{"id":2830,"date":"2015-10-03T23:25:59","date_gmt":"2015-10-03T19:25:59","guid":{"rendered":"https:\/\/rakhesh.com\/?p=2830"},"modified":"2015-10-03T23:25:59","modified_gmt":"2015-10-03T19:25:59","slug":"notes-on-nlb-vmware-etc","status":"publish","type":"post","link":"https:\/\/rakhesh.com\/windows\/notes-on-nlb-vmware-etc\/","title":{"rendered":"Notes on NLB, VMware, etc"},"content":{"rendered":"

Just some notes to myself so I am clear about it while reading about it. In the context of this VMware KB article – Microsoft NLB not working properly in Unicast mode<\/a>.<\/p>\n

Before I get to the article I better talk about a regular scenario. Say you have a switch and it’s got a couple of devices connected to it. A switch is a layer 2 device – meaning, it has no knowledge of IP addresses and networks etc. All devices connected to a switch are in the same network. The devices on a switch use MAC addresses to communicate with each other. Yes, the devices have IPv4 (or IPv6) addresses but how they communicate to each other is via MAC addresses.<\/p>\n

Say Server A (IPv4 address 10.136.21.12) wants to communicate with Server B (IPv4 address 10.136.21.22). Both are connected to the same switch, hence on the same LAN. Communication between them happens in layer 2. Here the machines identify each other via MAC addresses, so first Server A checks whether it knows the MAC address of Server B. If it knows (usually coz Server A has communicated with Server B recently and the MAC address is cached in its ARP table) then there’s nothing to do; but if it does not, then Server A finds the MAC address via something called ARP (Address Resolution Protocol). The way this works is that Server A broadcasts to the whole network that it wants the MAC address of the machine with IPv4 address 10.136.21.22 (the address of Server B). This message goes to the switch, the switch sends it to all the devices connected to it, Server B replies with its MAC address and that is sent to Server A. The two now communicate – I’ll come to that in a moment.<\/p>\n

When it’s communication from devices in a different network to Server A or Server B, the idea is similar except that you have a router connected to the switch. The router receives traffic for a device on this network – it knows the IPv4 address – so it finds the MAC address similar to above and passes it to that device. Simple.<\/p>\n

Now, how does the switch know which port a particular device is connected to. Say the switch gets traffic addresses to MAC address 00:eb:24:b2:05:ac – how does the switch know which port that is on? Here’s how that happens –<\/p>\n