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NSX-T Transport VLANs and Inter TEP Communication | LAB2PROD

NSX-T Transport VLANs and Inter TEP Communication | LAB2PROD

Inter TEP Communication, Sharing Transport VLANs: Unpacking One of The Latest Features In NSX-T 3.1

This post will include a brief recap of how Tunnel Endpoints  (TEP) and Transport VLANs have had to be configured for communication in prior releases of NSX-T. It will then focus on how the release of NSX-T 3.1 has changed the way Tunnel Endpoints for Edge Appliances that reside on NSX-T hosts can be configured, and how inter TEP communication can be achieved.

Table of Contents

  • What are TEPs and Transport VLANs?
  • How they communicate: Both host to host and host to Edge Appliances
  • How TEPs need to be configured when the Edge Appliances reside on a host transport node in releases prior to NSX-T 3.1
  • A streamlined method to TEP configuration NSX-T 3.1

Note: All screenshots in this article have been taken in an NSX-T 3.1 environment.

The video below shows Tunnel Endpoint and Logical Routing operations, it also walks through some troubleshooting.

NSX-T Tunnel Endpoints (TEPs) and Logical Routing // DEEP DIVE!! // DC VMUG 2022

This is a demonstration of NSX-T Tunnel Endpoints (TEPs) and Logical Routing, delivered live at DC V...

What are Tunnel Endpoints (TEPs) and Transport VLANs?

TEP’s are a critical component of NSX-T, each hypervisor participating in NSX-T (known as a host transport node) and Edge Appliance will have at minimum one TEP IP address assigned to it.  The host transport nodes and Edge Appliances may have two for load balancing traffic, this is generally referred to as multi-TEP.  

Transport VLANs is just what NSX-T calls the VLAN used for the TEPs, these are mapped using profiles and then attached to the host transport nodes and edge nodes.

TEP IP Assignment:

  • Host transport node TEP IP addresses can be assigned to hosts statically, using DHCP or using predefined IP Pools in NSX-T.
  • Edge Appliances can have their TEP IP addresses assigned statically or using a pool.

TEP’s are the source and destination IP addresses used in the IP packet for communication between host transport nodes and Edge Appliances.  Without correct design, you will more than likely have communication issues between your host transport nodes participating in NSX-T and your Edge Appliances, this will lead to the GENEVE tunnel between endpoints being down.  If your GENEVE tunnels are down you will NOT have any communication between the endpoints on either side of the downed tunnel.

management host nsxcli get logical-switch mac-table to show the inner mac outer mac and outer ip
The below images show the mac address table for a logical switch on a host transport node.

Notice how here we have a column for Inner MAC, Outer MAC and Outer IP under the LCP section, also how the MAC addresses have been spread across the two TEP IPs. If we dig a little deeper, we can start by identifying what exactly those inner MACs are, where they came from and then repeat the same for the outer MAC.

management host nsxcli get logical-switch to identify the switch
Let’s start by identifying the logical switch in question.

Now that we have the logical switch’s name and UUID we can dig a little deeper.

nsx management host nsxcli get logical-switch arp-table no entries
There are a couple of ways to do this, but since we have the name of the logical switch, I will continue to use nsxcli.  The next command we can run is ‘get logical-switch 6b151015-ab77-46bc-b4bd-62281ee7d6ec arp-table’. Change the UUID to suit your environment.

Wait a second.. why are there no entries, especially since we know there is workload on this segment.  Chances are, either the Virtual Machine (VM) has not sent an Address Resolution Protocol (ARP) request or it had and the table has since been flushed.  We can repopulate this table by simply initiating a ping from a VM on this segment.

The below image shows the same table after I initiated a ping from a VM on the segment.

nsx management host get logical-switch arp-table with entries

Excellent, now we have an entry in the table, if we scroll up to the first image you can now see that the inner MAC is the same as the MAC address from this entry.  So that means the inner MAC is the source VM’s vmnic’s MAC address.

