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

IS-IS Underlay

Carry VTEP reachability with IS-IS, 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 · isis-underlayIllustrative topology
Physical topology of isis-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 · Level 2 IS-IS and a numbered link

feature isis
router isis UNDERLAY
  net 49.0001.0000.0000.0001.00
  is-type level-2
  log-adjacency-changes
interface loopback0
  ip address 10.255.0.1/32
  ip router isis UNDERLAY
interface Ethernet1/1
  no switchport
  mtu 9216
  ip address 192.0.2.0/31
  isis network point-to-point
  ip router isis UNDERLAY
  no shutdown

All six switches use one Level 2 topology and unique system IDs. The NET’s system ID distinguishes each device; its final selector is 00. IS-IS establishes its link adjacency independently of the IPv4 subnet, then advertises IPv4 reachability for the VTEP loopbacks. The actual forwarding packets are still IP/UDP VXLAN, not IS-IS PDUs.

Verify adjacency and equal-cost reachability

LEAF1 · illustrative output, selected fields
LEAF1# show isis adjacency
System ID       Type Interface State
0000.0000.0001  L2   Eth1/1   UP
0000.0000.0002  L2   Eth1/2   UP
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, [115/21], isis-UNDERLAY, L2
    *via 192.0.2.8, Eth1/2, [115/21], isis-UNDERLAY, L2

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 isis
router isis UNDERLAY
  net 49.0001.0000.0000.0001.00
  is-type level-2
  log-adjacency-changes
interface loopback0
  ip address 10.255.0.1/32
  ip router isis UNDERLAY
interface Ethernet1/1
  no switchport
  mtu 9216
  ip address 192.0.2.0/31
  isis network point-to-point
  ip router isis UNDERLAY
  no shutdown
interface Ethernet1/2
  no switchport
  mtu 9216
  ip address 192.0.2.2/31
  isis network point-to-point
  ip router isis UNDERLAY
  no shutdown
interface Ethernet1/3
  no switchport
  mtu 9216
  ip address 192.0.2.4/31
  isis network point-to-point
  ip router isis UNDERLAY
  no shutdown
interface Ethernet1/4
  no switchport
  mtu 9216
  ip address 192.0.2.6/31
  isis network point-to-point
  ip router isis UNDERLAY
  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 isis
router isis UNDERLAY
  net 49.0001.0000.0000.0002.00
  is-type level-2
  log-adjacency-changes
interface loopback0
  ip address 10.255.0.2/32
  ip router isis UNDERLAY
interface Ethernet1/1
  no switchport
  mtu 9216
  ip address 192.0.2.8/31
  isis network point-to-point
  ip router isis UNDERLAY
  no shutdown
interface Ethernet1/2
  no switchport
  mtu 9216
  ip address 192.0.2.10/31
  isis network point-to-point
  ip router isis UNDERLAY
  no shutdown
interface Ethernet1/3
  no switchport
  mtu 9216
  ip address 192.0.2.12/31
  isis network point-to-point
  ip router isis UNDERLAY
  no shutdown
interface Ethernet1/4
  no switchport
  mtu 9216
  ip address 192.0.2.14/31
  isis network point-to-point
  ip router isis UNDERLAY
  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 isis
router isis UNDERLAY
  net 49.0001.0000.0000.0003.00
  is-type level-2
  log-adjacency-changes
interface loopback0
  ip address 10.255.0.3/32
  ip router isis UNDERLAY
interface Ethernet1/1
  no switchport
  mtu 9216
  ip address 192.0.2.1/31
  isis network point-to-point
  ip router isis UNDERLAY
  no shutdown
interface Ethernet1/2
  no switchport
  mtu 9216
  ip address 192.0.2.9/31
  isis network point-to-point
  ip router isis UNDERLAY
  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 isis
router isis UNDERLAY
  net 49.0001.0000.0000.0004.00
  is-type level-2
  log-adjacency-changes
interface loopback0
  ip address 10.255.0.4/32
  ip router isis UNDERLAY
interface Ethernet1/1
  no switchport
  mtu 9216
  ip address 192.0.2.3/31
  isis network point-to-point
  ip router isis UNDERLAY
  no shutdown
interface Ethernet1/2
  no switchport
  mtu 9216
  ip address 192.0.2.11/31
  isis network point-to-point
  ip router isis UNDERLAY
  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 isis
router isis UNDERLAY
  net 49.0001.0000.0000.0005.00
  is-type level-2
  log-adjacency-changes
interface loopback0
  ip address 10.255.0.5/32
  ip router isis UNDERLAY
interface Ethernet1/1
  no switchport
  mtu 9216
  ip address 192.0.2.5/31
  isis network point-to-point
  ip router isis UNDERLAY
  no shutdown
interface Ethernet1/2
  no switchport
  mtu 9216
  ip address 192.0.2.13/31
  isis network point-to-point
  ip router isis UNDERLAY
  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 isis
router isis UNDERLAY
  net 49.0001.0000.0000.0006.00
  is-type level-2
  log-adjacency-changes
interface loopback0
  ip address 10.255.0.6/32
  ip router isis UNDERLAY
interface Ethernet1/1
  no switchport
  mtu 9216
  ip address 192.0.2.7/31
  isis network point-to-point
  ip router isis UNDERLAY
  no shutdown
interface Ethernet1/2
  no switchport
  mtu 9216
  ip address 192.0.2.15/31
  isis network point-to-point
  ip router isis UNDERLAY
  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 · corresponding unnumbered transport interface

