Spanning Tree help

Unanswered Question
Aug 15th, 2008

Hi, I have got 2 Cisco 3560's trunked together, and the only Spanning info I can see on each are:

spanning-tree mode pvst

spanning-tree extend system-id

There is nothing under any ports etc, sunch as spanning-tree portfast

Should I be adding anything else to stop IP Frame loops?

I have this problem too.
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Daniele Giordano Fri, 08/15/2008 - 00:13

No, on your switch is enable pvst (per vlan spanning tree) that maintains a spanning tree instance for each VLAN configured in the network switch.

Use "show spanning-tree" command to display spanning-tree information.

whiteford Fri, 08/15/2008 - 00:18

Thanks, at he moment these 2 switches are one big VLAN, on the 2nd switch I will need to create a new VLAN.

Thing is my LAN connects to switch 1 and I'm wondering how the trunk will make be made aware of this new VLAN?

At the moment I just have:

interface GigabitEthernet0/2

description trunk to 3550

switchport trunk encapsulation dot1q

switchport mode trunk

Hi There

By default all traffic will be carried over a trunk link and will be tagged with the VLAN ID to distinguish between traffic from different VLAN's. However I think that you will have to create the new VLAN on both switches, even though you may have no ports in this VLAN on switch 1.

Of course as you are using dot1q encapsulation, native VLAN (VLAN 1 by default) traffic is passed untagged across the trunk. When the receiving switch sees the untagged packets, it then knows that this traffic is destined for the native VALN.

You will also have to consider intervlan routing. Do you want the hosts on the "one big VLAN" to communicate with host on this new VLAN?

Then you will need to introduce layer 3 functionality, if you do not already have it. If switch 1 connects to a router, then you can configure the new VLAN information there. If not you will have to turn on routing on switch 1 to route traffic between the existing "big VLAN" and the new VALN.

It would help if you could provide some information about your topology, as in what is switch 1 connected to (if anything)? Do you have a flat LAN (Single VALN) or is your LAN comprised of multiple VLANS? If yes, how is intervlan routing handled? Will traffic from outside of your LAN (remote offices, etc) need to access this new VLAN?

Best Regards,


whiteford Fri, 08/15/2008 - 00:49

So Would I need to tag the VLAN traffic for the new VLAN I want to create as it will go to the native vlan instead?

If so how?


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