What are the advantages to using VLANS

Posted on 2014-03-01
Medium Priority
Last Modified: 2014-03-07
I think I need to revert back to networking 101. I have three buildings connected via fiber optic. All three buildings are on the same domain. Each building has a DC. One big subnet. All three buildings are using ProCurve switches.

7 computers in building 1, 40 computers in building 2 and 70 computers in building 3. Total of 117 computers. It use to be with hubs that only one packet would get through to the destination and all the other packets on the line would get rejected and have to retry. Is that still the case with switches? Is the way I have it set up wrong?

A few departments in building 3 are complaining of speed issues with SQL. The SQL server is in there building. I have checked server and switch utilization and everything seems find. I am afraid that just the way I have the network designed (117 computers on the same segment) might be causing the problem. Thoughts?

Would using VLANS help?
Question by:LockDown32
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LVL 13

Assisted Solution

ktaczala earned 500 total points
ID: 39897372
Vlans Isolate Traffic, for instance,
VLan1: could handle daily traffic, like internet, file share, printing, domain authentication.
VLan2: would be set up the handle something like a camera surveillance system so the High traffic of pushing live images wouldn't affect the Vlan1 traffic.
VLan3: might be configured to handle something like manufacturing plant CNC network.

I'd look more at the SQL setup.  How powerful is the SQL Server?  Is it standalone? Clustered? how many connected users at one time?
LVL 15

Author Comment

ID: 39897400
Interesting feedback. I was looking at the setup and was really questioning the need for VLANS. To me 120 computers on the same LAN isn't really a lot. Especially since there is very, very little cross traffic between buildings. So as a rule are VLANS designed for much larger networks then what I have?

   The SQL Server(s) should be adequate. One is a single Xeon E3-1220, 16GB memory and an Adaptec 5405 in a Raid 5 with three 15000 rpm SAS drives. It only services 7 users. The other is a monster Intel. Two physical Xeon 5400 series CPS, 24GB memory and another Adaptec 5405 in a Raid 5 with three 15000 rpm SAS drives. It services maybe 20 users.

   Both departments are complaining of speed issues to their respective SQL servers. Of course there is a lot more to consider. Workstation specs and everything else but the only other things these two departments have in common is that they are in the same building on the same LAN. I am running out of places to look.
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LVL 15

Author Comment

ID: 39897675
Both apps are from major external vendors. Both vendors have been on the respective SQL servers and can't find anything wrong (imagine that).
LVL 57

Assisted Solution

giltjr earned 1000 total points
ID: 39897687
VLAN's can serve many purposes.

A VLAN as a layer 2 broadcast domain.   Although 120 devices is not a lot and "...  there is very, very little cross traffic between buildings." The problem is that any and all broadcast traffic will flow between the buildings.

It not necessary the number of devices that determine when to use VLAN's.  We have some VLAN's that have only 1 or 2 devices on them.  Why?  Because they need to be separated for security reasons from our other networks.  

How are the switches cables?  Say if you have a single switch in each building and they are daisy chained like:

SW1 <---> SW2 <---> SW3

Then all traffic from a device on SW1 to a device on SW3 must pass through SW2.

Switches are WAY different from hubs.  As you stated a hub sends everything to every port and each device must look at every frame to decide if it is supposed to do something with it or not.

A switch will learn what MAC addresses are on which ports, by VLAN, and only send traffic out a port if the destination MAC address is on that port.  However if the frame is a broadcast frame, it will go out all ports in within that VLAN.

Now you could separate each building into unique VLAN's.  However if the switches are daisy chained as in my earlier example, you still have some of the same problem.  You have reduced the L2/L3 broadcast domains.  So the amount of traffic flowing the all the switches will be reduced some.  However you still have a possible problem as   the between SW1 and SW3 still must pass through SW2.

What I would look at first is how are the switches connected.

If SW3 traffic must pass through SW2 to get to SW1  and SW1 is where the MSSQL server sits.  I would look at  the port utilization the port that connects SW1 and SW2.

What is the link speed between the switches?
LVL 31

Assisted Solution

masnrock earned 500 total points
ID: 39902018
Switches are far more intelligent than hubs. Hubs will literally broadcast out every packet, whereas switches will learn over time where devices are and understand how to route the traffic, so there are far fewer broadcasts.

