PacketLab
Protocol Library/Routing & Switching/VXLAN
VXLAN · Routed underlay deployments

OSPF Underlay

Carry VTEP reachability with OSPF, first over /31 links and then with an independent unnumbered variant.

Two spines, four leaves

Example platform: Nexus 9300/9000v · NX-OS 9.3(9) command model. Commands are platform-specific. Use a release and hardware/image with the stated VXLAN, multicast and routing features. All outputs and packet excerpts below are constructed teaching examples, not lab recordings.

Every leaf has one link to each spine. SPINE1/2 are transport nodes and redundant EVPN reflectors; leaves terminate VNI 10010. The numbered and unnumbered recipes are independent alternatives, not cumulative commands. Hosts H1–H4 use 10.10.10.11–14/24 on leaf Ethernet1/3, with no gateway required for this same-subnet test.

Independent lab · ospf-underlayIllustrative topology
Physical topology of ospf-underlay; addressing and roles appear in the following table.
Solid lines are physical connections; crossings are not junctions. The tenant service terminates on leaves, not on transit spines. Port addresses and host settings are listed below.
Physical linkAddress at first endpointAddress at second endpoint
SPINE1 Ethernet1/1 ↔ LEAF1 Ethernet1/1192.0.2.0/31192.0.2.1/31
SPINE1 Ethernet1/2 ↔ LEAF2 Ethernet1/1192.0.2.2/31192.0.2.3/31
SPINE1 Ethernet1/3 ↔ LEAF3 Ethernet1/1192.0.2.4/31192.0.2.5/31
SPINE1 Ethernet1/4 ↔ LEAF4 Ethernet1/1192.0.2.6/31192.0.2.7/31
SPINE2 Ethernet1/1 ↔ LEAF1 Ethernet1/2192.0.2.8/31192.0.2.9/31
SPINE2 Ethernet1/2 ↔ LEAF2 Ethernet1/2192.0.2.10/31192.0.2.11/31
SPINE2 Ethernet1/3 ↔ LEAF3 Ethernet1/2192.0.2.12/31192.0.2.13/31
SPINE2 Ethernet1/4 ↔ LEAF4 Ethernet1/2192.0.2.14/31192.0.2.15/31
DeviceLoopback0 / router ID
SPINE110.255.0.1/32
SPINE210.255.0.2/32
LEAF110.255.0.3/32
LEAF210.255.0.4/32
LEAF310.255.0.5/32
LEAF410.255.0.6/32

Build the numbered /31 underlay

SPINE1 · OSPF routing process and a numbered link

feature ospf
router ospf UNDERLAY
  router-id 10.255.0.1
interface loopback0
  ip address 10.255.0.1/32
  ip router ospf UNDERLAY area 0.0.0.0
interface Ethernet1/1
  no switchport
  mtu 9216
  ip address 192.0.2.0/31
  ip ospf network point-to-point
  ip router ospf UNDERLAY area 0.0.0.0
  no shutdown

Apply the corresponding /31 endpoint and OSPF configuration on every leaf–spine link. All six devices share area 0; router IDs and loopbacks remain unique. The point-to-point type avoids a DR/BDR election on these routed links. Both ends agree on MTU, area and network type.

Verify adjacency and equal-cost reachability

LEAF1 · illustrative output, selected fields
LEAF1# show ip ospf neighbors
Neighbor ID     State      Address      Interface
10.255.0.1     FULL/-     192.0.2.0   Eth1/1
10.255.0.2     FULL/-     192.0.2.8   Eth1/2
LEAF1 · illustrative output, selected fields
LEAF1# show ip route 10.255.0.6
10.255.0.6/32, ubest/mbest: 2/0
    *via 192.0.2.0, Eth1/1, [110/81], ospf-UNDERLAY, intra
    *via 192.0.2.8, Eth1/2, [110/81], ospf-UNDERLAY, intra

The reduced outputs show expected roles, not recorded counters or timers. Equal metric/eligible paths permit ECMP; a single flow need not use both uplinks. Confirm each selected path has the required MTU.

