Connect 2 networks

[Stephen Sisson]

We have two networks that need connecting and having problems.

Network 1 is using subnet 192.168.1.0/24 on Vlan 1 and we need to add network 2 to network one on the same subnet 192.168.1.0/24, the problem is network 2 3750 switch on Vlan 1 is using 10.1.1.0 .

I created a new vlan on network 2 3750 switch called Vlan 242 with ip address 192.168.1.241 /24, we see the trunk come up and protocol is up. I’m not able to ping 192.168.1.1 from network 2 3750 switch.

After checking I see spanning tree is pruning the trunked port for vlan 242.

I’m trying to connect both networks on the same subnet 192.168.1.x I can’t change network 1 vlan 1 as other equipment is connected to that network, we can’t change network 2 3750 switch vlan 1 10.1.1.x as other equipment is connected to that network.

What’s required to make both networks talk on the same subnet without changing the whole network to make this work

Thank you

[Rick Morris]

Switch A has 192.168.1.0/24 - VLAN 1

Switch B has 192.168.1.0/24 - VLAN 2

Can you provide the SVI config for both?

Also, do you have the VLAN's defined in the vlan database?

Can you provide the config used for the trunk?

[Stephen Sisson]

network 1 cisco 3560 has Vlan 1 with subnet 192.168.1.1/24 - Network 2 Vlan 1 has 10.1.1.30 /24 I know most networks use the same Vlan when possible so all equipment is on the same subnet.

Network 1 3560 switch

interface Vlan1

ip address 192.168.1.12 255.255.255.0

interface GigabitEthernet0/1

description <**To_GADRSW00_GI1/0/50**>

switchport trunk encapsulation dot1q

switchport trunk allowed vlan 1,2,193,241

switchport mode trunk

switchport nonegotiate

speed nonegotiate

Network 2 3750 switch

interface Vlan242

ip address 192.168.1.241 255.255.255.0

interface GigabitEthernet1/0/50

description <**To_SCCUL3560-TC3_Switch_GI01**>

switchport trunk encapsulation dot1q

switchport trunk native vlan 242

switchport trunk allowed vlan 193,241,242

switchport mode trunk

switchport nonegotiate

speed nonegotiate

I'm not sure about the Vlan defined in the vlan database

Thank you

[Rick Morris]

On switch 2 you are not allowing vlan 1 and on switch 2 you are not allowing vlan 241

Are you able to allow all or do you need to prune the vlans?

[Stephen Sisson]

I thought we only trunk the Vlans on the switch going to the other switch - like network 1 vlan 1, trunk to network 2 - network 2 3750 trunk vlan 241, 242 to network 1 - let me go try this, by the way we do no prunning that's going on through spanning tree.

Thank you

[Rick Morris]

I apologize you are correct, you add the vlans you want to allow accross not for incoming, you would remove any incoming that you did not want to see on that trunk.

[stubinski]

Hi,

You do not want to have the same network, 192.168.1.0 /24, setup on two different switches in the same network, unless you are merging (then you need something to nat the traffic).

You will either need to break up the /24 network so 192.168.1.0 /25 is on one side and 192.168.1.128 /25 is on the other side or you will need a device to nat for you.

A better alternative (IMHO) would be if this is a small enough network, you could move one or both switches off vlan 1 to a new vlan even though you mentioned you didn't want to. If you have the changes scripted beforehand (copy and paste the config in notepad and make reuired changes) it would cause minimal downtime. I would make sure to be consoled into the switch to make sure things went smooth. If you went this route, you could setup the gateway on one switch and trunk the traffic from the other switch. It would make your life much simpler.

Hope this helps

[Stephen Sisson]

Thanks for showing away to make this work; we need what we have to work by connecting the two networks through the cisco switches. Can anyone show how to make two cisco switches using different subnets on Vlan 1, allowing both to talk with same subnet 192.168.1.0 network. I'm trunking Vlan 1 but spanning tree continues to block the port, we have the 3560 Vlan 1 using 192.168.1.0 network and cisco 3750 Vlan 1 using 10.1.1.0 network - need both on the 192.168.0.0 network.

Thank you

[Jon Marshall]

Stephen

It's still not clear what you want to try and do.  When you say "talk with the same subnet" what exactly do you mean. From what i can follow you have

switch1 - vlan 1 using subnet 192.168.1.0/24

switch2 - vlan 1 using subnet 10.1.1.0/24

now if you only have vlan 1 on both swithes and you wanted the subnets to communicate you could have a routed link between the 2 switches and that would work but that my have knock on effects to what else you are doing in your network.

You cannot simply trunk the switches and expect it to work because you have different subnets for vlan 1 on each switch.

Could you perhaps clarify ?

Jon

[Stephen Sisson]

Hello,

We are trying to bring the two networks together so everyone on the 10.1.1.0 network can talk with the 192.168.1.0 network on network 1.

We have two companies we purchased and need to connect the network 1 on Vlan 1 192.168.1.0/24 to network 2 Vlan 1 10.1.1.0 so all users on network 2 can use the network 1 subnet - we need some users and servers to use the new network 192.168.1.0 on the 3750 stack, because network 1 is using Vlan 1 and network 2 is using Vlan 1 with 10.1.1.0 I'm having trouble connecting them.

