Managed Switch: help me understand


To effectively use the Network Load Balancing (NLB) capability that is built into Windows, it's important that the servers sharing the common virtual IP address be plugged into a hub, not a switch. The reason for this is that these servers share a common virtual MAC address. A switch would get confused when it tries to "learn" the appropriate port for the virtual MAC address. This is why a whitepaper that I recently read recommends:

- plugging the servers in the cluster into a hub - uplinking that hub into a switch

This ensures that each server in the cluster is able to see each packet destined for the virtual MAC address. This is all pretty standard stuff, but here's the thing: I don't want to have to mount a separate hub in my rack for each cluster. Can I accomplish the same thing with a "managed switch"? I've never used a managed switch before, so I'm not sure.

Let's say I have a managed switch with 24 ports. I fill up ports 1-16 with Ethernet cables from various machines on my LAN... but I then plug in the two servers in my cluster into ports 23-24. I want ports 23-24 to act like a hub that is unlinked to the rest of the switch. Eventually, the switch would learn that the shared MAC address of the cluster lives behind ports 23-24 and all traffic destined for that MAC address would be

*broadcast* to those two ports. Does that make sense?

I'm not sure what the appropriate jargon/terminology is for this type of setup, but I've done some research and I think it's called a VLAN. The crucial point to note here, though, is that ports 23-24 would act as a hub (all traffic is broadcast) instead of a switch.

All comments are appreciated.


Reply to
Sal Bompensaro
Loading thread data ...

Many switches also allow you to put specific ports in "monitor mode" - where the ports you specify will get all traffic that the switch sees.


Reply to
Kameron Gasso

Cool, I guess that's what I'm looking for, as long as I can make the "monitor mode" only apply to the ports in my VLAN instead of the whole switch.

Reply to
Sal Bompensaro

I think You have to use PORT TRUNKING for this purpose, not VLAN.

Reply to
Stanislaw Chmielarz

Naw,..Port Trunking is a VLAN thing where the router uses the same physical interface (with same cable and switch port) to serve multiple VLANs. The line between it and the switch becomes the Trunk Line. Investigate the term "Router on a stick" for more details.

For load balancing, the right way, requires a switch that is specifically designed for load balancing multiple NICs.

Reply to
Phillip Windell

A URL to the paper would help someone help you....

But what you really need is for the switch to deliver a copy of each inbound packet to the NIC cards at each server.

depending on how you configure the cluster, this is either a conventional unicast MAC address, or a multicast address. Multicast will work with a switch, but has its own issues,

a hub does this in the hardware, but since hubs top out at 100 Mbps, with modern servers and Gigabit capable interfaces, the hub may limit overall throughput.

You need a switch that can handle your cluster setup(s), and you may need each cluster in its own subnet.

Yes it does do some of what you want - but no setting up a VLAN does not make the switch behave as a hub - it makes it behave as a collection of separate switches.

You would isolate the cluster within a VLAN, sort out a way to send cluster traffic to all attached ports, but then you add a router (or the routing inside your layer 3 switch) to connect that VLAN as a separate subnet to the rest of your network.

If you use unicast cluster MAC addresses, then you have to configure the switch to replicate traffic to that MAC address to each relevant port. If you use this setup, then you dont have to isolate clusters on their own subnets / vlans, but it is going to make it easier to fault find, configure and change later if you do.

If you use multicast cluster MAC Addresses, then you need a router (or layer

3 capable switch) willing to ARP and work with a multicast MAC address, or that allows you to set that mapping manually.

Either way you need switch(es) with high end capabilities that allow for the setup you choose. i suggest you get your vendor to spec them for this purpose and agree and actually do a functional test for the load balancing working correctly before you pay for any hardware.

Reply to
stephen Forums website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.