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 Spine01 configuration. This spine acts as:

• 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