Building a Cisco VXLAN EVPN Fabric with NX-OSv – Part 1: Underlay Network & Interface Configuration (OSPF)

Introduction

In modern data center networks, scalability and flexibility are critical. Traditional Layer 2 designs struggle with limitations like spanning tree and VLAN constraints. This is where VXLAN (Virtual Extensible LAN) combined with EVPN (Ethernet VPN) comes in.

In this blog, we’ll walk through a Cisco NX-OSv VXLAN EVPN fabric, focusing specifically on the Underlay configuration:

Underlay routing core (OSPF)
EVPN control plane (BGP Route Reflector)
Multicast RP (Anycast RP for VXLAN BUM traffic)

Topology Overview

Spine01 and Spine02 connects to multiple leaf switches via Layer 3 links.
Each link uses a /31 subnet for efficient IP usage.
Loopback interfaces are used for:
- Router ID
- BGP peering
- VXLAN tunnel endpoints (in leafs)

1. Network Interfaces Configuration

The underlay provides IP reachability between all VTEPs.

Interfaces Configuration

Each interface toward leafs is configured as Layer 3:

Key Points:

no switchport → Enables Layer 3 mode
/31 addressing → Efficient point-to-point links
MTU 9216 → Required for VXLAN encapsulation
Repeat for all spine-to-leaf links

Spine01

hostname Spine01
interface ethernet 1/1
 no switchport
 ip address 10.1.1.0/31
 mtu 9216
 no shutdown
 
interface ethernet 1/2
 no switchport
 ip address 10.1.1.2/31
 mtu 9216
 no shutdown

interface ethernet 1/3
 no switchport
 ip address 10.1.1.4/31
 mtu 9216
 no shutdown

interface ethernet 1/4
 no switchport
 ip address 10.1.1.6/31
 mtu 9216
 no shutdown

Loopback Interface

interface loopback 0
 ip address 1.1.1.1/32
 no shutdown

Used for:

Router ID
BGP peering source

Spine02

hostname Spine02
interface ethernet 1/1
 no switchport
 ip address 10.1.2.0/31
 mtu 9216
 no shutdown
 
interface ethernet 1/2
 no switchport
 ip address 10.1.2.2/31
 mtu 9216
 no shutdown
 
interface ethernet 1/3
 no switchport
 ip address 10.1.2.4/31
 mtu 9216
 no shutdown
 
interface ethernet 1/4
 no switchport
 ip address 10.1.2.6/31
 mtu 9216
 no shutdown

Loopback Interface

interface loopback 0
 ip address 2.2.2.2/32
 no shutdown

Used for:

Router ID
BGP peering source

Leaf01

hostname Leaf01
interface ethernet 1/1
 no switchport
 ip address 10.1.1.1/31
 mtu 9216
 no shutdown

interface ethernet 1/2
 no switchport
 ip address 10.1.2.1/31
 mtu 9216
 no shutdown

Loopback Interface

interface loopback 0
 ip address 11.11.11.11/32
 no shutdown

Used for:

Router ID
BGP peering source

Leaf02

hostname Leaf02
interface ethernet 1/1
 no switchport
 ip address 10.1.1.3/31
 mtu 9216
 no shutdown

interface ethernet 1/2
 no switchport
 ip address 10.1.2.3/31
 mtu 9216
 no shutdown

Loopback Interface

interface loopback 0
 ip address 22.22.22.22/32
 no shutdown

Used for:

Router ID
BGP peering source

B-Leaf01

hostname B-Leaf01
interface ethernet 1/1
 no switchport
 ip address 10.1.1.5/31
 mtu 9216
 no shutdown

interface ethernet 1/2
 no switchport
 ip address 10.1.2.5/31
 mtu 9216
 no shutdown

Loopback Interface

interface loopback 0
 ip address 33.33.33.33/32
 no shutdown

Used for:

Router ID
BGP peering source

B-Leaf02

hostname B-Leaf02
interface ethernet 1/1
 no switchport
 ip address 10.1.1.7/31
 mtu 9216
 no shutdown

interface ethernet 1/2
 no switchport
 ip address 10.1.2.7/31
 mtu 9216
 no shutdown

Loopback Interface

interface loopback 0
 ip address 44.44.44.44/32
 no shutdown

Used for:

Router ID
BGP peering source

2. Underlay Network Configuration (OSPF)

Spine01

feature ospf
router ospf 1
 router-id 1.1.1.1