vmnic displays MTU 1500 (or 0) instead of 9000 during MTU 9000 setup in ESXi

Products

• VMware vCenter Server
• VMware vSphere ESXi

Issue/Introduction

Symptoms:

CDP information shows that MTU configured is 1500 (or sometimes 0) while physical switch port is configured with MTU 9000. 

Environment

• VMware ESXi Server 7.x
• VMware ESXi Server 8.x

Resolution

To validate the MTU configuration:

• Confirm with esxcfg-nics -l that cards are on MTU 9000.

Use # pktcap-uw --uplink vmnicX --dstmac 01:00:0c:cc:cc:cc --capture UplinkRcvKernel -o /vmfs/volumes/<datastore>/<folder name>/vmnicXCDP.pcap to capture CDP packets.

• Review in Wireshark to confirm information provided by CDP.
• If further investigation is needed, contact Cisco.

Additional Information

• Host-to-Host Ping Tests (Same Chassis): Perform a vmkping -I <vmk#> <destination host IP> -d -s 8972 between ESXi hosts within the same UCS chassis to verify intra-chassis Jumbo Frame connectivity.
• Host-to-Host Ping Tests (Different Chassis): Extend tests to hosts in different UCS chassis (if applicable) within the same UCS domain to verify inter-chassis Jumbo Frame connectivity.
• Inter-UCS Domain Ping Tests (If Applicable): Perform vmkping -I <vmk#> <destination host IP> -d -s 8972  between hosts in different UCS domains to confirm end-to-end Jumbo Frame support across domains.

For more information, refer to the Cisco documentation below.

Configure UCS with VMware ESXi End-to-End Jumbo MTU