Test Jumbo Frames (MTU 9000) between ESXi hosts

When we want to enable Jumbo-Fames on VMware vSphere, it must be enabled on

• physical switches
• virtual switches - VMware Distributed Switch (VDS) or VMware Standard Switch (VSS)
• VMkernel interfaces where you would like to use Jumbo-Frames (typically NFS, iSCSI, NVMeoF, vSAN, vMotion)

Let's assume it is configured by network and vSphere administrators and we want to validate that vMotion network between two ESXi hosts supports Jumbo Frames. Let's say we have these two ESXi hosts

• ESX11 has IP address 10.160.22.111 on vMotion vmk interface within vMotion TCP/IP stack. 
• ESX12 has IP address 10.160.22.112 on vMotion vmk interface within vMotion TCP/IP stack.

Ping is a good network diagnostic tool for this purpose. It uses ICMP protocol within IP protocol. So, what is the maximum size of IP/ICMP packet? With Jumbo Frame we have 9000 bytes for Layer 2 (Ethernet Frame) payload and within Ethernet Frame is IP packet cariing ICMP packet with Echo Request. So, here is the calculation. 

9000 (MTU) - 20 (IP header) - 8 (ICMP header) = 8972 bytes

 

Let's do ping with 8972 bytes payload and with flag -d (fragmentation disabled).

[root@esx11:~] ping -I vmk1 -S vmotion -s 8972 -d 10.160.22.112
PING 10.160.22.112 (10.160.22.112): 8972 data bytes
8980 bytes from 10.160.22.112: icmp_seq=0 ttl=64 time=0.770 ms
8980 bytes from 10.160.22.112: icmp_seq=1 ttl=64 time=0.637 ms
8980 bytes from 10.160.22.112: icmp_seq=2 ttl=64 time=0.719 ms

We can see succesful test of large ICMP packets without fragmentation. We validated that ICMP packets with size 8972 bytes can be transfered over the network without fragmentation. That's the indication that Jumbo Frames (MTU 9000) are enabled end-to-end.

Now let's try to cary ICMP packets with size 8973 bytes.

[root@esx11:~] ping -I vmk1 -S vmotion -s 8973 -d 10.160.22.112
PING 10.160.22.112 (10.160.22.112): 8973 data bytes
sendto() failed (Message too long)
sendto() failed (Message too long)
sendto() failed (Message too long)

We can see that ICMP packets with size 8973 bytes cannot be transfered over the network without fragmentation. This is expected behavior and it proves that we know what we do.