Keep the overlay separate

LEAF1 · EVPN loopback peers

feature bgp
nv overlay evpn
router bgp 65000
  neighbor 10.255.0.1
    remote-as 65000
    update-source loopback0
    address-family l2vpn evpn
      send-community
      send-community extended
  neighbor 10.255.0.2
    remote-as 65000
    update-source loopback0
    address-family l2vpn evpn
      send-community
      send-community extended

Both spines reflect EVPN to all four leaves. The complete recipe configures the local access VLAN, VNI and BGP ingress replication. No PIM is required for this chosen replication method. The underlay protocol change does not require changing VNI 10010 or host membership.

Complete independent lab configurations

Start with clean isolated lab devices. These are the complete scenario configurations, not changes to apply on top of a different lesson. Physical ports must match the address table. Virtual-image data-plane capabilities can differ from physical switches; these examples have not been executed on live devices.

Download configuration reference

SPINE1

hostname SPINE1
feature ospf
router ospf UNDERLAY
  router-id 10.255.0.1
interface loopback0
  ip address 10.255.0.1/32
  ip router ospf UNDERLAY area 0.0.0.0
interface Ethernet1/1
  no switchport
  mtu 9216
  ip address 192.0.2.0/31
  ip ospf network point-to-point
  ip ospf cost 40
  ip router ospf UNDERLAY area 0.0.0.0
  no shutdown
interface Ethernet1/2
  no switchport
  mtu 9216
  ip address 192.0.2.2/31
  ip ospf network point-to-point
  ip ospf cost 40
  ip router ospf UNDERLAY area 0.0.0.0
  no shutdown
interface Ethernet1/3
  no switchport
  mtu 9216
  ip address 192.0.2.4/31
  ip ospf network point-to-point
  ip ospf cost 40
  ip router ospf UNDERLAY area 0.0.0.0
  no shutdown
interface Ethernet1/4
  no switchport
  mtu 9216
  ip address 192.0.2.6/31
  ip ospf network point-to-point
  ip ospf cost 40
  ip router ospf UNDERLAY area 0.0.0.0
  no shutdown
feature bgp
nv overlay evpn
router bgp 65000
  router-id 10.255.0.1
  address-family l2vpn evpn
  neighbor 10.255.0.3
    remote-as 65000
    update-source loopback0
    address-family l2vpn evpn
      send-community
      send-community extended
      route-reflector-client
  neighbor 10.255.0.4
    remote-as 65000
    update-source loopback0
    address-family l2vpn evpn
      send-community
      send-community extended
      route-reflector-client
  neighbor 10.255.0.5
    remote-as 65000
    update-source loopback0
    address-family l2vpn evpn
      send-community
      send-community extended
      route-reflector-client
  neighbor 10.255.0.6
    remote-as 65000
    update-source loopback0
    address-family l2vpn evpn
      send-community
      send-community extended
      route-reflector-client

SPINE2

hostname SPINE2
feature ospf
router ospf UNDERLAY
  router-id 10.255.0.2
interface loopback0
  ip address 10.255.0.2/32
  ip router ospf UNDERLAY area 0.0.0.0
interface Ethernet1/1
  no switchport
  mtu 9216
  ip address 192.0.2.8/31
  ip ospf network point-to-point
  ip ospf cost 40
  ip router ospf UNDERLAY area 0.0.0.0
  no shutdown
interface Ethernet1/2
  no switchport
  mtu 9216
  ip address 192.0.2.10/31
  ip ospf network point-to-point
  ip ospf cost 40
  ip router ospf UNDERLAY area 0.0.0.0
  no shutdown
interface Ethernet1/3
  no switchport
  mtu 9216
  ip address 192.0.2.12/31
  ip ospf network point-to-point
  ip ospf cost 40
  ip router ospf UNDERLAY area 0.0.0.0
  no shutdown
interface Ethernet1/4
  no switchport
  mtu 9216
  ip address 192.0.2.14/31
  ip ospf network point-to-point
  ip ospf cost 40
  ip router ospf UNDERLAY area 0.0.0.0
  no shutdown
feature bgp
nv overlay evpn
router bgp 65000
  router-id 10.255.0.2
  address-family l2vpn evpn
  neighbor 10.255.0.3
    remote-as 65000
    update-source loopback0
    address-family l2vpn evpn
      send-community
      send-community extended
      route-reflector-client
  neighbor 10.255.0.4
    remote-as 65000
    update-source loopback0
    address-family l2vpn evpn
      send-community
      send-community extended
      route-reflector-client
  neighbor 10.255.0.5
    remote-as 65000
    update-source loopback0
    address-family l2vpn evpn
      send-community
      send-community extended
      route-reflector-client
  neighbor 10.255.0.6
    remote-as 65000
    update-source loopback0
    address-family l2vpn evpn
      send-community
      send-community extended
      route-reflector-client

