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New Member

Frequency of broadcasts in a switched network without VLANS

I am thinking of segmenting my switched network of about 200 PCs using VLANS for the different departments. However, I would like to show my boss that the switch is heavily broadcasting in the present configuration. My question is, what is the frequency and duration of broadcasts in a switched network without VLANs? Also how do I measure the amount of traffic on the switch before and after segmenting into VLANs?

Thanks in advance!

4 REPLIES
Bronze

Re: Frequency of broadcasts in a switched network without VLANS

I believe the following URL will help with the question of how to measure the amount of traffic on the switch before and after the implementation of VLAN's. The frequency and duration of broadcast would depend on the amount of traffic actually coming across the switch.

http://www.cisco.com/warp/public/473/103.html

Cisco Employee

Re: Frequency of broadcasts in a switched network without VLANS

By default all our switchports are in VLAN 1. So your theory of no VLANs is not correct. A switch maintains CAM(MAC address) table for every device connected to it. Unless the destination MAC address is not known, the switch will not send out broadcasts. To me, 200 users in one VLAN is not totally unreasonable.

Creating multiple VLANs is definetely a good idea, but you'll need a router to route between the VLANs if you want any inter VLAN communication.

New Member

Re: Frequency of broadcasts in a switched network without VLANS

One VLAN is as good as having no VLANs! Anyway, suppose I have a router for inter VLAN communication, will Windows 9x/2k detect all the computers connected to the switch even if they are in different VLANs when I browse the network using Network Neighbourhood/My Network Places or will it show only those computers in the same VLAN? Keeping that information will require frequent broadcasts wouldn't it, because MAC table entries will age out when not used frequently.

Another thing about switches....if two computers try to connect to the same third computer at exactly the same time, wouldn't a collission occur?

Cisco Employee

Re: Frequency of broadcasts in a switched network without VLANS

Not exactly. By defnition, a collision occurs when two hosts try to use a shared transmission medium (cable) simultaneously.

So, if two computers sent data out two a third computer on the same shared segment at the same time, then yes, a collision would occur.

If it's a switched LAN or full-duplex, a collision will not occur.

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