VLAN - CCENT

[Helterskelter07]

I believe I may have got myself in a fizz about VLANS, please can someone assist with a simple explanation.

L2 switchs, With 2 Vlans configued as below, has 2 seperate broadcast domains, however to get an internet gateway L3 would both Vlans need to be on the subnet as the defauls gateway router. So you do not subnet the Vlans ?

NETWOK 172.16.30.0 /27

Vlan 1 - 172.16.30.0 - 31 (-2 for broadcast and Network address)

Vlan 2 - 172.16.30.32 - 63 (-2 for broadcast and Network address)

Router 1 : 172.16.30.65

My logic says that as the L3 default gateway is on a different subnet to VLAN 1 & 2 they will not see the DG on R1.

So in order for this to work both VLAN 1 & 2 need to be in the same subnet as R1, but they will be seperate broadcast domains, so a PC on VLAN 1 will not see a PC on VLAN2 even if they are on the same subnet.

Is this correct ?

Thanks

Duncan

[Richard Burts]

Duncan

You have several parts right, and some others need a little clarification.

First and most important when you configure the layer 2 switch with two vlans, and assign some switch ports to one vlan and other switch ports to the other vlan then you have created two broadcast domains. And a PC in one vlan will not be able to see or communicate with a PC in the other vlan - not until we get layer 3 routing into the picture.

For the router to be able to supply routing to both vlans then both vlans must have a connection to the router. There are a couple of ways to do that.

- you could connect an access port in the first vlan to one router interface and you could connect another access port in the second vlan to another router interface. Then the router could route between them.

- you could configure one switch port as a trunk port to carry both vlans and connect that switch port to the router. Then you have to configure the router to recognize the vlans on the trunk. You do this by configuring subinterfaces on the router. This is the more common solution but is a bit more complex to configure.

You are correct that if you configure the router with Router 1 : 172.16.30.65 then neither of the vlans on the switch will be able to use that as their default gateway.

HTH

Rick

[paul driver]

Hello duncan

It all depends what device is doing inter vlan routing!

If the router is performing this then the router would have multiple interfaces or subinterfaces configured for each vlan And then your access switches would have an interface and default-gateway pointing to the one of the interface/ subinterfaces IP address

The other way would be to have one switch performing inter vlan routing:
This would involve that switch enabling ip routing and having multiple svi interfaces with ip addresses for each vlan, with a default or dynamic route to the wan router for external access.

Each access switch would use a trunked interface connected to the l3 switch agin with a ip default-gateway pointing to the svi ip address of the l3 switch.

Res
Paul


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[paul driver]

Apologies Richard
you beat me to the post -didn't mean to post over you.

Res
Paul

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[Helterskelter07]

Thank you both for the explanation, I take my first Cisco exam next week and am really enjoying the hands on which I understand I will get more of in the ICND2.

So I have my home lab a switch 2950 with Vlan 1 & Vlan 2 and my 2600 router which has 2 x FE interfaces.

Option 1 use a FE interface per Vlan (each of these interfaces would need to be set as the DG for the Vlan)

Option 2 port trunking, is this spanning tree? I have only covered theory on this so far to stop boadcast storms

Thanks

Duncan

[Richard Burts]

Paul

NP. It happens frequently - sometimes I am the one who got there first and sometimes I am the one who got there second. In any case the original poster gets the benefit of having opinions from two different people. And I am encouraged that we both were giving fairly similar advice.

Duncan

A 2950 switch and a 2600 router are a good place to start as you start to learn Cisco networking. The 2600 may support trunking on its FastEthernet interface depending on the version of code that it is running and also the feature set of code that it is running. The easy way to figure out what you have got is to post the output of show version from the router.

The link that I am posting is a fairly good one that talks about trunking using 2600 and 2950. It talks about both ISL trunking and dot1Q trunking. ISL is quite old and I advise that you not pay much attention to that part in the article. But it is helpful in identifying the requirements for code and for its discussion and example configuration.

http://www.cisco.com/en/US/tech/tk389/tk815/technologies_configuration_example09186a00800949fd.shtml

HTH

Rick

[Mithali_Scott]

The Cisco Certified Entry Networking Technician certification is the first stage of Cisco's certification system. The CCENT certification is an interim step to Associate level or directly with CCNA and CCDA certifications.