LEAF1

hostname LEAF1
feature ospf
router ospf UNDERLAY
  router-id 10.255.0.3
interface loopback0
  ip address 10.255.0.3/32
  ip router ospf UNDERLAY area 0.0.0.0
interface Ethernet1/1
  no switchport
  mtu 9216
  ip address 192.0.2.1/31
  ip ospf network point-to-point
  ip ospf cost 40
  ip router ospf UNDERLAY area 0.0.0.0
  no shutdown
interface Ethernet1/2
  no switchport
  mtu 9216
  ip address 192.0.2.9/31
  ip ospf network point-to-point
  ip ospf cost 40
  ip router ospf UNDERLAY area 0.0.0.0
  no shutdown
feature bgp
nv overlay evpn
router bgp 65000
  router-id 10.255.0.3
  address-family l2vpn evpn
  neighbor 10.255.0.1
    remote-as 65000
    update-source loopback0
    address-family l2vpn evpn
      send-community
      send-community extended
  neighbor 10.255.0.2
    remote-as 65000
    update-source loopback0
    address-family l2vpn evpn
      send-community
      send-community extended
feature nv overlay
feature vn-segment-vlan-based
vlan 10
  vn-segment 10010
interface Ethernet1/3
  switchport
  switchport mode access
  switchport access vlan 10
  spanning-tree port type edge
  no shutdown
interface nve1
  no shutdown
  source-interface loopback0
  host-reachability protocol bgp
  member vni 10010
    ingress-replication protocol bgp
evpn
  vni 10010 l2
    rd auto
    route-target import 65000:10010
    route-target export 65000:10010

LEAF2

hostname LEAF2
feature ospf
router ospf UNDERLAY
  router-id 10.255.0.4
interface loopback0
  ip address 10.255.0.4/32
  ip router ospf UNDERLAY area 0.0.0.0
interface Ethernet1/1
  no switchport
  mtu 9216
  ip address 192.0.2.3/31
  ip ospf network point-to-point
  ip ospf cost 40
  ip router ospf UNDERLAY area 0.0.0.0
  no shutdown
interface Ethernet1/2
  no switchport
  mtu 9216
  ip address 192.0.2.11/31
  ip ospf network point-to-point
  ip ospf cost 40
  ip router ospf UNDERLAY area 0.0.0.0
  no shutdown
feature bgp
nv overlay evpn
router bgp 65000
  router-id 10.255.0.4
  address-family l2vpn evpn
  neighbor 10.255.0.1
    remote-as 65000
    update-source loopback0
    address-family l2vpn evpn
      send-community
      send-community extended
  neighbor 10.255.0.2
    remote-as 65000
    update-source loopback0
    address-family l2vpn evpn
      send-community
      send-community extended
feature nv overlay
feature vn-segment-vlan-based
vlan 10
  vn-segment 10010
interface Ethernet1/3
  switchport
  switchport mode access
  switchport access vlan 10
  spanning-tree port type edge
  no shutdown
interface nve1
  no shutdown
  source-interface loopback0
  host-reachability protocol bgp
  member vni 10010
    ingress-replication protocol bgp
evpn
  vni 10010 l2
    rd auto
    route-target import 65000:10010
    route-target export 65000:10010

