  - name: zfs
    version: 2.2.2-0ubuntu9.4
    remote: false
  - name: cephobject
    version: 17.2.9
    remote: true
  - name: powerflex
    version: 1.16 (nvme-cli)
    remote: true

Issue description

When a VM is created on an OVN network, LXD correctly applies the OVN network MTU to the guest NIC. In this case, the OVN network is configured with bridge.mtu=8942, and the guest VM NIC comes up with MTU 8942 as expected.

However, the corresponding host-side tap* interface for that VM is created with MTU 1500 instead of the effective OVN/guest NIC MTU. As a result, the VM advertises and transmits with MTU 8942, but traffic hits a 1500-byte bottleneck at the host TAP device.

In a jumbo underlay deployment, this breaks OVN-backed VM connectivity for larger packets even though:

• the physical uplink/underlay is jumbo-capable
• the OVN network MTU is configured correctly
• the guest NIC MTU is configured correctly

Manually changing the host-side TAP MTU to match the OVN/guest MTU immediately fixes the problem.

• This appears specific to VMs, where the host-side endpoint is a TAP device.
• The problem is not with the guest NIC MTU, which is set correctly.
• The problem is not with the physical underlay MTU, which is jumbo-capable.
• The failure is caused by the host-side TAP interface remaining at MTU 1500.
• This also happens when bridge.mtu is set to a different value, e.g. 1442.

Expected behavior:
The host-side TAP interface created for a VM on an OVN network should inherit or be programmed with the effective OVN/NIC MTU, rather than remaining at the default 1500.

Steps to reproduce

1. Create or use a jumbo-capable uplink/underlay network, for example with MTU 9000.

2. Create an OVN network attached to that uplink, with a reduced OVN MTU to account for encapsulation, for example:

   • uplink MTU: 9000
   • OVN network bridge.mtu: 8942

3. Launch a VM on that OVN network.

4. Inside the VM, verify that the guest NIC has the OVN MTU:
   ip link show

5. On the host, identify the corresponding host-side tap* interface for that VM and verify its MTU:
   ip link show tapXXXX

6. Observe that:

   • the guest NIC MTU is 8942
   • the host-side tap MTU is 1500

7. Attempt jumbo traffic from the VM, for example with DF set and a payload that fits 8942 but not 1500.

8. Observe that the traffic fails.

9. On the host, manually set the TAP MTU to match the OVN/guest MTU:
   ip link set dev tapXXXX mtu 8942

10. Retry the same traffic.

11. Observe that traffic now succeeds.

Information to attach

• Any relevant kernel output (dmesg)
• Instance log (lxc info NAME --show-log)
• Instance configuration (lxc config show NAME --expanded)
• Main daemon log (at /var/log/lxd/lxd.log or /var/snap/lxd/common/lxd/logs/lxd.log)
• Output of the client with --debug
• Output of the daemon with --debug (or use lxc monitor while reproducing the issue)