How to leverage NATed T1 Gateway for overlapping networks over a Transit Connect

In my last post, I have showed you how to access a SDDC overlapped segment from a VPC behind a TGW attach to the SDDC through a Route Based VPN.

In this blog post, I am going to cover how to leverage the NATed T1 Gateway for connection to an overlapping segment from a VPC behind a TGW connected to my SDDC over a Transit Connect (vTGW).

There are slight differences between both configuration. The main one is that static routing in the vTGW peering attachment is used instead of dynamic routing. The second one is we will have to use route aggregation on the SDDC.

Route Summarization (eg. Aggregation)

A question, I have heard for a long time from my customers is when are we going to support route summarization. Route summarization — also known as route aggregation — is a method to minimize the number of entries in routing tables for an IP network. It consolidates selected multiple routes into a single route advertisement.

This is now possible since M18 with the concept of Route Aggregation!

Route Aggregation will summarize multiple individual CIDRs into a smaller number of advertisements. This is possible for Transit Connect, Direct Connect endpoints and the Connected VPC .

In addition, since the launch of multi CGW, it’s mandatory for CIDRs sitting behind a non default CGW to be able to advertised them.

This is going to be mandatory in this use case as I am using a NATed T1 Compute Gateway with an overlapping segments that I want to access over a DNAT rule.

Implementing the topology

SDDC Group and Transit Connect

In this evolution of the lab, I have created an SDDC Group called “Chris-SDDC-Group” and attach my SDDC to it.

If you remember well from my previous post, SDDC Groups are a way to group SDDCs together for ease of management.

Once I created the SDDC group, it has deployed a VMware Managed Transit Gateway (Transit Connect) that I have then peered to my native TGW.

I have entered the information needed including the VPC CIDR ( that stands behind the peered TGW.

Keep in mind that the process of establishing the peering connection may take up to 10′ to complete. For more detailed information on how to setup this peering check my previous blog post here.

Creating the Route Aggregation

In order to create the route aggregation, I have had first to open the NSX UI as the setup is done over the Global configuration menu from the Networking tab which is only accessible through the UI.

I created a new aggregation prefix list called DNATSUBNET by clicking on the ADD AGGREGATION PREFIX LIST …

I have created the DNATSUBNET with the prefix to advertised it over Transit Connect

with the following prefix (CIDR):

To finish, I have then created the route aggregation configuration. For that, I have first given it a name, selected the INTRANET as the endpoint, and selected the prefix list created earlier.

Checking the Advertised route over the vTGW

In order to make sure the route aggregation works well, I have verified it in both the VMC UI at the Transit Connect level on the Advertised Routes tab:

and in the SDDC group UI from the Routing tab that displays the Transit Connect (vTGW) route Table:

Adding the right Compute Gateway Firewall rule

In this case, as I am using a vTGW peering attachment, there is no need to create an additional Group with the VPC CIDR as there is an already created group called “Transit Connect External TGW Prefixes” that I can used.

I have utilized the same group with the CIDR used to hide the overlapped segment with the DNAT rule.

I then have created the Compute Gateway Firewall rule called ‘ToDNAT‘ with the group “Transit Connect External TGW Prefixes” as the source and the group “NatedCIDRs” as the destination with ‘SSH’ and ‘ICMP ALL’:

Testing the topology

Checking the routing

The routing in the VPC didn’t change. We only need to add a static route back to the SDDC NATed segment CIDR:

Next is to check the TGW routing table to make sure there is also a route to the SDDC Nated CIDR through the peering connection.

We have to add a static route with a route back to the NATed CIDR:

I confirmed there was a route in the Default Route table of the Transit Gateway:

Pinging the VM in the SDDC from the VPC

To test my lab connectivity, I have connected to the instance created in the AWS native VPC ( and try to ping the Ip address and it worked again!

In this blog post, I have demonstrated how to connect to an overlapped SDDC segment by creating an additional NATed T1 Compute Gateway. In this lab topology, I have tested connectivity from a native VPC behind a TGW connected to my SDDC over a Transit Connect (vTGW).

I hope you enjoyed it!

In my next blog post, I will show you how to establish a communication between a subnet in a VPC and an SDDC segment that are overlapping over a Transit Connect, stay tune!

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