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New Member

VLAN configuring headache

I am thanking those in advance with taking the time to look at this situation, and render and suggestions. I am trying to deal with what I've been dealt with in my network. I've attached  a ROUGH sketch that lays out what I'm looking at:

We have a large network. It's been segmented at various area. THe main network I'm calling Network A. From the CORE switch of Network A, it is connected to a smaller CORE switch of Network B. The overall network is flat; no routing occurs on this network. Access Switches are trunked to the Distro switches, and Distro on up to the Core switch, then up to the network PRIMARY CORE, and out to the world...

So, a long time ago there used to be a LOT of devices on Network B. But not as many as Network A. There now is only 4 devices left on Network B; a Core Switch, a Distro Switch, and 2 Access switches. The Core and DIstro are in the same rack. They feed the two access switches over fiber trunk links.

As I said, Network A and Network B aren't routed. However, they have two devices between them that ARE routed via static routes between the two devices. These are only two static routes, and they are basically acting as a PPP link between the two routes.

So, on Network A, there exists VLAN 60. VLAN 60 is used for specific devices on that network. VLAN 60 is assigned an IP address, and these certain comms devices are attached to the network via VLAN 60. They pull their IP addresses from DHCP, which has a VLAN 60 scope.

It has now been determined that one of these comms devices needs to be put on Network B. VLAN 60 has never existed on Network B. I started to think this would be easy; I'd just change the VTP domain on Network B to the VTP domain on Network A (only 4 network devices left on B), let VTP do it's magic, and go about my business. So, I changed the VTP domain name to match Network A's...made them all clients..and TADA! Nope..nothing changed I'm studying for CCNP Switch. I JUST read this about a month ago, and did labs on it..let me go back and read up....oops! You can only use VTP across trunk links. So, because the connection between Network A and Network B is a routed link, VTP won't go between the two, correct?

So, now that I'm stuck with that, I'm figuring how to configure VLAN 60 on Network B so that it will play with the other devices on VLAN 60 on Network A. On network A, VLAN 60 has an IP address of Other devices have IP addresses from that VLAN via DHCP. Do I just create VLAN 60 on Network B, give it an IP address within the same subnet, and let the magic happen? Or am I thinking too hard, and trying to over think this?

Everyone's tags (3)
New Member

VLAN configuring headache

Where vlan 60 is assigned on the network A ? On the core router ?

If so, I believe Core in the segment A is able to comunicate with the routed segment ?

There are two options:

1. Create SVi on the core routers and put Vlan 60 in the proper ports. Then create SVI for vlan 60 on B network and put the appropriate ports in the vlan 60. It will create routable interfaces which you will be able to interconnect via routed segment - via routing protocol or static routes.

2. Create SVI on routed segment and trunk this back to Core in network A. This IP address will be a routed point that will bypass the routed segmment and then by having the same config on the right part of your diagram, the traffic will arrive on SVI, then will go over trunk and then to the proper access port with Vlan 60 on segment B.

All depends on the hardware you have in your network which you have not mentioned in your post.

New Member

VLAN configuring headache


Thanks for the reply.

Network A has a 6509 as Core. Network B has a 3750G as a core. There is no routing table/routing protocol going on between these two. The routing table shows the next hop from Network B as directly connected. The two devices that are the routed segment...there's another non-routed device in there as well, but I can't discuss further on that. Just know that it's a "routed segment".

So, Network A has an SVi called VLAN 60, and it's got the IP address of When you do a sho ip int bri, vlan 60 shows up in the list as up/up. Ports on Network A that have these comms devices on it are put into VLAN 60. Network A has it's own VTP domain.

Network B has it's own VTP domain. I was just going to put the 4 devices into Network A's VTP domain, but then the IP addressing is different between the two.

I'm guessing I can just create VLAN 60 (to remain consistant) on Network B, give it SVi, and go from there. I'll just have to give it an IP address consistent from the Network B side.

If that darn routed segment/devices weren't there, it would go so much better.


Re: VLAN configuring headache

   I think you have to clarify what that routed link is between the 2 cores. Is it just a direct link between the 2 cores ?  What are the core switches model wise.  It really depends on that routed link .   When you say there are 2 devices between them what does that mean ?  Is that routed link how you will be attaching the 2 cores or are you running a new link ?

New Member

Re: VLAN configuring headache

See, I can't go further into that routed segment..I can't discuss it.

All I can say about it is it is a statically routed segment. No routing protocols used. There are a couple static routes in the table at each end. Basically the two devices point to each other. Can't go into what else they are.

I know it would be helpful with this problem. It's maddening to me

But know that Network A connects to Network B via a "magic" statically routed segment.

The interface that is on Network A that points to this has an SVi that is pointing to it. Network B has same thing. The default gateway on  Network B is, which is on the right side device of the "routed" segment. See updated image.

New Member

VLAN configuring headache

Yes, by removing the routing segment you would just have a pure trunk and vtp across and job done, everybody happy.

Create SVI for network B (replica of network A) on 3750 switch, then just run trunk and access ports and i think it will solve the problem.

You may will have to put a default static route on 3750 pointing back (or default gateway however I think default gateway may be good for the layer 2 switch with no routing capabilities) to the routed segment and a static route on the routing segment pointing back to segment B.

Let me know how you getting on.


New Member

VLAN configuring headache


While I would love to remove the routed segment..let's call it "propriatery" and it has to stay where it is.

If I mirror the SVI like Network A...

I can't use the IP that is already assigned to the SVI on the Network A side. So, if I understand correctly, I can use another IP within that scope, and everything "should" be happy, correct?

Like I said, the distro and core switches on Network B aren't routed. The routing table shows everything directly connected. There is NO routing statements on them. THere is just the default gateway pointing to the next hop on the "magic' segment.

On that device, there is a static route pointing to Network B.

New Member

Re: VLAN configuring headache

Oh yes, by mirroring I meant same setup, different details . Probably you will be using a new subnet as per your attached diagram.

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