LEAF3

hostname LEAF3
feature ospf
router ospf UNDERLAY
  router-id 10.255.0.5
interface loopback0
  ip address 10.255.0.5/32
  ip router ospf UNDERLAY area 0.0.0.0
interface Ethernet1/1
  no switchport
  mtu 9216
  ip address 192.0.2.5/31
  ip ospf network point-to-point
  ip ospf cost 40
  ip router ospf UNDERLAY area 0.0.0.0
  no shutdown
interface Ethernet1/2
  no switchport
  mtu 9216
  ip address 192.0.2.13/31
  ip ospf network point-to-point
  ip ospf cost 40
  ip router ospf UNDERLAY area 0.0.0.0
  no shutdown
feature bgp
nv overlay evpn
router bgp 65000
  router-id 10.255.0.5
  address-family l2vpn evpn
  neighbor 10.255.0.1
    remote-as 65000
    update-source loopback0
    address-family l2vpn evpn
      send-community
      send-community extended
  neighbor 10.255.0.2
    remote-as 65000
    update-source loopback0
    address-family l2vpn evpn
      send-community
      send-community extended
feature nv overlay
feature vn-segment-vlan-based
vlan 10
  vn-segment 10010
interface Ethernet1/3
  switchport
  switchport mode access
  switchport access vlan 10
  spanning-tree port type edge
  no shutdown
interface nve1
  no shutdown
  source-interface loopback0
  host-reachability protocol bgp
  member vni 10010
    ingress-replication protocol bgp
evpn
  vni 10010 l2
    rd auto
    route-target import 65000:10010
    route-target export 65000:10010

LEAF4

hostname LEAF4
feature ospf
router ospf UNDERLAY
  router-id 10.255.0.6
interface loopback0
  ip address 10.255.0.6/32
  ip router ospf UNDERLAY area 0.0.0.0
interface Ethernet1/1
  no switchport
  mtu 9216
  ip address 192.0.2.7/31
  ip ospf network point-to-point
  ip ospf cost 40
  ip router ospf UNDERLAY area 0.0.0.0
  no shutdown
interface Ethernet1/2
  no switchport
  mtu 9216
  ip address 192.0.2.15/31
  ip ospf network point-to-point
  ip ospf cost 40
  ip router ospf UNDERLAY area 0.0.0.0
  no shutdown
feature bgp
nv overlay evpn
router bgp 65000
  router-id 10.255.0.6
  address-family l2vpn evpn
  neighbor 10.255.0.1
    remote-as 65000
    update-source loopback0
    address-family l2vpn evpn
      send-community
      send-community extended
  neighbor 10.255.0.2
    remote-as 65000
    update-source loopback0
    address-family l2vpn evpn
      send-community
      send-community extended
feature nv overlay
feature vn-segment-vlan-based
vlan 10
  vn-segment 10010
interface Ethernet1/3
  switchport
  switchport mode access
  switchport access vlan 10
  spanning-tree port type edge
  no shutdown
interface nve1
  no shutdown
  source-interface loopback0
  host-reachability protocol bgp
  member vni 10010
    ingress-replication protocol bgp
evpn
  vni 10010 l2
    rd auto
    route-target import 65000:10010
    route-target export 65000:10010
HostAddressGateway
H110.10.10.11/24none
H210.10.10.12/24none
H310.10.10.13/24none
H410.10.10.14/24none

Use the independent unnumbered alternative

SPINE1 · replace the numbered link in the independent alternative

interface Ethernet1/1
  no ip address
  ip unnumbered loopback0
  ip ospf network point-to-point
  ip router ospf UNDERLAY area 0.0.0.0

All eight routed links in the alternative borrow their local device’s Loopback0 address. This saves the sixteen /31 interface addresses. Unique loopbacks remain advertised. The table above describes the numbered lab; it is not the addressing of the unnumbered alternative. This platform model supports one unnumbered routed adjacency per device pair; use a supported L3 port-channel design for parallel physical members between the same pair.

