Chapter 1: Setting Up the Underlay for EVPN VXLAN Fabric

Objective

This lab focuses on configuring the IP network underlay required for an EVPN VXLAN fabric. Participants will learn to establish a scalable and resilient underlay using OSPF as the Interior Gateway Protocol (IGP).

The lab covers critical aspects such as IP addressing strategies for point-to-point links and loopback interfaces, MTU considerations for VXLAN traffic, and OSPF configuration best practices in a spine-leaf topology.

By the end of this lab, participants will have a fully functional OSPF-based underlay, providing the necessary reachability for VTEP communication.

Key Features Implemented

• IP Addressing Strategy:

  • Loopback interface configuration for Router IDs (RIDs) and VTEP source interfaces.
  • Learner's choice of point-to-point link addressing:
    • /30 subnets
    • /31 subnets
    • IP Unnumbered

• MTU Configuration:

  • Setting appropriate MTU on fabric links to accommodate VXLAN overhead.

• OSPF as Underlay IGP:

  • Configuration of OSPF in a spine-leaf topology.
  • Utilizing OSPF point-to-point network type on inter-switch links.
  • Ensuring full reachability for all underlay infrastructure IPs.

Topology

This lab utilizes a standard spine-leaf Clos topology based on the provided diagram:

Configuration Tasks

Task 1: Configure hostnames on all switches.

(No specific example given, assume standard hostname configuration commands per vendor.)

Task 2: Loopback Interface Configuration

• Set up Loopback0 as the VTEP source on all leaf switches, using the format 10.200.200.xxx/32, where xxx corresponds to the Spine or Leaf number.
• Set up Loopback1 for OSPF Router ID on all switches, using the format 10.1.1.xxx/32, where xxx corresponds to the Spine or Leaf number.

Task 3: Point-to-Point Link IP Addressing (Learner's Choice)

Assign IP addresses to all spine-leaf physical links using subnets from the 10.1.1.0/24 network range, selecting one of the available methods for addressing.

• Option A: Using /30 Subnets
  • Example for LEAF-101 (Ethernet1/1) to SPINE-201 (Ethernet1/1): Network: 10.2.101.0/30
• Option B: Using /31 Subnets
  • More efficient for point-to-point links, providing 2 usable IPs. (RFC 3021)
• Option C: Using IP Unnumbered
  • Conserves IP addresses by "borrowing" an IP from a loopback interface.

Task 4: MTU Configuration

• VXLAN introduces about 50-54 bytes of overhead. To prevent fragmentation and accommodate potential jumbo frames, set the MTU to 9216 on all spine-leaf physical interfaces.

Task 5: OSPF Underlay Configuration

• Enable the OSPF feature.
• Configure the OSPF routing process named "UNDERLAY", and assign the Loopback1 IP address as the router ID on all switches.
• Enable OSPF on Loopback interfaces, advertising Loopback networks on all switches into OSPF Area 0.
• Enable OSPF on Spine-Leaf physical interfaces (Ethernet1/1 and Ethernet1/2, as applicable per device) and add them to OSPF Area 0, ensuring the OSPF network type is set to point-to-point.

Final Configurations

hostname LEAF-101

interface loopback0
  ip address 10.200.200.101/32
  ip router ospf UNDERLAY area 0.0.0.0

interface loopback1
  ip address 10.1.1.101/32
  ip router ospf UNDERLAY area 0.0.0.0

interface Ethernet1/1
  no switchport
  mtu 9216
  medium p2p
  ip unnumbered loopback1
  ip ospf network point-to-point
  ip router ospf UNDERLAY area 0.0.0.0
  no shutdown

interface Ethernet1/2
  no switchport
  mtu 9216
  medium p2p
  ip unnumbered loopback1
  ip ospf network point-to-point
  ip router ospf UNDERLAY area 0.0.0.0
  no shutdown

feature ospf

router ospf UNDERLAY
  router-id 10.1.1.101

hostname LEAF-102

interface loopback0
  ip address 10.200.200.102/32
  ip router ospf UNDERLAY area 0.0.0.0

interface loopback1
  ip address 10.1.1.102/32
  ip router ospf UNDERLAY area 0.0.0.0

interface Ethernet1/1
  no switchport
  mtu 9216
  medium p2p
  ip unnumbered loopback1
  ip ospf network point-to-point
  ip router ospf UNDERLAY area 0.0.0.0
  no shutdown

interface Ethernet1/2
  no switchport
  mtu 9216
  medium p2p
  ip unnumbered loopback1
  ip ospf network point-to-point
  ip router ospf UNDERLAY area 0.0.0.0
  no shutdown

feature ospf

router ospf UNDERLAY
  router-id 10.1.1.102

hostname SPINE-201

interface loopback1
  ip address 10.1.1.201/32
  ip router ospf UNDERLAY area 0.0.0.0

interface Ethernet1/1
  no switchport
  mtu 9216
  medium p2p
  ip unnumbered loopback1
  ip ospf network point-to-point
  ip router ospf UNDERLAY area 0.0.0.0
  no shutdown

interface Ethernet1/2
  no switchport
  mtu 9216
  medium p2p
  ip unnumbered loopback1
  ip ospf network point-to-point
  ip router ospf UNDERLAY area 0.0.0.0
  no shutdown

feature ospf

router ospf UNDERLAY
  router-id 10.1.1.201

hostname SPINE-202

interface loopback1
  ip address 10.1.1.202/32
  ip router ospf UNDERLAY area 0.0.0.0

interface Ethernet1/1
  no switchport
  mtu 9216
  medium p2p
  ip unnumbered loopback1
  ip ospf network point-to-point
  ip router ospf UNDERLAY area 0.0.0.0
  no shutdown

interface Ethernet1/2
  no switchport
  mtu 9216
  medium p2p
  ip unnumbered loopback1
  ip ospf network point-to-point
  ip router ospf UNDERLAY area 0.0.0.0
  no shutdown

feature ospf

router ospf UNDERLAY
  router-id 10.1.1.202

Verification Commands

• show ip interface brief || show interface Ethernet1/1
  • Verify IP Interfaces and MTU.
• show ip ospf neighbor
  • Verify OSPF Neighbors.
• show ip ospf interface brief || show ip ospf interface Ethernet1/1
  • Verify OSPF Interface Configuration. (Expected: Network Type should be P2P for spine-leaf links)
• show ip route || show ip route ospf
  • Verify OSPF Routing Table. Ensure all Loopback0 and Loopback1 IPs are learned.
• Ping Commands (for End-to-End Reachability)
  • Verify End-to-End Reachability (Critical for VTEPs). Ping between Loopback0 interfaces of LEAF-101 and LEAF-102.
  • Ping from any switch to any other switch's Loopback1.
  • Example: ping 10.200.200.102 source 10.200.200.101 (adjust IPs as per your config)
• show running-config ospf
  • Review OSPF Configuration.

Expected Outcomes

• All Spine and Leaf switches (SPINE-201, SPINE-202, LEAF-101, LEAF-102) have correct IP addresses configured.
• MTU on all spine-leaf physical interfaces (e.g., eth1/1, eth1/2 on relevant devices) is set to the configured jumbo value (9216).
• OSPF adjacencies are FULL between all directly connected Spine and Leaf switches.
• OSPF network type for all spine-leaf interfaces is POINT_TO_POINT.
• OSPF routing tables contain routes to all Loopback0 (VTEP source IPs) and Loopback1 (RIDs).
• Full IP reachability is confirmed between all Loopback0 interfaces and, critically, between Loopback1 interfaces on LEAF-101 and LEAF-102 (and other switch loopbacks).