interface Ethernet1/1
  no ip address
  ip unnumbered loopback0
  medium p2p
  isis network point-to-point
  ip router isis UNDERLAY

Apply the same model to every transport link in the separate alternative recipe. IS-IS’s link-layer adjacency does not require the link endpoints to have distinct /31 IPv4 subnets, but the platform must support its unnumbered IP-forwarding model. Advertise the unique Loopback0 addresses and verify the resolved next-hop adjacency; do not infer packet forwarding from an UP IS-IS adjacency alone.

LEAF1 · illustrative output, selected fields
LEAF1# show ip route 10.255.0.6
10.255.0.6/32, ubest/mbest: 2/0
    *via 10.255.0.1, Eth1/1, isis-UNDERLAY, L2
    *via 10.255.0.2, Eth1/2, isis-UNDERLAY, L2
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 isis
router isis UNDERLAY
  net 49.0001.0000.0000.0001.00
  is-type level-2
  log-adjacency-changes
interface loopback0
  ip address 10.255.0.1/32
  ip router isis UNDERLAY
interface Ethernet1/1
  no switchport
  mtu 9216
  ip unnumbered loopback0
  medium p2p
  isis network point-to-point
  ip router isis UNDERLAY
  no shutdown
interface Ethernet1/2
  no switchport
  mtu 9216
  ip unnumbered loopback0
  medium p2p
  isis network point-to-point
  ip router isis UNDERLAY
  no shutdown
interface Ethernet1/3
  no switchport
  mtu 9216
  ip unnumbered loopback0
  medium p2p
  isis network point-to-point
  ip router isis UNDERLAY
  no shutdown
interface Ethernet1/4
  no switchport
  mtu 9216
  ip unnumbered loopback0
  medium p2p
  isis network point-to-point
  ip router isis UNDERLAY
  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 isis
router isis UNDERLAY
  net 49.0001.0000.0000.0002.00
  is-type level-2
  log-adjacency-changes
interface loopback0
  ip address 10.255.0.2/32
  ip router isis UNDERLAY
interface Ethernet1/1
  no switchport
  mtu 9216
  ip unnumbered loopback0
  medium p2p
  isis network point-to-point
  ip router isis UNDERLAY
  no shutdown
interface Ethernet1/2
  no switchport
  mtu 9216
  ip unnumbered loopback0
  medium p2p
  isis network point-to-point
  ip router isis UNDERLAY
  no shutdown
interface Ethernet1/3
  no switchport
  mtu 9216
  ip unnumbered loopback0
  medium p2p
  isis network point-to-point
  ip router isis UNDERLAY
  no shutdown
interface Ethernet1/4
  no switchport
  mtu 9216
  ip unnumbered loopback0
  medium p2p
  isis network point-to-point
  ip router isis UNDERLAY
  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 isis
router isis UNDERLAY
  net 49.0001.0000.0000.0003.00
  is-type level-2
  log-adjacency-changes
interface loopback0
  ip address 10.255.0.3/32
  ip router isis UNDERLAY
interface Ethernet1/1
  no switchport
  mtu 9216
  ip unnumbered loopback0
  medium p2p
  isis network point-to-point
  ip router isis UNDERLAY
  no shutdown
interface Ethernet1/2
  no switchport
  mtu 9216
  ip unnumbered loopback0
  medium p2p
