VXLAN

Mohsin Raza January 06, 2025

VXLAN

Virtual Extensible LAN in Cisco ACI is a network overlay protocol that enables scalable, flexible, and efficient Layer 2 connectivity across Layer 3 networks. Cisco ACI uses VXLAN as the underlying transport mechanism to provide connectivity between endpoints, allowing for application-centric policy enforcement and scalable multi-tenancy in modern data centers.

VXLAN
Fig 1:VXLAN

Here’s a detailed explanation of VXLAN in Cisco ACI:

1. Overview of VXLAN

  • VXLAN is a tunneling protocol that encapsulates Layer 2 Ethernet frames into Layer 3 UDP packets.
  • It uses a VXLAN Network Identifier (VNI), a 24-bit field, to identify Layer 2 segments (up to 16 million).
  • VXLAN provides the ability to extend Layer 2 segments across Layer 3 boundaries, enabling features like workload mobility and multi-tenancy.

2. Role of VXLAN in Cisco ACI

  • In ACI, VXLAN serves as the data plane protocol for transporting traffic across the fabric.
  • It eliminates the need for traditional VLAN-based networking, which is limited to 4096 VLANs.
  • VXLAN enables the ACI fabric to support large-scale multi-tenancy and overlay networks.

3. Key Components of VXLAN in ACI

  1. Endpoint:

    • Devices like servers or virtual machines connected to ACI leaf switches.

  2. Endpoint Group (EPG):

    • Logical groups of endpoints that share similar policies.

  3. Bridge Domain (BD):

    • A Layer 2 forwarding construct in ACI that maps to a VXLAN VNI.

  4. VXLAN VNI:

    • A unique identifier assigned to each bridge domain for traffic encapsulation.

  5. Tunnel Endpoint (VTEP):

    • VXLAN encapsulation/decapsulation occurs at the ACI fabric nodes (leaf or spine switches).
    • Each leaf switch functions as a VTEP.

  6. Multicast or Anycast:

    • In ACI, multicast is avoided by using a control plane-based learning mechanism instead of traditional flood-and-learn VXLAN.

4. VXLAN Encapsulation in ACI

  • Encapsulation Process:

    • When traffic enters the ACI fabric, the leaf switch encapsulates the original Ethernet frame in a VXLAN header.
    • The VXLAN header contains the VNI to identify the target bridge domain.

  • Decapsulation Process:

    • The receiving leaf switch decapsulates the VXLAN packet to extract the original Ethernet frame.

  • Transport Mechanism:

    • VXLAN packets are transported over a Layer 3 IP-based underlay network using UDP.

5. VXLAN Control Plane in ACI

Cisco ACI uses a COOP (Council of Orchestration Protocol) control plane for endpoint learning and VXLAN tunnel management, instead of traditional flood-and-learn techniques. Here's how it works:

  1. Endpoint Learning:

    • When a leaf detects an endpoint, it registers the endpoint information (MAC/IP) with spine switches using COOP.

  2. VTEP Address Resolution:

    • When a leaf switch needs to send traffic to a remote endpoint, it queries the spine switches to learn the VTEP (remote leaf) responsible for the target endpoint.

  3. Efficient Forwarding:

    • VXLAN tunnels are established between VTEPs (leaf switches) dynamically, based on endpoint communication requirements.

6. Benefits of VXLAN in ACI

  1. Scalability:

    • Supports up to 16 million logical networks with 24-bit VNI, compared to 4096 VLANs in traditional networks.

  2. Multi-Tenancy:

    • Logical isolation of tenants using separate bridge domains and VXLAN VNIs.

  3. Workload Mobility:

    • Seamless movement of workloads across the fabric without changing IP or MAC addresses.

  4. Reduced Flooding:

    • ACI eliminates traditional VXLAN flooding (e.g., ARP or broadcast) by using a centralized control plane.

  5. Dynamic Tunnels:

    • VXLAN tunnels are created dynamically between leaf switches as needed.

7. Monitoring VXLAN in ACI

  • APIC GUI:
    • Provides insights into VXLAN tunnels, VNIs, and endpoint mappings.
  • CLI Commands:
    • show vxlan encapsulation: Displays active VXLAN tunnels.
    • show endpoint: Lists learned endpoints and their associated VNIs.
  • Telemetry and Logs:
    • Tracks VXLAN-related events for troubleshooting.

8. VXLAN Use Cases in ACI

  1. Data Center Interconnect (DCI):

    • Extends ACI fabric across geographically distributed data centers.

  2. Hybrid Cloud Networking:

    • Connects on-premises ACI fabrics to cloud environments using VXLAN overlays.

  3. Microsegmentation:

    • Isolates workloads at the Layer 2 level using VNIs.

  4. Application Mobility:

    • Ensures uninterrupted application connectivity during migration.

9. Challenges Addressed by VXLAN in ACI

  • VLAN Limitations: Overcomes the 4096 VLAN ID limit.
  • Broadcast Flooding: Uses a control-plane approach to eliminate inefficient flooding.
  • Static Configuration: Automates tunnel creation and management.

VXLAN in Cisco ACI is not just a tunneling protocol but an integral part of the architecture that enables ACI's scalability, automation, and application-centric design. It forms the foundation for modern, flexible, and policy-driven networking in ACI environments.

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