Here is an interesting issue I hit today. Installing VMware vCloud Director 5.1 with vShield Manager 5.1.2a was giving me problems creating the VXLAN backend. Each time it would fail to install the VIB with “vib-module for agent not installed on host …. (vshield-vxlan-service)”. Lots of searching and an SR to VMware was raised. Then, just after raising the SR, I found my answer, thanks to this post

http://browse.feedreader.com/c/vClouds/289627048

Snipped here in case it vanishes from feedreader….

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When I tried to enable VXLAN in my vCloud Director setup I got the following error: “VIB module for agent is not installed on host vShield-VXLAN-Service”

This is solved by installing the VXLAN VIB which is available as a download on your vShield Manager: https://vsm-ip/bin/vdn/vibs/5.1/vxlan.zip on all your ESXi hosts in your VCD Cluster.

I used vCenter Update Manager to update all my ESXi hosts. no reboot is required.

After that, login to your  vCloud Networking and Security appliance (previously knows as vShield Manager), navigate to Networks, Network Virtualization and click on Preparation. Click Resolve to prepare your hosts.

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Hope this note helps someone.