Re: NLB heartbeat clarification please.

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Hi Johnny,

To multicast or not to multicast, that is the question.

So NLB will create a new virtual MAC address and assign it to the
network card that has NLB enabled, and all the hosts in the NLB cluster
will use this virtual MAC. All incoming packets are received by all
nodes in the cluster, and the NLB driver on each node is responsible
for determining which packets are for that server.

In unicast mode, NLB replaces the network card's original MAC with
the virtual MAC.

In multicast mode, NLB adds a new virtual MAC to the network card and
also keeps the card's original MAC (two addresses per NIC).

The benefit of unicast mode (the default mode) is that it will work out
of the box with all routers/switches (only 1 MAC address/per NIC). The
disadvantage is that since all nodes in the cluster all have the same
MAC/IP address they will NOT have the ability to communicate with each
other via their NLB bound NIC, so a second network card is required for
ANY other communication between the servers.

Now multicast mode does not have the limitation since the servers can
communicate with each other via their original addresses of their NLB
network cards; each server's NLB network card operating in multicast
mode has two MAC addresses (the real one and virtual one) which can
cause some problems since most routers will reject the ARP replies sent
by nodes in the cluster. The router sees the response to the ARP
request that contains a unicast IP address with a multicast MAC address
and the router considers this to be invalid and rejects the update to
the ARP table so basically you'll need to manually configure the ARP
entries on the router.

Hope this helps...

-nm

.

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