Specifications

HP Virtual Connect for Cisco Network Administrators (version 1.2x) page 37
VC Managed MAC Addresses
One of the many features provided by Virtual Connect is the ability to “manage” the blade server
MAC addresses. Specifically, Virtual Connect ‘manages’ the blade server MAC addresses. Virtual
Connect does not ‘virtualize’ the blade server MAC addresses. Many VC implementers don’t
appreciate the difference between ‘virtualized’ MAC addresses and ‘managed’ MAC addresses.
A ‘virtualized’ MAC address is a MAC address that is not really owned and used by a physical NIC.
Often, a virtualized MAC address is a MAC address that replaces the real MAC address of a physical
NIC without the server’s knowledge. In other words, the server thinks it is communicating on the
network with MAC address X, however, some device is replacing real MAC address X with a virtual
MAC address Y. Many of the benefits of MAC address management are lost in this type of
implementation.
A ‘managed’ MAC address is a MAC address that really is owned and used by a physical NIC. The
server has simply been assigned, by the administrator, to use a specific MAC address on a specific
physical NIC port. This managed MAC address, for all intents and purposes, appears to the server as
the MAC address that was burned into the physical NIC at the factory.
The benefits of VC Managed MAC addresses are:
• WYSIWYG - What You See (on the server) Is What You Get (on the network)
There is no discrepancy between what the server thinks its MAC address is and what the
external network sees as the server’s MAC address. This dramatically reduces the
complexity of troubleshooting network related issues.
• Server application licensing is maintained after hardware changes
Many server application licensing mechanisms can key off the server’s MAC addresses.
If the server’s MAC address changes (replacing a failed NIC, booting server image on a
different physical server, etc.), then the application licensing may require re-licensing
using the new MAC address. Virtualized MAC addresses do not prevent this problem.
However, VC’s use of managed MAC addresses does prevent this problem since the
server image will always see the VC managed MAC address regardless of which physical
server it is running on.
• No Performance impact on network devices
Virtualized MAC addresses can require that a network device (e.g. switch) manipulate
every frame a server transmits to replace the server’s MAC address with the virtualized
MAC address. Also, when the source MAC address is edited by the network device, the
frames checksum (CRC) has to be recomputed by the network device. The more frames
a server transmits, the more work the network device has to do, which can have an
impact on the performance of the network device. Alternatively, VC’s use of managed
MAC addresses means the server transmits with the managed MAC address. No device
on the network (VC or switch) is required to manipulate the server’s frames. This results in
absolutely no performance impact on the network.
When a blade server with VC managed MAC addresses is removed from the enclosure (or when a
VC Server Profile is unassigned from a server blade while still in the enclosure) the blade server
automatically reverts back to the actual MAC addresses burned into the physical NICs at the
factory. This prevents any issues with duplicate MAC addresses on the network caused by moving
blade servers around within the data center.
Virtual Connect Manager (VCM) provides three domain-wide choices for managing the blade
server MAC addresses: