In January 2014 I wrote a blog post about network flooding because of Windows NLB clusters in unicast mode. Yesterday, Windows NLB, HP switches and I met again.
After moving a customers core network from HP 5400zl switches to two IRF stacks with HP 7506 switches, multiple Windows NLB clusters stopped working. Because the Windows NLB used multicast operation mode, it was instantly clear that the switches were the problem.
The explanation is easy: By default, a Comware based switch does not learn multicast MAC addresses. And because of this, the switch does not add them to the ARP table. And you can’t add static multicast MAC address entries. You have to disable the ARP entry check.
To do so, you have to login and change to the system-view. Then disable the ARP entry check option, which is enabled by default.
<HP> system-view [HP] undo arp check enable
A few seconds after issuing the command, the NLB clusters started working again and I saw ARP entries with MAC addresses starting with 03-BF.
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