LEAF1 · illustrative output, selected fields
LEAF1# show ip ospf neighbors
Neighbor ID     State      Address       Interface
10.255.0.1     FULL/-     10.255.0.1   Eth1/1
10.255.0.2     FULL/-     10.255.0.2   Eth1/2
Complete independent lab configurations

Start with clean isolated lab devices. These are the complete scenario configurations, not changes to apply on top of a different lesson. Physical ports must match the address table. Virtual-image data-plane capabilities can differ from physical switches; these examples have not been executed on live devices.

Download configuration reference

SPINE1

hostname SPINE1
feature ospf
router ospf UNDERLAY
  router-id 10.255.0.1
interface loopback0
  ip address 10.255.0.1/32
  ip router ospf UNDERLAY area 0.0.0.0
interface Ethernet1/1
  no switchport
  mtu 9216
  ip unnumbered loopback0
  ip ospf network point-to-point
  ip ospf cost 40
  ip router ospf UNDERLAY area 0.0.0.0
  no shutdown
interface Ethernet1/2
  no switchport
  mtu 9216
  ip unnumbered loopback0
  ip ospf network point-to-point
  ip ospf cost 40
  ip router ospf UNDERLAY area 0.0.0.0
  no shutdown
interface Ethernet1/3
  no switchport
  mtu 9216
  ip unnumbered loopback0
  ip ospf network point-to-point
  ip ospf cost 40
  ip router ospf UNDERLAY area 0.0.0.0
  no shutdown
interface Ethernet1/4
  no switchport
  mtu 9216
  ip unnumbered loopback0
  ip ospf network point-to-point
  ip ospf cost 40
  ip router ospf UNDERLAY area 0.0.0.0
  no shutdown
feature bgp
nv overlay evpn
router bgp 65000
  router-id 10.255.0.1
  address-family l2vpn evpn
  neighbor 10.255.0.3
    remote-as 65000
    update-source loopback0
    address-family l2vpn evpn
      send-community
      send-community extended
      route-reflector-client
  neighbor 10.255.0.4
    remote-as 65000
    update-source loopback0
    address-family l2vpn evpn
      send-community
      send-community extended
      route-reflector-client
  neighbor 10.255.0.5
    remote-as 65000
    update-source loopback0
    address-family l2vpn evpn
      send-community
      send-community extended
      route-reflector-client
  neighbor 10.255.0.6
    remote-as 65000
    update-source loopback0
    address-family l2vpn evpn
      send-community
      send-community extended
      route-reflector-client

SPINE2

hostname SPINE2
feature ospf
router ospf UNDERLAY
  router-id 10.255.0.2
interface loopback0
  ip address 10.255.0.2/32
  ip router ospf UNDERLAY area 0.0.0.0
interface Ethernet1/1
  no switchport
  mtu 9216
  ip unnumbered loopback0
  ip ospf network point-to-point
  ip ospf cost 40
  ip router ospf UNDERLAY area 0.0.0.0
  no shutdown
interface Ethernet1/2
  no switchport
  mtu 9216
  ip unnumbered loopback0
  ip ospf network point-to-point
  ip ospf cost 40
  ip router ospf UNDERLAY area 0.0.0.0
  no shutdown
interface Ethernet1/3
  no switchport
  mtu 9216
  ip unnumbered loopback0
  ip ospf network point-to-point
  ip ospf cost 40
  ip router ospf UNDERLAY area 0.0.0.0
  no shutdown
interface Ethernet1/4
  no switchport
  mtu 9216
  ip unnumbered loopback0
  ip ospf network point-to-point
  ip ospf cost 40
  ip router ospf UNDERLAY area 0.0.0.0
  no shutdown
feature bgp
nv overlay evpn
router bgp 65000
  router-id 10.255.0.2
  address-family l2vpn evpn
  neighbor 10.255.0.3
    remote-as 65000
    update-source loopback0
    address-family l2vpn evpn
      send-community
      send-community extended
      route-reflector-client
  neighbor 10.255.0.4
    remote-as 65000
    update-source loopback0
    address-family l2vpn evpn
      send-community
      send-community extended
      route-reflector-client
  neighbor 10.255.0.5
    remote-as 65000
    update-source loopback0
    address-family l2vpn evpn
      send-community
      send-community extended
      route-reflector-client
  neighbor 10.255.0.6
    remote-as 65000
    update-source loopback0
    address-family l2vpn evpn
      send-community
      send-community extended
      route-reflector-client

