Recently ran into an issue where some of the VMs on an ESX host didn’t have network connectivity. Checking out the virtual switch settings I saw the following:

~ # esxcfg-vswitch -l
Switch Name Num Ports Used Ports Configured Ports MTU Uplinks
vSwitch0 128 4 128 1500 vmnic0

PortGroup Name VLAN ID Used Ports Uplinks
VM Network 1 1 vmnic0
Management Network 50 1 vmnic0

From the above we can see that “VM Network” is the PortGroup setup for our VMs. One thing to notice is that we are actually Tagging this PortGroup with VLAN 1. We usually call this VST (Virtual Switch Tagging). VMware KB 1004074 actually describes VST in detail.

Next thing I wanted to check was the physical switch configuration. To do that we can use CDP (Cisco Discovery Protocol) to first find out what physical switch port we are connected to:

~ # vim-cmd hostsvc/net/query_networkhint
(vim.host.PhysicalNic.NetworkHint) [
(vim.host.PhysicalNic.NetworkHint) {
dynamicType = <unset>,
device = "vmnic0",
...
...
connectedSwitchPort = (vim.host.PhysicalNic.CdpInfo) {
dynamicType = <unset>,
cdpVersion = 0,
timeout = 0,
ttl = 169,
samples = 2982,
devId = "core_switch",
address = "192.168.9.18",
portId = "GigabitEthernet0/27",

More information regarding CDP can be seen in VMware KB 1007069. From the above output we can see that we were connected to “GigabitEthernet0/27”. So we logged into the switch to check out the configuration and we saw the following:

core_switch# show run int gi0/27
interface GigabitEthernet0/27
switchport trunk encapsulation dot1q
switchport mode trunk
switchport trunk allowed vlan 1,50,60,128
spanning-tree portfast trunk

So this was a trunk port and it’s allowing/expecting vlans 1,50,60,128 to be tagged. Now with VLAN 1 it’s a special case. From “Configuring EtherChannel and 802.1Q Trunking Between Catalyst L2 Fixed Configuration Switches and a Router (InterVLAN Routing)”:

Note: Native VLAN is the VLAN that you configure on the Catalyst interface before you configure the trunking on that interface. By default, all interfaces are in VLAN 1. Therefore, VLAN 1 is the native VLAN that you can change. On an 802.1Q trunk, all VLAN packets except the native VLAN are tagged

Since VLAN 1 is usually the default VLAN, when you trunk the default VLAN it will actually not tag that VLAN. So what we did was, remove the VLAN tag from our PortGroup, so it looked like this after wards:

~ # esxcfg-vswitch -l
Switch Name Num Ports Used Ports Configured Ports MTU Uplinks
vSwitch0 128 4 128 1500 vmnic0

PortGroup Name VLAN ID Used Ports Uplinks
VM Network 0 1 vmnic0
Management Network 50 1 vmnic0

So now it was kind of setup as EST (External Switch Tagging). More information regarding EST can be seen in VMware KB 1004127. After making that change the VMs were able to get online without any issue.


Published by Karim Elatov

13 September 2012

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