VLANs

Daniel333

I am just not getting the point of a VLAN I guess.

Why make this logical grouping in the first place? If you have 2 workstations plugged into a Switch set to 10.1.0.x/16 and two more workstations in that same 1900 series switch set to 10.2.0.x/16. They are on serperate subnets so they can't communicate anyway.

And you should not have to worry about their broadcasts and collisions, as the switch should see and learn all the MAC addresses and filter them, right?

What am I missing here? I have reread Cisco Intro Chapter 10-Virtual LANs and Trunking twice now.

thanks in advance

remyforbes777

You answered your own question. In order for the switch to learn which subnets are connected to which ports it has to send out a broadcast to all ports to find this. With VLANs you minimize the broadcast domains. If you are in VLAN 2 and it needs to find out which port a node is on in VLAN 2 it will only send broadcast out of the ports associated with VLAN 2 and no other ports meaning it doesn't send broadcasts out to all ports thus minmizing your broadcast domains.

dis1931

I am just not getting the point of a VLAN I guess.

Why make this logical grouping in the first place?

This is mostly to segregate traffic. Broadcasts are segregated. You can setup security and access lists based on these different networks. I'm sure you know the canned answers if you are studying for the CCNA.

If you have 2 workstations plugged into a Switch set to 10.1.0.x/16 and two more workstations in that same 1900 series switch set to 10.2.0.x/16. They are on serperate subnets so they can't communicate anyway.

You are right they will not be able to communicate. Well you will most likely want them to communicate. Let's say the accounting and engineering dept. have different subnets...eventually the engineering dept. needs to spend money and needs to access accounting documents to request money. Just an example. You will need a router with lots of interfaces...one for each different network at your site....Where a vlan network only needs one interface to the router over a trunk link. Also, chances are your seperate vlans(networks) need to share some resources such as servers, printers, etc...How will you assign addresses on your network? How will DHCP know where to assign what address to in your network? A VLAN can be setup with a range of DHCP addresses that are assigned automatically where in your scenario they would all need to be manually configured to work. I'm sure there are more reasons but those are some good ones.

Daniel333

Thanks guys, still working on it. I don't think I need to know how to set them up for the CCNA-Intro exam, but sure is helping to to actually try and do it. I am using the guide on this very site, at http://www.techexams.net/techlabs/ccna/lab-switch-basics.shtml for the "how to" and I think the IP addresses in the example are off (copy/paste below). Anyhow, I think I had better grab the ICND book and see if I can find a little more detail on this VLan stuff. Before I try and set one up again. As this has been a wasted couple hours. But at least I know WHY we use VLANs, I just need to figure out the how from here.

thanks again

copy/paste from http://www.techexams.net/techlabs/ccna/lab-switch-basics.shtml
STEP 3. Configure an IP address for the switch
To be able to manage the switch using telnet, you will need to configure it with an IP address. Instead of assigning an IP address to one of the switch ports, we are going to assign an IP address to the Management VLAN.

Use the following commands to assign the IP address 192.168.1.254 to VLAN 1, which is the management VLAN by default:
TEswitch1(config)#interface vlan 1
TEswitch1(config-if)#ip address 192.168.0.9 255.255.255.0

If you need to be able to connect to the switch from other networks, you will also need to configure a default gateway address. For example, if the switch is connected to a router with the IP address 192.168.1.254, use the following command, in Global Configuration mode, to use it as the default gateway:
TEswitch1(config-if)#exit
TEswitch1(config)#ip default-gateway 192.168.0.254

Webmaster

Looks like I made some typo there. The IP address in the command (192.168.0.9) is the correct one for int vlan 1, and the same goes for the default gateway example (as it has to be in the same subnet so 192.168.0.254 and not 192.168.1.254. I'll correct in the TechLab in a sec.

Thanks,
Johan