That leaves us having to identify the remaining two outer MAC’s, this is pretty easy to do.  There are a couple of ways to do this, but let’s continue with nsxcli and the logical switch we have been working with.

nsxcli management host get logical-switch vtep-table showing vtep ip and vtep mac mapping

This time we can run ‘get logical-switch  6b151015-ab77-46bc-b4bd-62281ee7d6ec vtep-table’, you should see output similar to the below image.

management host esxcfg-vmknic -l showing nsx-t vmkernel

The above output may not be enough to determine which outer MAC belongs to each host transport node. To verify this, you can exit out of nsxcli, type in ‘esxcfg-vmknic -l’, and see the below output. Here we can see the outer MAC’s are the MAC addresses of the vmkernel nics used for NSX-T, vmk10 and vmk11. Therefore, we can deduce that payload from within the NSX-T domain is encapsulated in a packet header that uses the source and destination TEP IP and MAC addresses. 

How TEPs Communicate?

NSX-T TEPs communicate with each other by using their source and destination addresses.  The TEPs can be in Transport VLANs (different subnets) and still communicate, if this is part of your requirements then you must ensure those Transport VLANs (subnets) are routable and can communicate.  

 NSX-T TEPs use GENEVE encapsulation and communicate over UDP port 6081, as a bare minimum during the design of NSX-T for your environment you must ensure this port is open.  Keep in mind this article covers the TEP networks (Transport VLANs), there are still other ports that need to be opened for NSX-T to function as intended, these ports are documented here VMware Ports: NSX-T Datacenter. You must also ensure that you have your MTU configured in your physical fabric with a minimum of 1600 bytes, however the VMware Validated Design recommends setting the MTU to 9000.  This needs to be set end to end in your physical environment, that is, where ever you have NSX-T enabled hosts and edge appliances running. If this is not met, you will end up with packet fragmentation and this will likely cause your tunnels to remain down and/or other unwanted behaviour.
Below is an example of an IP packet that is being sent from one host transport node to the other using nsxcli on the host.  You can perform a similar action in your environment by running ‘start capture interface vmnic4 direction dual file capture expression srcip′
nsxcli capture to show tep communication across the transport VLAN and tunnel is up
The example below is the same output from above, however outputting the capture to a file and opened in Wireshark for easier reading.

Once the packet is decapsulated the payload is exposed and actioned accordingly.  This article focusses on TEPs and will not go into logical routing and switching of payload data.

Below is the ARP table from the top of rack switch, here you can see it has learnt the TEP IP addresses from the hosts, the MAC addresses listed are the MAC addresses of the hosts vmkernel ports or the edge appliances vmnic.
Top of rack switch arp table to show host and edge TEPs
For those that like visuals, this is a quick diagram of how the hosts are wired and a topology of the lab that will be used in this article.

diagram showing host transport nodes and edge transport nodes wired together with the top of rack switch
The above has demonstrated how hosts communicate via their TEP IP addresses, however this behaviour is the same for host to edge communication as well.

How TEPs need to be configured when Edge Appliances reside on a host transport node in prior releases NSX-T 3.1

In prior releases of NSX-T when deploying either a fully collapsed cluster (host/compute/edge) or compute and edge cluster, and you were running a single N-VDS, it was a requirement to have the NSX-T Edge TEPs (Transport VLAN) on a different subnet to the host transport nodes Transport VLAN. If you had more than 2 physical NICs and had a vDS for all management traffic and edges plumbed into it and NSX-T had a dedicated set of NICs for it’s N-VDS you could utilize the same Transport VLAN for host transport node TEPs and edge appliances. 
The IP packets had to be routed from the host transport nodes to the Edge appliance and vice versa, that is the GENEVE packets had to ingress/egress on a physical uplink from the host as there was no datapath within the host for the GENEVE tunnels.  
If this did not occur, the packets were dropped and this was due to the fact that the packets are sent internally to the transport node with the same VLAN ID (Transport VLAN) unencapsulated and subsequently dropped.  
Note: In NSX-T 3.1 inter TEP communication will not work if you use a vDS port group.  In order for inter TEP communication to work in this release, you must use a VLAN-backed segment created in NSX-T.
The next set of images will show an example of the port group configuration and of two packet captures, one on a host transport node and the other on the Edge Appliance. In this scenario, the edge is configured with two vDS trunk portgroups and it’s TEP IP addresses are in the same Transport VLAN as the host transport nodes.
image showing nsx-t edge transport nodes uplink portgroups for correct wiring

