Packet loss across different VLANs on the same core switch

Packet loss between VLANs on the same switch is often caused by misconfigurations in inter-VLAN routing, duplex mismatches, trunking issues, or hardware/software forwarding limitations.

Common Causes

1. Duplex and Speed Mismatches If a port on the switch or connected device is set to half-duplex while the other side is full-duplex, packet loss can occur, especially under higher traffic loads. This typically affects inter-VLAN traffic more noticeably than intra-VLAN traffic because inter-VLAN packets traverse the Layer 3 routing path, which is more sensitive to collisions and retransmissions . 2. Inter-VLAN Routing Issues When a Layer 3 switch performs inter-VLAN routing, misconfigurations can force packets to be routed via the CPU (slow path) instead of hardware-based Multilayer Switching (MLS). This can drastically reduce forwarding rates and cause packet drops between VLANs, while traffic within the same VLAN remains unaffected . 3. Trunk and VLAN Tagging Problems Incorrect trunk configuration or mismatched VLAN IDs can result in dropped packets. For example, if a trunk port does not allow a specific VLAN or if the native VLAN is inconsistent across switches, inter-VLAN traffic may be dropped. Similarly, in virtualized environments, mismatches between ESXi portgroups and physical switch VLAN tagging can lead to packet loss . 4. Overloaded or Oversubscribed Links High traffic on uplinks or inter-VLAN routing interfaces can cause congestion, leading to packet drops. This is more pronounced when multiple VLANs share the same physical interface or when the switch CPU is handling routing in software . 5. Physical Layer Issues Faulty cables, NICs, or connectors can cause unidirectional traffic or intermittent packet loss. Even if intra-VLAN traffic appears normal, inter-VLAN traffic may be affected if it traverses a problematic link .

Troubleshooting Steps

1. Check Port Duplex and Speed Settings – Ensure all ports, including uplinks and server connections, are set to the same duplex and speed, preferably auto-negotiation or explicitly full-duplex .
2. Verify VLAN and Trunk Configuration – Confirm that all VLANs are allowed on trunk ports, native VLANs match, and encapsulation types (802.1Q or ISL) are consistent .
3. Monitor Switch Counters – Look for errors, collisions, or drops on interfaces involved in inter-VLAN traffic .
4. Test Inter-VLAN Routing Path – Use ping and traceroute between VLANs to identify where packet loss occurs. If the CPU is handling routing, consider enabling hardware-based MLS or optimizing routing paths .
5. Inspect Physical Layer – Check cables, NICs, and switch ports for faults or intermittent connectivity issues .
6. Virtual Environment Checks – Ensure virtual portgroups and physical switch ports are configured compatibly for VLAN tagging, especially in ESXi or NSX environments .

Summary

Packet loss between VLANs on the same core switch is usually due to duplex mismatches, trunk misconfigurations, inter-VLAN routing via software, or physical layer issues. By systematically checking port settings, VLAN/trunk configurations, switch counters, and routing paths, you can isolate and resolve the problem while ensuring smooth inter-VLAN communication.