Do I need to create loopback IP address on servers when using LD?

[zhichao]

Beside ASLB mode, do I need to create loopback IP address for the servers when using Local Director?

Understand LD does not use NAT for IP address, so the server may drop the packet when it sees the packet's destination is not for its own IP.

And how to set the loopback IP on Windows servers? I can only create the secondary IP under the ethernet NIC configuration, and this always cause the two web servers to complain about duplicated IP address.

[Gilles Dufour]

the LD will do NAT.

So no need for loopback.

Gilles.

[zhichao]

Hi, but what I know is at least ASLB is not doing NAT.

And I could not find any paper mentions the LD changes the IP address (NAT) for the servers. Can you provide me some info? thanks!

[Gilles Dufour]

there is the directed mode and dispatched mode.

The first one NAT and not the 2nd one.

More info at :

http://www.cisco.com/en/US/products/hw/contnetw/ps1894/products_installation_guide_chapter09186a008007f3de.html

Gilles

[ahojmark · Accepted solution]

The case of the loopbacks depends on whether you run LDIR in dispatched mode or directed mode.

Directed mode is a form of NAT where the LDIR changes the destination IP address in the incoming packet to that of the real server chosen.

Dispatched mode is sort of like 'Layer 2 NAT' or 'MAC NAT', because the LDIR changes the destination MAC address but not the destination IP address. For the server to actually pick up a packet adressed to an IP address not configured on the ingress interface, it will need to have this address configured somewhere else, and this is where the loopback adapter comes into play.

It is true that with ASLB, you need to run dispatched mode. This is due to how the rewrite is implemented on the C6K (which is needed for ASLB).

Regarding the Windows configuration, you need to install the loopback adapter. How this is done depends on the Windows version, but basically you need to install the driver manually, as if you had added a physical adapter to the machine and were to manually install a driver. You'll find the loopback adapter under the 'hardware' manufacturer Microsoft in the Add Hardware configuration dialog. When the new 'hardware' has been installed, you can configure an IP address on it like any other NIC.

So to answer your question: No you do not *need* to use loopbacks when using LDIR, but this depends on the LDIR configuration.

HTH,

-A