edge node vrf 0 get interfaces to show tep addresses

This is a screenshot of the edge appliances TEP IP interfaces.

Host transport node with TEP IP’s in the same range.

nsx management host esxcfg-vmknic -l showing host tep addresses and interfaces

vCenter view showing the Edge appliances are in fact sitting on the management host which is prepared for NSX-T.

vCenter view showing edge nodes sitting on management host prepped for nsx

Now you should have a clearer picture of how the environment is set up. The next part of the article will show you that with this setup, inter TEP communication does not work, i.e. the Edge Appliance sits on a host transport node and both are on the same Transport VLAN, and the packet is not routed externally and brought back into the edge.

Provided below is a packet capture from the host transport node, here you can see a packet being sent from TEP IP to (Edge TEP IP).
management host nsxcli packet capture to show geneve tunnel using same transport vlan and vds portgroup
Provided below is another packet capture on the Edge appliance,if you want to single out a TEP interface, SSH into the appliance as admin, type in vrf 0 and then get interfaces. In the list, there is a field called interface. Copy the interface ID of the one you wish to run the capture on. The command to use is “start capture interface <id> direction dual file <filename>”.
nsx-t edge node packet capture showing geneve tunnel bfd messages

This is a WireShark view of the same to make reading easier.

wireshark view of the edge packet capture nsx-t geneve bfd tunnel status
Here is what you will see in the NSX-T Manager UI.
nsx-t manager ui showing tunnel status down when using the same tep vlan and vds portgroup

Testing the same configuration with the Edge Appliances in a different Transport VLAN

I won’t go into changing the uplink profile and updating the IP settings on the node itself, but in the below screenshot you can see the addresses for the TEP interface on en1 have changed from 192.168.65.x to 192.168.66.x.
nsx-t edge appliance tep transport vlan on different vlan to host tep transport vlan

Immediately the tunnels come up.

nsx-t manager ui showing tunnels up when using different transport vlans

The below packet capture on Edge also shows the control messages are passing correctly now and that the packets are no longer being dropped.

nsx-t edge packet capture showing tunnel up with bfd messages when using different transport vlans

In summary, to configure TEPs when Edge Appliances reside on a host transport node in release prior to NSX-T 3.1, ensure they are not on the same Transport VLAN. However, if they are, ensure they are not on the same vDS/N-VDS to have the GENEVE tunnel up.

Additionally, it has been demonstrated that inter TEP communication did not work in prior versions to NSX-T 3.1.

How has this changed in NSX-T 3.1

Inter TEP communication has been added in the latest release of NSX-T. This means you can have your edge TEPs and host transport node TEPs in the same Transport VLAN. 
As we saw earlier in this post, even in NSX-T 3.1 when the edge TEPs are in the same Transport VLAN as the host TEPs and not on a separate vDS and set of NICs, inter TEP communication does not work. You may have also noticed, above the vDS portgroups for the edges there were two additional VLAN-backed segments created in NSX-T, they were labelled ‘edge-seg-1’ and ‘edge-seg-2’.
The screenshot below shows their configuration in NSX-T.
nsx-t manager ui showing vlan-backed segments created for edge uplinks for inter tep
Next I will attach these segments as the uplinks for the edge transport node VM’s and reconfigure the profiles to use the same Transport VLAN (65).
The below screenshot is of en1 and it’s NIC configuration.
nsx-t edge vm vmnic vcenter configuration

TEP interfaces on the edge appliance, showing the same IP range.

nsx-t vrf 0 get interfaces showing same tep vlan ip range as host transport nodes transport vlan

And now if we go back into the UI, the tunnels are up.

nsx-t manager ui showing inter tep communication working tunnels up same transport vlan using vlan-backed segments

And here is a packet capture on the edge to show the BFD control messages coming through.

nsx-t packet capture showing inter tep commincation using the same transport vlan with bfd messages up


With the release of NSX-T 3.1, inter TEP communication is now possible.  That means we are able to use the same Transport VLAN for host transport node TEP IP addresses and Edge appliances.  The caveat is, you must use VLAN-Backed segments created in NSX-T for this communication to work.  Inter TEP communication is not a must, you may still use different VLANs for TEP addressing, however this is been a feature that has been long awaited and will make some customers very happy!

4 thoughts on “NSX-T Transport VLANs and Inter TEP Communication | LAB2PROD

    • Author gravatar

      Hello, I really appreciate your work! it’s just Awesome.

      Please tell me, there is ay requierement for the vmk10 about where to place it (in term of portgroup) and what could be the best design for that?

      In factI’m using NSX-T3.0 , i had a communication problem between edge and TN and i changed the mapping of vmk10 from none to “Seg_TEP” (its and Overlay Seg). Due to that, Node tunnel gone down and now I’m not able to replace vkm10 to previous location (Port group= None) so that I can at least get TN Tunnel UP and theh apply your tutorial to make then communication with edge as you explained here.
      Also, i’m using

      • Author gravatar

        Hi Wilfried,
        Are you attempting to run a single VLAN for tunnel endpoints and configure inter tep?

        If so, you will need to create a VLAN-backed segment and set it to trunked 0-4094. In vCenter, attach this new portgroup to the edge VM. Then tag the transport VLANs in the uplink profile for the transport node. If you run into an issue where the VLAN-backed segment is not showing in vCenter to be able to attach it to the edge node, this is likely because host transport node does not belong to the same VLAN transport zone you created the segment in.

        If you are not attempting to configure inter TEP communication, the portgroup does not need to be a VLAN backed and can be a standard VDS portgroup.

        So your edge appliances vNIC attachments should look like this for inter TEP communication;
        – vnic0 – management portgroup
        – vnic1 – VLAN-backed trunked portgroup
        – vnic2 – VLAN-backed trunked portgroup

        Edge nic assignment for standard TEP communication:
        – vnic0 – management portgroup
        – vnic1 – VDS trunked portgroup
        – vnic2 – VDS trunked portgroup

        Then in System > Fabric > Profiles > Uplink Profiles; create the uplink profiles for your edges and host transport nodes, both should specify the same VLAN if using inter TEP, if not configuring inter TEP specify the correct VLAN ID for each.

        Hopefully this clears things up for you.

    • Author gravatar

      Great article Shank!!
      In my lab environment I am using NSX-T 3.2 version with VDS prepared host transport nodes. Two of those are added to both type of TZs (Overlay and VLAN). The Edges is running on Host transport node and it’s Mgmt. and Uplink interfaces are connected to VDS created DVPG. But the Tunnel interface for TEP is connected to NSX-T created VLAN segment. Both Host TNs and Edge TNs are sharing the same TEP IP Pool since they are in the same VLAN. But still the tunnel status is showing down from edge and host TN side. Any input from your side would be much appreciated.

      • Author gravatar

        The Edge node should have 3 vNICs, one connected to a management portgroup and two additional vNIC interfaces that will be attached to trunking portgroups. Both of these interfaces should be attached to a VLAN-backed segment configured in trunking 0-4094 and should be created in NSX-T.

        Once you complete that, continue to wire the Tier-0 as per normal, using separate VLAN-backed segments for T0 uplink connectivity. Do not use the trunking portgroups you created for the Edge vNICs.

        Let me know if that helps, I will look to rewrite this article to include the Edge wiring.

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