CCNA questions- setup a vlan

Worrying about routing between VLANs

Since a VLAN's primary job is to segment traffic, proper planning is in order. How will this VLAN communicate with the rest of the network? Routing is it's only option so this will need to be looked at. Trunking will only come into play when spanning switches with your VLAN. Using VLAN's is a form of QOS, your providing more bandwidth to the individuals that need it. In other words, if you have 2 application servers used by 50 people in the company, it would be a good idea to create two VLANS, the servers and server users on one, and the rest of the users on another. Then you route between them whenever other resources are needed. This assures more bandwidth to users of the application.

If your switch(s) is "non-blocking" in its architecture - it should not matter whether you have three vlans or one from a bandwidth perspective.  One should probably put servers on their own vlan from an organizational standpoint.  But it will not give the users more bandwidth for their application.    The whole concept of "non-blocking" guarantees that.

And putting users of particular servers on a special VLAN sounds like a nightmare.  In a typical company it would mean assigning a different VLAN  to different ports based on the user requirements.  What a headache!  Users changing cubes/offices, new employees, promotions, transferrs.  Eek.  My whole day would be spent messing around with the switch.  Too busy for that.  I'd set a range of ports or a whole switch in an IDF to one VLAN.  Make as few exceptions as possible.

Then what's the sense of having VLANS, I can do the same thing by segmenting them by IP address? That's what VLANS are for, to segment traffic away from the rest of the network to ensure quality communication. This allows for increased bandwidth since all traffic is kept local to the VLAN. If your going to put your whole company on one VLAN, you just defeated the purpose of using them.

First let's review just what is a VLAN.  Basically all that happens is a tag is added to a data frame which identifies it as a member of one VLAN vs another.  This has no impact on the available bandwidth at the port nor at the bus.  It used to be the case that having all hosts on one VLAN afforded a perhaps significant speed advantage in that you avoided a layer 3 router hop.  But with the advent of layer 3 "wire speed" switching - that advantage is negligible.  

Of course if you had your users on one VLAN and the Apps on another VLAN (a most typical scenario) one would need to plan for the appropriate amount of pipe between the two.  If the users and servers were all one switch - that throughput would be the throughput of the backplane.  If (more likely) the users were on one switch and the servers on another switch and another VLAN - you could use gig E on fiber or even multiple gig E on fiber trunks.  

Mmedwid, yes, let us review what a VLAN is. Below is an experpt from Cisco documentation.

"A virtual LAN (VLAN) is a switched network that is logically segmented by function, project team, or application, without regard to the physical locations of the users. Any switch port can belong to a VLAN, and unicast, broadcast, and multicast packets are forwarded and flooded only to stations in the VLAN. Each VLAN is considered a logical network, and packets destined for stations that do not belong to the VLAN must be forwarded through a router or bridge."

With this in mind, bandwidth is increased since the only traffic that is received will either be local to the VLAN or routed. Since traffic is local, broadcasts, which are the biggest users of bandwidth, are limited to the VLAN. Segmenting trafic in this way by creating smaller broadcast domains increases network efficiency and manageability. It doesn't matter what the backplane or trunking is, if I were to put 1000 users on switches with no VLANs the performance of the network would be terrible. You also only put applications on their own VLAN whenever remote hosts need to use them, otherwise they will be on the same VLAN as the majority of users. This allows for better performance to the application.

mikecr,  

The entire reason for a switched network is so that the collision domain for unicast traffic is limited to the individual segment - host to switch port.  That traffic is then forwarded only to the destination port.  NOT every port on the VLAN.  Why would you buy a switch if unicast traffic was heard by every nic on the VLAN?  Performance would be terrible.  In fact  even if you use multicast - only workstations that announce themselves as listeners will receive the multicast traffic - NOT every port on the VLAN (assuming one has switched on CGMP - else you'll see big problems.)  

Performance with 1000 users with a switch on one VLAN would be absolutely fine.  The one switch example is not practical because a 6509 fully populated with 8  48 port blades would bring you 384 users.  But let's take three switches and trunk them together and put them all on one VLAN.  There would  be no problem with this at all because the workstation nics would not hear each others traffic.  

The exception is broadcast traffic. But if broadcast traffic is the largest portion of volume on your network - you've got other problems.  Broadcast traffic is typically around 5% of the traffic in TCP/IP environments.   At least those I've seen.  

I see actually the largest Cisco 6509 now has 13 slots - so one could get up to 624 users on it.  The system has a 256GBps switch fabric.  All folks on one VLAN...no problem.  

http://www.cisco.com/warp/customer/cc/pd/si/casi/ca6000/index.shtml

Nice review of all related topics including layer 2 and 3 switching, microsementation, design et al...

http://www.cisco.com/univercd/cc/td/doc/cisintwk/idg4/nd2002.htm#xtocid229273

http://www.cisco.com/univercd/cc/td/doc/cisintwk/idg4/nd2002.htm

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mikecr, there will be a separate question with points for your help.

Okay, no problem..