VLAN basics help

[dan_track]

Hi

I have a 3750 router, with about 5 vlans on there. The problem I have is that if I plug in a workstation in say vlan 4 it can't see workstations in vlan 5.

Could someone please tell me why it can't see them, and how I can fix this on the 3750? Are there any spanning tree problems I should be aware of?

Thanks in advance for your help.

Dan

[Richard Burts]

Dan

If you plug the workstation in on vlan 4 and it can not see workstations in vlan 5, does it see workstations in other vlans or does it not see workstations in any other vlan? Knowing this will help to identify the problem.

I am going to guess that the workstation does not see workstations in any other vlan. If that is the case then the most likely cause of this is a mismatch of configuring the default gateway of the workstation. Can you post the IP address, mask, and gateway of the PC and the IP address of the 3750 for the vlan in which the workstation is connected?

HTH

Rick

[dan_track]

Hi Rick,

Thanks for the reply.

The PC get's its IP address from the dhcp server in vlan 5. How do I find out the IP address of the 3750?

Thanks

Dan

[mahmoodmkl]

HI

u can use the command show run or show interface.

there u can look for interface vlan under it u can find the ip address assigned to any particular vlan SVI.

THANKS

Mahmood

[dan_track]

Hi

I managed to get the details:

The IP address would be: 10.200.208.x (any ip address)

subnet mask: 255.255.255.0

Gateway: 10.200.200.4

Vlan 4 IP: 10.200.200.3/24

Vlan 5 IP: 10.200.208.3/24

I have two switches. The 3750 has all the ports configured to be VLAN 4, while the other switch (3560) has all the ports configured to be in Vlan 5.

Many Thanks

Dan

[Richard Burts]

Dan

Thanks for posting the additional information. It confirms my first guess about the problem being a mismatch in configuring the workstation address, mask, and gateway. The workstation address and mask put it into subnet 10.200.208.0/24 but its gateway is in 10.200.200.0

There are several things about what you have posted that I do not understand. If the workstation is connected in vlan 4 how does it get an address from the dhcp server in vlan 5?

Also the workstation default gateway of 10.200.200.4 is not the address that you show for the VLAN IP. What is it? Is it the address of some other router in the network?

Perhaps if we get these worked out we will find a solution for your problem.

HTH

Rick

[dan_track]

Hi Rick

Apologies for not being clear. The workstation is in vlan 5 when working. It doesn't work in vlan 4, although I can't see why. It can get from vlan 5 to vlan 4 but not the other way around.

Any further explanations would be appreciated.

Thanks

Dan

[mahmoodmkl]

HI

This is the natural behaviour if u implement vlans.

as each vlan is a seprate broadcast domain workstations from one vlan will not see the workstations in other vlan.If u want to enable workstations in different vlans to see each other u need a routing device to route between different vlans.

And i think u r 3750 can become a routing device as it is a multilayer switch.

Thanks

Mahmood

[dan_track]

Hi

What would I need to put on the router to allow the traffic to pass between vlans. Can you give me a config example please.

Many Thanks

Dan

[mahmoodmkl]

HI

u can go like this if u r having a router on a stick scenario.

router(config)int f0/0

router(config-if)no ip address

router(config-if)int f0/0.1

router(config-subif)encapsulation dot1q

router(config-subif)ip address

this will be the gateway address for u r clients.

if u r using a multilayer switch then

u can use

interface vlan

ip address

no shut

u have to do same for all the vlans which u have configured.

Thanks

Mahmood

[dan_track]

Hi

Thanks for the info.

I just checked another router I have in a different environment which also has differnet vlans. When I do sh run on the router (3640) I can't see any configured vlans. Which means it should route traffic between vlans, however routing is happening. For example one of the LAN links is in the vlan 108 while another port has a vlan of 102. Would you know how this is working, or will I find out once I can show the vlan's on the router?

Thanks again

Dan

[mahmoodmkl]

HI

I think u r having two ports on u r router which are connected to two ports in the switches in the respective vlans i e. 108,102 in this scenario u dnot need any sub-interfaces as u r physical interfaces are doing the routing for the vlans.

i think u r able to route between these two vlans only am i right.

as u r router interfaces will be configured as the default-gatway for u r vlans it will be doing routing as IP Routing will be enabled in u r router by default and the router sees the two networks as directly connected.

So it is natural it has to work.

Thanks

Mahmood

[devang_etcom]

hi dan,,,

to make two vlan communicate u need to have layer 3 device which is router...and u need to have intervlan routing technology...which enable hosts in two different valn make communication...

here u need to connect switch to the router...and then on router and have to create subinterfaces...

the intervlan routing documents can be easily available on cisco website...so if u search then u will get it easily and if u need further help then i can...

this is the link where u can have sample configuration...http://www.cisco.com/en/US/tech/tk389/tk815/technologies_configuration_example09186a008019e74e.shtml

rate the post is it helps

regards

Devang

regards

Devang