LEAF1

hostname LEAF1
feature ospf
router ospf UNDERLAY
  router-id 10.255.0.3
interface loopback0
  ip address 10.255.0.3/32
  ip router ospf UNDERLAY area 0.0.0.0
interface Ethernet1/1
  no switchport
  mtu 9216
  ip unnumbered loopback0
  ip ospf network point-to-point
  ip ospf cost 40
  ip router ospf UNDERLAY area 0.0.0.0
  no shutdown
interface Ethernet1/2
  no switchport
  mtu 9216
  ip unnumbered loopback0
  ip ospf network point-to-point
  ip ospf cost 40
  ip router ospf UNDERLAY area 0.0.0.0
  no shutdown
feature bgp
nv overlay evpn
router bgp 65000
  router-id 10.255.0.3
  address-family l2vpn evpn
  neighbor 10.255.0.1
    remote-as 65000
    update-source loopback0
    address-family l2vpn evpn
      send-community
      send-community extended
  neighbor 10.255.0.2
    remote-as 65000
    update-source loopback0
    address-family l2vpn evpn
      send-community
      send-community extended
feature nv overlay
feature vn-segment-vlan-based
vlan 10
  vn-segment 10010
interface Ethernet1/3
  switchport
  switchport mode access
  switchport access vlan 10
  spanning-tree port type edge
  no shutdown
interface nve1
  no shutdown
  source-interface loopback0
  host-reachability protocol bgp
  member vni 10010
    ingress-replication protocol bgp
evpn
  vni 10010 l2
    rd auto
    route-target import 65000:10010
    route-target export 65000:10010

LEAF2

hostname LEAF2
feature ospf
router ospf UNDERLAY
  router-id 10.255.0.4
interface loopback0
  ip address 10.255.0.4/32
  ip router ospf UNDERLAY area 0.0.0.0
interface Ethernet1/1
  no switchport
  mtu 9216
  ip unnumbered loopback0
  ip ospf network point-to-point
  ip ospf cost 40
  ip router ospf UNDERLAY area 0.0.0.0
  no shutdown
interface Ethernet1/2
  no switchport
  mtu 9216
  ip unnumbered loopback0
  ip ospf network point-to-point
  ip ospf cost 40
  ip router ospf UNDERLAY area 0.0.0.0
  no shutdown
feature bgp
nv overlay evpn
router bgp 65000
  router-id 10.255.0.4
  address-family l2vpn evpn
  neighbor 10.255.0.1
    remote-as 65000
    update-source loopback0
    address-family l2vpn evpn
      send-community
      send-community extended
  neighbor 10.255.0.2
    remote-as 65000
    update-source loopback0
    address-family l2vpn evpn
      send-community
      send-community extended
feature nv overlay
feature vn-segment-vlan-based
vlan 10
  vn-segment 10010
interface Ethernet1/3
  switchport
  switchport mode access
  switchport access vlan 10
  spanning-tree port type edge
  no shutdown
interface nve1
  no shutdown
  source-interface loopback0
  host-reachability protocol bgp
  member vni 10010
    ingress-replication protocol bgp
evpn
  vni 10010 l2
    rd auto
    route-target import 65000:10010
    route-target export 65000:10010

