This part of the troubleshooting series can be sub-divided into following sections. Refer Index page for basic principles, HCX command line basics and to access other parts in the series.
1. VM on extended segment unreachable
- Check the basics – whether the HCX Network Extension tunnels are up, the extension status is Complete, NIC is connected to the VM?
- Can the VMs on the extended segment, one on-prem and one in cloud ping each other? Is the gateway reachable? If not, can VMs on the same side ping each other?
- If ping works and intermittent application connectivity issues are seen – check if path MTU discovery works fine. Refer this article for the things that can break PMTUD.
2. DHCP not working
VMs on the extended segment migrated to cloud does not receive IPs anymore from the on-prem DHCP server. Check the segment security as described in this article or disable DHCP server block in a custom profile. DHCP server block is enabled by default in the default profile.
3. Network cannot be extended
Symptom “Network used as backing for overlay network cannot be extended” shown under the networks
Possible causes
- On-prem management VMkernel and networks used by HCX cannot be extended and they are rightfully tagged as ineligible
- Verify pre-requisites and limitations for network extension
- Apart from the above mentioned networks, ineligibility is generally shown for NSX networks. Verify if On-prem NSX manager is registered with HCX and HCX web and app services restarted
- If extending NSX logical segments from on-prem, ensure overlay-transport zone is selected under compute profile and service mesh (In cases where there is no NSX on-prem, distributed portgroups are extended and the relevant Distributed vSwitch is selected during this step).
Thanks to VMware HCX team, field teams, PMs, support, customers for all of the above inputs. Please add your feedback in the comments to include additional issues and fixes.
Prasanna 
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