  isis network point-to-point
  ip router isis UNDERLAY
  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 isis
router isis UNDERLAY
  net 49.0001.0000.0000.0004.00
  is-type level-2
  log-adjacency-changes
interface loopback0
  ip address 10.255.0.4/32
  ip router isis UNDERLAY
interface Ethernet1/1
  no switchport
  mtu 9216
  ip unnumbered loopback0
  medium p2p
  isis network point-to-point
  ip router isis UNDERLAY
  no shutdown
interface Ethernet1/2
  no switchport
  mtu 9216
  ip unnumbered loopback0
  medium p2p
  isis network point-to-point
  ip router isis UNDERLAY
  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 isis
router isis UNDERLAY
  net 49.0001.0000.0000.0005.00
  is-type level-2
  log-adjacency-changes
interface loopback0
  ip address 10.255.0.5/32
  ip router isis UNDERLAY
interface Ethernet1/1
  no switchport
  mtu 9216
  ip unnumbered loopback0
  medium p2p
  isis network point-to-point
  ip router isis UNDERLAY
  no shutdown
interface Ethernet1/2
  no switchport
  mtu 9216
  ip unnumbered loopback0
  medium p2p
  isis network point-to-point
  ip router isis UNDERLAY
  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 isis
router isis UNDERLAY
  net 49.0001.0000.0000.0006.00
  is-type level-2
  log-adjacency-changes
interface loopback0
  ip address 10.255.0.6/32
  ip router isis UNDERLAY
interface Ethernet1/1
  no switchport
  mtu 9216
  ip unnumbered loopback0
  medium p2p
  isis network point-to-point
  ip router isis UNDERLAY
  no shutdown
interface Ethernet1/2
  no switchport
  mtu 9216
  ip unnumbered loopback0
  medium p2p
  isis network point-to-point
  ip router isis UNDERLAY
  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

For a missing IS-IS adjacency, inspect Level 2 agreement, system-ID uniqueness, link type, interface activation and authentication. If adjacency is UP but the remote /32 is missing, inspect the link-state database and loopback participation. Repair the intended NET/interface statements, then verify the IPv4 route, NVE peer and host exchange. Repeatedly assigning the same NET on all devices is not a shortcut for placing them in one fabric.

Further reading

Key Takeaways

  • IS-IS builds the underlay topology and advertises IPv4 VTEP reachability.
  • System IDs must be unique even within the same area design.
  • This example uses Level 2 and point-to-point routed links.
  • Unnumbered IPv4 forwarding is a platform capability, not an inference from IS-IS adjacency.
  • Verify adjacency, route resolution and overlay delivery independently.

IS-IS Underlay Quiz

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

1. What must be unique in each NET?
2. Which level is configured?
3. Does IS-IS transport tenant Ethernet frames itself in this lab?
4. What still needs verification after an UP adjacency?
5. What is removed from the unnumbered physical link?
6. Can all six devices use the same NET safely?