We have twenty or more Vlan's on each network - so changing either switch to something other than Vlan 1 can cause downtime for that whole subnet.

We have five cisco 3560 trunking on network 1 and everyone able to use anything on the 192.168.1.0 network, we need to add network 2 - new company to company 1 192.168.1.0.

After trunking the two switches I put IP route 192.168.1.0 255.255.255.0 next hop to 192.168.1.x network - still not able to ping anything on that subnet, not even the next hop cisco switch.

Thank you

[Jon Marshall]

Okay, are there are any other vlans on the new 3750 other than vlan 1 ?

Jon

[Stephen Sisson]

We have about twenty Vlans on the 3750 switches

[Jon Marshall]

Stephen

Okay, a couple of solutions spring to mind -

1) if both switches are routing for vlans then we could use a new vlan to interconnect the switches and then route from one switch to the other. This would work if both switches are already routing which it sounds like they might be. You could then run a routing protocol between the 2 switches so each network knows about each other subnets.

2) you remove the L3 SVI from the 3750 for vlan 1 and add the IP (from the 10.x.x.x subnet) as a secondary IP to the vlan 1 interface on the 3560 and then connect the switches with a link in vlan 1. However you still have the issue of other subnets routing to each other.

3) you could simply create a  new vlan on the 3750 and assign all the vlan 1 ports into that vlan but you would need to do that for all switches connected to the 3750 as well.

Basically we still don't have enough information. So can you please answer all of the following -

1) does the 3560 route for the existing vlans. If not which device does and how is the 3560 connected to it ?

2) does the 3750 route for the new vlans. If not which device does and how is the 3750 connected to it ?

3) i am assuming that the only connection between existing and new is between the 3560 and 3750. Is this correct ?

4) you say there are multiple vlans on the new switches. Do you only want vlan 1 on the new switches to communicate or all of them. I'm assuming there is no communication at present. If there is then please clarify how that is working.

The simplest solution is if the 3560 routes for the existing vlans and the 3750 routes for the new vlans. Then we can simply route between the 2 switches and only have routes for the vlans you want to communicate. There would be no downtime as such with this solution. But the setup my not be suited to this which is why i want to understand exactly what you have.

Jon

[Stephen Sisson]

Hello

1) Does the 3560 route for the existing Vlan’s. If not which device does and how is the 3560 connected to it ? Yes routing for three subnets to 192.168.1.1 another 3560 switch with routing - all switches having routing

2) does the 3750 route for the new Vlan’s. If not which device does and how is the 3750 connected to it ? Yes routing ten networks, routing to the new MPLS network to other locations.

3) i am assuming that the only connection between existing and new is between the 3560 and 3750. Is this correct ? Yes Sir, we purchased this new company last year, and trying to connect them through the two switches 3560 - 3750

4) you say there are multiple Vlan’s on the new switches. Do you only want vlan 1 on the new switches to communicate or all of them. I'm assuming there is no communication at present. If there is then please clarify how that is working. We only need this one subnet 192.168.1.x network to connect but this can change - we need users on the 192.168.1.x network to talk with equipment on the network 2 10.1.1.x network and with the routing in place on the 3560 they can ping devices on the 10.1.1.x network through the 3750 switch network 1.

I figured you may say we need to change one of the switches from the current Vlan 1 subnet to use another that matches all other equipment on either network. I need to figur what is less work then to change the new network 3750 switches or network 1 on the 3560 switches. I'm still not sure why we can't add a vlan like we did Vlan 242 with 192.168.1.x to the 3560 by trunking between the two networks and this work, originally thought the problem is Native Vlan's on both networks because they both are using Vlan 1, now I'm not sure and still confused.

Thanks for trying

[Jon Marshall · Accepted solution]

The main problem is vlan 1 using 2 different subnets. Based on your answers i would simply do this -

choose and unused address range eg. 192.168.5.0

on the 3560

gi0/1 <-- connects to 3750

no switchport 

ip address 192.168.5.1 255.255.255.252

on the 3750

int gi0/1 <-- connects to 3560

no switchport

ip address 192.168.5.2 255.255.255.252

routing

=====

you could run a dynamic routing protocol but as you only want the 2 subnets to talk to each other -

3560

ip route 10.1.1.0 255.255.255.0 192.168.5.2

3750

ip route 192.168.1.0 255.255.255.0 192.168.5.1 

the above assumes 192.168.1.0/24 is only on your existing switches and 10.1.1.0/24 is only on your new switches.

So with the above you route between switches and vlan 1 is a different vlan 1 on the 3560 and the 3750. If you then want to route between more subnets you can either add more statics or run a dynamic routing protocol. You should be able to implement this with no downtime to your network.

However as a more long term solution you may want to move all routing for vlans to one switch. If you did then you could trunk between the switches but you still have the vlan 1 issue. The solution would be to either -

1) run a secondary 10.1.1.x address on the vlan 1 interface on the 3560 (remember you are routing all vlans now on the 3560)

or

2) as you say create a new vlan and allocate all the vlan 1 ports in the new switches into that vlan. You could then trunk between the switches but you would still want to centralise all the routing ie. move all the L3 SVIs on the 3750 to the 3560 or vice-versa.

Obviously 1) and 2) will require downtime so you would need to plan accordingly.

Jon