VXLAN/EVPN Connectivity Disrupted by Underlay Route Modification and Faulty Overlay ECMP Hardware Entry

In the ever-evolving landscape of data center networking, VXLAN (Virtual Extensible LAN) and EVPN (Ethernet VPN) have emerged as pivotal technologies for building scalable and flexible network architectures. However, as with any complex system, these technologies are not immune to disruptions and failures. This article delves deep into a specific scenario where VXLAN/EVPN connectivity is disrupted due to underlay route modification and faulty overlay ECMP (Equal-Cost Multi-Path) hardware entries. We will explore the intricacies of this issue, its potential causes, and the best practices for mitigation and resolution.

Understanding VXLAN and EVPN

Before we dive into the specific problem at hand, it’s crucial to establish a solid understanding of VXLAN and EVPN technologies and their roles in modern data center networks.

VXLAN (Virtual Extensible LAN)

VXLAN is a network virtualization technology that encapsulates Layer 2 Ethernet frames within Layer 3 UDP packets. This encapsulation allows for the extension of Layer 2 networks across Layer 3 boundaries, effectively overcoming the limitations of traditional VLANs in large-scale data center environments.

Key features of VXLAN include:

• 24-bit VXLAN Network Identifier (VNI), allowing for up to 16 million logical networks
• Overlay network that operates independently of the underlying physical network (underlay)
• Support for multi-tenancy and network segmentation
• Improved scalability and flexibility in network design

EVPN (Ethernet VPN)

EVPN is a control plane protocol that works in conjunction with VXLAN to provide efficient and scalable Layer 2 and Layer 3 network virtualization. It uses MP-BGP (Multiprotocol Border Gateway Protocol) for distributing MAC and IP address information across the network.

Key benefits of EVPN include:

• Unified control plane for both Layer 2 and Layer 3 services
• Efficient handling of multi-destination traffic
• Support for multi-homing and load balancing
• Reduced flooding and improved network efficiency

The Anatomy of VXLAN/EVPN Connectivity

To understand how underlay route modifications and faulty overlay ECMP hardware entries can disrupt VXLAN/EVPN connectivity, we need to examine the components and processes involved in establishing and maintaining this connectivity.

Underlay Network

The underlay network in a VXLAN/EVPN architecture is the physical IP network that provides connectivity between VXLAN Tunnel Endpoints (VTEPs). Typically, this is an IP fabric built using protocols such as OSPF, IS-IS, or BGP. The stability and performance of the underlay network are crucial for the proper functioning of the VXLAN overlay.

Overlay Network

The overlay network is the logical network created by VXLAN encapsulation. It operates on top of the underlay and is responsible for carrying the encapsulated traffic between VTEPs. The overlay network is managed by EVPN, which uses MP-BGP to distribute reachability information for MAC and IP addresses.

ECMP (Equal-Cost Multi-Path)

ECMP is a routing strategy that allows for the use of multiple best paths to a destination. In VXLAN/EVPN networks, ECMP is often used in both the underlay and overlay to provide load balancing and improve network utilization.

The Problem: Connectivity Disruption

Now that we have established the foundational concepts, let’s examine the specific problem of VXLAN/EVPN connectivity disruption caused by underlay route modification and faulty overlay ECMP hardware entries.

Scenario Description

In this scenario, a data center network experiences intermittent connectivity issues between VXLAN segments. Upon investigation, it is discovered that the problem is twofold:

1. Underlay route modifications are causing instability in the physical network.
2. Faulty overlay ECMP hardware entries are leading to incorrect forwarding decisions in the VXLAN overlay.

Impact on Network Performance

The combination of these issues results in the following symptoms:

• Intermittent packet loss between VXLAN segments
• Increased latency for inter-VXLAN traffic
• Inconsistent load balancing across available paths
• Potential service disruptions for applications relying on VXLAN connectivity

Root Cause Analysis

To effectively address the connectivity disruption, it’s essential to perform a thorough root cause analysis of both the underlay route modification and the faulty overlay ECMP hardware entries.

Underlay Route Modification

The instability in the underlay network can be attributed to several factors:

• Frequent route flaps due to misconfigured timers or unstable links
• Incorrect route advertisements caused by misconfigured routing policies
• Hardware failures or software bugs in underlay devices
• Suboptimal routing design leading to unnecessary route churn

These issues can result in constant updates to the routing tables, causing temporary loss of connectivity and increased CPU