LEAF3

hostname LEAF3
feature ospf
router ospf UNDERLAY
  router-id 10.255.0.5
interface loopback0
  ip address 10.255.0.5/32
  ip router ospf UNDERLAY area 0.0.0.0
interface Ethernet1/1
  no switchport
  mtu 9216
  ip unnumbered loopback0
  ip ospf network point-to-point
  ip ospf cost 40
  ip router ospf UNDERLAY area 0.0.0.0
  no shutdown
interface Ethernet1/2
  no switchport
  mtu 9216
  ip unnumbered loopback0
  ip ospf network point-to-point
  ip ospf cost 40
  ip router ospf UNDERLAY area 0.0.0.0
  no shutdown
feature bgp
nv overlay evpn
router bgp 65000
  router-id 10.255.0.5
  address-family l2vpn evpn
  neighbor 10.255.0.1
    remote-as 65000
    update-source loopback0
    address-family l2vpn evpn
      send-community
      send-community extended
  neighbor 10.255.0.2
    remote-as 65000
    update-source loopback0
    address-family l2vpn evpn
      send-community
      send-community extended
feature nv overlay
feature vn-segment-vlan-based
vlan 10
  vn-segment 10010
interface Ethernet1/3
  switchport
  switchport mode access
  switchport access vlan 10
  spanning-tree port type edge
  no shutdown
interface nve1
  no shutdown
  source-interface loopback0
  host-reachability protocol bgp
  member vni 10010
    ingress-replication protocol bgp
evpn
  vni 10010 l2
    rd auto
    route-target import 65000:10010
    route-target export 65000:10010

LEAF4

hostname LEAF4
feature ospf
router ospf UNDERLAY
  router-id 10.255.0.6
interface loopback0
  ip address 10.255.0.6/32
  ip router ospf UNDERLAY area 0.0.0.0
interface Ethernet1/1
  no switchport
  mtu 9216
  ip unnumbered loopback0
  ip ospf network point-to-point
  ip ospf cost 40
  ip router ospf UNDERLAY area 0.0.0.0
  no shutdown
interface Ethernet1/2
  no switchport
  mtu 9216
  ip unnumbered loopback0
  ip ospf network point-to-point
  ip ospf cost 40
  ip router ospf UNDERLAY area 0.0.0.0
  no shutdown
feature bgp
nv overlay evpn
router bgp 65000
  router-id 10.255.0.6
  address-family l2vpn evpn
  neighbor 10.255.0.1
    remote-as 65000
    update-source loopback0
    address-family l2vpn evpn
      send-community
      send-community extended
  neighbor 10.255.0.2
    remote-as 65000
    update-source loopback0
    address-family l2vpn evpn
      send-community
      send-community extended
feature nv overlay
feature vn-segment-vlan-based
vlan 10
  vn-segment 10010
interface Ethernet1/3
  switchport
  switchport mode access
  switchport access vlan 10
  spanning-tree port type edge
  no shutdown
interface nve1
  no shutdown
  source-interface loopback0
  host-reachability protocol bgp
  member vni 10010
    ingress-replication protocol bgp
evpn
  vni 10010 l2
    rd auto
    route-target import 65000:10010
    route-target export 65000:10010
HostAddressGateway
H110.10.10.11/24none
H210.10.10.12/24none
H310.10.10.13/24none
H410.10.10.14/24none

Repair the layer that failed

An ExStart/Exchange adjacency points toward MTU and database exchange checks; a missing adjacency calls for link, OSPF interface, area, authentication and timer checks. If neighbors are Full but a VTEP /32 is absent, inspect loopback advertisement and SPF state. Restore the matching MTU and OSPF interface statements, then validate the installed route and both endpoint directions. Do not use an MTU-ignore shortcut as proof that VXLAN packet sizes fit.

Further reading

Key Takeaways

  • OSPF carries VTEP loopback reachability in this underlay.
  • Point-to-point /31 links avoid subnet address waste and DR/BDR elections.
  • All six router IDs and loopbacks must remain unique.
  • Unnumbered links borrow local loopback addresses and use a different address model.
  • Full adjacency, installed route and tenant delivery are separate checks.

OSPF Underlay Quiz

Six questions · four options each · select one answer per question. Submit all answers to see your score and explanations.

1. What does OSPF advertise for VXLAN transport?
2. Why select OSPF point-to-point on these links?
3. How many /31 interface addresses do eight numbered links use?
4. What does an unnumbered interface borrow here?
5. Full adjacency but no remote VTEP route: what is useful to check?
6. Does mtu-ignore establish usable VXLAN MTU?