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# Troubleshooting Guide: Cisco NX-OS VXLAN EVPN
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This guide provides useful Cisco NX-OS commands for troubleshooting the spine-and-leaf VXLAN EVPN topology.
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This guide provides useful Cisco NX-OS commands for troubleshooting the spine-and-leaf VXLAN EVPN topology, as well as packet capture techniques for debugging network connectivity.
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## Table of Contents
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-[Packet Capture and Traffic Analysis](#packet-capture-and-traffic-analysis)
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-[BGP EVPN Control Plane](#bgp-evpn-control-plane)
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-[VXLAN Overlay](#vxlan-overlay)
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-[VLAN Configuration](#vlan-configuration)
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-[Port Configuration](#port-configuration)
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-[General System Information](#general-system-information)
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## Packet Capture and Traffic Analysis
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### Devstack Node - Monitoring ARP Traffic
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Monitor ARP traffic on the devstack node to troubleshoot connectivity between the overcloud and baremetal nodes.
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#### Monitor ARP on br-ex (OVS Bridge)
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```bash
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stack@devstack:~$ sudo tcpdump -i br-ex -envv arp
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```
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Shows ARP traffic on the OVS external bridge. You should see VLAN-tagged ARP requests from baremetal nodes coming in, and ARP replies from the router going out.
Shows ARP traffic on the physical trunk interface that connects to leaf01. This helps verify if packets are actually leaving/entering the physical interface with correct VLAN tags.
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**What to look for:**
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-**Ingress**: ARP requests from baremetal nodes should arrive with VLAN tags
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-**Egress**: ARP replies from router should leave with VLAN tags
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- If you see traffic on br-ex but NOT on trunk0, there's an OVS flow issue
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- If VLAN tags are present on trunk0 but missing when they arrive at the switch, there's an undercloud Neutron trunk issue
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#### Monitor All Traffic on trunk0
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```bash
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stack@devstack:~$ sudo tcpdump -i trunk0 -envv
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```
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Shows all traffic including STP, LLDP, and data packets. Useful for verifying physical connectivity.
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### Cisco Switch - Monitoring Traffic
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Monitor traffic on Cisco NX-OS switches using the built-in ethanalyzer tool.
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#### Monitor ARP on a Specific Interface
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```bash
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leaf01# ethanalyzer local interface front-panel eth1/3 display-filter arp
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leaf01# ethanalyzer local interface front-panel eth1/4 display-filter arp
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```
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Captures and displays ARP traffic on the specified front-panel interface (e.g., Ethernet1/3, Ethernet1/4).
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**Expected output:**
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```
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2026-03-17 23:16:05.641404 fa:16:3e:12:24:7c -> 22:dd:04:01:1b:08 ARP 10.0.5.1 is at fa:16:3e:12:24:7c
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```
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**What to check:**
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- Are ARP packets arriving at the switch?
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- Are VLAN tags present or stripped?
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- Is bidirectional ARP traffic visible (both requests and replies)?
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#### Monitor All Traffic on an Interface
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```bash
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leaf01# ethanalyzer local interface front-panel eth1/3
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```
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Shows all traffic including STP, ARP, and data packets. Press Ctrl+C to stop.
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#### Limit Number of Packets Captured
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```bash
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leaf01# ethanalyzer local interface front-panel eth1/3 limit-captured-frames 20
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```
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Captures only 20 frames and then stops automatically.
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#### Monitor Specific Protocol
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```bash
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# Monitor only ICMP traffic
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leaf01# ethanalyzer local interface front-panel eth1/3 display-filter icmp
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# Monitor only IPv4 traffic
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leaf01# ethanalyzer local interface front-panel eth1/3 display-filter ip
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```
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### Debugging VLAN Tag Issues
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If you suspect VLAN tags are being stripped or not applied correctly:
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1.**On devstack, capture on trunk0:**
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```bash
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sudo tcpdump -i trunk0 -envv 'vlan 103'
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```
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Verify that outbound traffic has VLAN tags
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2.**On leaf01, capture on the corresponding interface:**
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```bash
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ethanalyzer local interface front-panel eth1/4 display-filter arp
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```
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Check if the same traffic arrives with or without tags
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3.**Compare**:
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- If traffic leaves trunk0 tagged but arrives at the switch untagged → undercloud Neutron trunk issue
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- If traffic doesn't leave trunk0 at all → OVS flow issue on devstack
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- If traffic arrives tagged but isn't forwarded → switch VLAN configuration issue
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### Check MAC Learning on Switches
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After seeing ARP traffic, verify that the switches learned the MAC addresses:
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