The NetScaler VPX virtual appliance has some decisive differences from its MPX hardware counterparts. While the performance differences are well documented, some of the finer networking related points are a bit obscure and not readily discoverable. While none of them are likely to be show stoppers, it’s important to be familiar with the limitations.

Here’s a short table I’ve assembled describing the impact of the hypervisor on the NetScaler virtual appliance as compared to the network stack of the MPX:

Native 802.1q VLAN Tagging Tagging is defined on the hypervisor.XenServer is limited to 7 tagged networks and 16 on VMware.
Native 802.3ad Link Aggregation 802.3ad is not supported by XenServer.Source Level Balancing (SLB) NIC bonding is the closest parallel and offers NIC redundancy with great performance.But not all switches work well with SLB so be sure to test under load, plug both links into a single switch, or skip SLB entirely in favor of native NetScaler device failover.
Device Fail-over Failover is supported between VPX devices through NetScaler’s native redundancy mechanism.So there’s no need forXenMotion or VMotion support with the VPX.
Dedicated SSL Chipset No SSL chipsets are available to the VPX, but none the less, it is capable of 300 3DES and 1000 RC4 sessions.At double the VPN capacity, VPX offers an incremental upgrade path from Secure Gateway by providing a full SSL-VPN, Smart Access, and improved security.   Enterprises making a strategic investment will want the 10x+ SSL capacities provided from dedicated Cavium chipsets which can be found in the MPX harware appliances.

Licensing Changes in VPX 9.1 Build 100.3:

For VPX appliances only, the 9.1_100.3 license software will check the MAC address of the FIRST INTERFACE listed. In previous builds, the license software checked the MAC address of the NEWEST INTERFACE. For VPX customers who upgrade to 9.1_100.3, this change will invalidate licenses on VMs which had more than one interface. They will need to revisit licensing portal to re-host their license.CTX122426 – NetScaler VPX Licensing Guide has been updated with the rehosting instructions.

VPX owners are allowed to relicense their VPX system up to 3 times.

References:XenServer Administrator’s Guide: Chapter 4, Networking