EtherSwitch & VTP

[DSJONES10]

We have a 3745 with a NM-16ESW, when set as a VTP client it does not receive updates from a "server" - a 3550-12G.

The 3550-12G is connected to FE0/0 as a trunked link.

The router can ping the switch and vice versa, the equipment connected to the NM-16ESW cannot ping either the router or 3550.

Devices connected to the NM-16ESW can ping each other.

Any suggestions?

Thanks.

[Kevin Dorrell]

Please could you post the configs of the two units?

I'm not sure if you can make VTP work to a routed interface of the router. To create a trunk on a routed port normally requires me to manually configure a subinterface for each VLAN

I would have thought to make this work you would have to connect the 3550-12G to a switchport, then configure the switchport as a trunk in the normal way.

To connect the VLANs to the router, you will have to create VLAN virtual interfaces on the router.

Here is a useful document about the 16ESW:

http://www.cisco.com/warp/public/473/etherswitch_FAQ.html

Kevin Dorrell

Luxembourg

[DSJONES10]

Hi Kevin,

as per your suggestion I've attached the confs and vtp info.

I have a vg248 attached to FE3/0 - the problem area...

Thanks.

[nachieket]

Just check out the following:

1. If the trunk link is getting established properly or not. The options you have configured on the trunk ports should match.

2. If the IPs on both the switches are from the same subnet or not ! If they are from the same subnet then establishing the proper trunk link should allow them to ping each other. But if they are not from same subnet then you would need to create trunk to the router on 3550-12G and make sure you create subinterfaces for both the subnets defined on the different switches so that when you try to ping from NM-16ESW to 3550-12G or to router it will go to router first through trunk ports and do the routing and then come back to 3550-12G and then 3550-12G can respond back to icmp by the same way.

I hope this will help you out.

Nachiket Joshi

[DSJONES10]

There are 10 switches in total, hub-spoking to a pair of 3550-12g's.

The 3550-12g's are both vtp servers, all these switches can ping each other.

From one of the 3550-12g GE ports is a link to the 3745, its a trunk link. The router FE has sub-interfaces for the 3 key VLANs. All 3550 switches can ping the router and the effective default gateway of the other networks - on the routers FE.

The issue appears to be the etherswitch module within the router. Currently it does not communicate VTP wise either as a client or as an effective server.

[Kevin Dorrell]

As I said before, I think the problem is that VTP will not run through a routed interface on the 3745. You have the switch trunk connected to F0/0. Each VLAN is defined with a subinterface F0/0.n. I don't think VTP can create subinterfaces on your 3745.

OTOH, I've never tried running a trunk with VTP to a port of a 16ESW, but I don't see why it shouldn't work. Try connecting your router at a switch port.

The way I see it is that the F0/0 is essentially a layer 3 entity (supporting a collection of layer-3 subentities). It is designed for routing. The 16ESW switchports, on the other hand, are layer-2 entities. (Unless you use a recent version of IOS that allows you to turn them into routed ports). That is why to connect a set of them to the router (layer-3), you need a virtual interface to represent the VLAN.

Now VTP is a protocol that controls layer-2 trunking ...

Try using a switchport, defined as a trunk, to connect your router. See if VTP works then.

Kevin Dorrell

Luxembourg.

[DSJONES10]

I can sort of see what you're getting at however looking at it "logically" being a router-switch with the capability of defining VTP settings and VLANs within a routing environment - i.e. an understanding of L3 requirements and interVLAN communications, you'd have thought that would be it.

It certainly undertsood the existance of the host IP addresses as when I experimented with defining a VLAN 100 with a host address within the require subnet it balked - as expected.

Are you suggesting running a trunk connection from the EtherSwitch module into the core switch [the VTP server]?

[Kevin Dorrell]

Yes, that is exactly what I mean.

Let me try and persuade you in different terms. (I am doing this as much for my benefit as for yours, because I hope that if I am wrong someone will take the trouble to put me right.) I have attached a sketchy config to illustrate the point.

Think of the 16ESW as a small layer 2 switch. It can do VTP, VLANs, whatever, but not routing. You can set VLANs between the switch ports, and they will talk within the VLAN. You can connect them to a larger layer-2 network, like I have done for F3/15 in the example. Then it will do VTP.

But the switchports will not route between VLANs on their own. For that, you need to attach the VLANs to a router. You do that by creating a virtual VLAN interface for each VLAN you want to route (see my example config). The VLAN interface represents the VLAN to the router. It is on the VLAN interface that you put the router IP address, etc.

This much I know. But what I am a little hazy about is how a router interface like F0/0 fits into this scheme. I don't think it can do VTP. If it did, then every time you added a VLAN at the VTP server, it would have to create a new sub-interface. Where would it get the IP address from?

I am also unsure what happens if you have subinterfaces on an F0/0 as well as a VLAN virtual for that VLAN. Will they talk or not?

BTW, so as not to miss anything out, you can turn one of the layer-2 switchports on the 16ESW into a layer-3 routed interface by adding an IP address. You can even add sub-interfaces to it. Then it becomes a router interface rather than a switchport. You can only do that in IOS 12.2(15)ZJ and 12.3(4)T onwards, see http://www.cisco.com/warp/public/473/etherswitch_FAQ.html#qa3

Hope this gives you some goodstuff to experiment with. If I am wrong, then I'm sorry. If I'm right, please don't forget to "Rate this post" ;-).

Kevin Dorrell

Luxembourg

[DSJONES10]

Hello Kevin,

you were correct, it appears the etherswitch does "stand-alone" when it comes to VTP.

Thanks and keep up the excellent "support" work, Stuart

[Kevin Dorrell]

Thank you Stuart, I'm glad it was useful to you.

Kevin Dorrell

Luxembourg

[Kevin Dorrell]

Just a minor correction to me last posting: I have just read that if you turn a 16ESW switchport into a routed interface by adding an ip address and doing no switchport, then you cannot add subinterfaces to it.

http://www.cisco.com/warp/public/473/etherswitch_FAQ.html#qa15

I guess that was obvious if I think about it. When you are doing sub-interfaces to represent VLANs, you usually leave the ip address off the physical interface.

I guess what they are saying here is: yes, you can use a phsyical interface of a 16ESW as a routed interface, but it does not support 802.1Q in that mode. If you want it to be a trunk, you have to use it as a switchport.

Kevin Dorrell

Luxembourg