You could use VLANs if you desired, but it is not necessary in this case. What is the speed of the switch in building 3? Gigabit or 10/100?
LVL 15

Author Comment

ID: 39903084
OK. From the feedback above VLANS won't do me any good. Thanks for answering that question. The switches in all three buildings are 10/100/1000 but... only building 3 will support 1000. The cabling in building 1 and 2 is poor enough that I can't get away with 1000. I tried it a couple of times and a couple workstations just freaked.

   Which brings up another point. A couple devices in building 2 at fixed at 10. HVAC controllers. It use to be that having devices running 10 would kill the network. Is that still true?
LVL 57

Expert Comment

ID: 39903104
No, using switches basically isolates each switch port on its own "LAN segment".

Unless a port is shared between multiple devices, "up links" or "switch to switch" connections, traffic on one port does not necessary affect traffic on another port NORMALLY.

Traffic between SW1 and SW2 will flow at what ever speed they are connected to.  SW2 will buffer traffic that goes to the devices running at 10 Mbps.  

Now, if there is enough traffic to overflow the buffers on SW2, then traffic to the 10 Mbps devices could get flushed from the buffers and would need to be re-transmitted.  However, this would only affect the 10 Mbps devices typically.  Devices running at 100 Mbps would not really be affected.

Traffic from the 10 Mbps devices will flow at 10 Mbps to SW2, but will then flow at the SW1 to SW2 speed.
LVL 57

Expert Comment

ID: 39903114
Do devices in building #1 talk to other devices in building #1?

Do devices in building #2 talk to other devices in building #2?

Since the speed between building #1 and #2 is limited to 100 Mbps, you could be experiencing some problems with buffer overruns if you have devices running at 1000 Mbps that may send/receive a lot of data between those buildings.

If a device on building #1 is sending data at 1000 Mbps to the switch and that switch must buffer because the speed to building #2 is 100 Mbps, you could be seeing discards and thus retransmissions.

If your switches are managed you want to see if there are a lot of buffer drops/discards on the ports that connect the switches to each other.  Especially the switches that connect building #1 and building #2.
LVL 15

Author Comment

ID: 39903161
I think I missed the question. Each building is connected linearly via GB Fiber optic. Each building has its own server and there is very, very little cross traffic between buildings.

   They are managed switched. ProCurves. I have looked at the port statistics and utilization on all three switches until I am blue in the face. The servers don't show any sign of being taxed nor do the switches.

   The two departments that seem to complain the most both access different SQL servers. They say programs they run will either quit responding of close without being prompted but there is no rhyme or reason as to when the freeze or close. Some days things run fine. Other days are bad. I can't seem to find what is going on.
LVL 57

Accepted Solution

giltjr earned 1000 total points
ID: 39903229
O.K. I just re-read your post 39903084.  It's the cabling within building 1 and 2 that can't do 1 Gbps.  The cabling between the buildings can do 1 Gbps.

So you have 1 Gbps between all the building, but within building 1 and 2, it is 100 Mbps.

To summarize your problem:

Users in building 3 accessing a server in building 3 are complaining about poor response time and application freezes.

Everything else was to learn the difference between HUBS and switches.  O.K. got it now.

To start with for right now I would ignore anything about building 1 and building 2.  The application and problem seems to be within building #3.  We may need to come back to building #1 and/or #2 later on.  As it could be the application needs to access something within those buildings and that is what is causing the issue.

If the only complaints right now deal with the applications that access the SQL servers, I would start by having whomever controls those server look at them and see if they are having problems.  If there was a network problem, EVERYTHING would have a problem.  Not just specific applications.

I would look at running the following packet captures in building 3, not all at the same time, but at different times:

1) A standalone PC just passively capturing anything that comes out the port it is connected to.  This will help identify if there are any broadcast storms that could be affecting network performance.

2) Take the standalone PC and mirror the port that one of the SQL servers is connected to and capture packets.  See what is going on with it from a network point of view.

3) Do the same as #2 but for the other SQL server.

4) Do the same as #3, but for one of the desktops where the user complains the the most about poor performance and freezes.

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