Trunk VLAN across multiples switches/routers

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Jul 24th, 2007
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I have two Cisco 4507R devices. VLAN 100 exist on Router A and is trunked to Router B. I need to trunk VLAN 100 to Switch A which is connected to Router B. I am having trouble because no matter what I do I can not get the device hooked to Switch A to function on the network or even so much as ping the gateway on Router A.

I can provide configs if necessary.

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rajinikanth Tue, 07/24/2007 - 19:11
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Yes please provide configs it might help to detect the problem

Amit Singh Tue, 07/24/2007 - 20:30
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Config is necessary but the most important is the network diagram, which can help us understanding your topplogy first. Once we have the topology we will be able to better help you.

-amit singh

sri.chalasani Wed, 07/25/2007 - 06:17
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I have attached the config (slightly edited) and a basic diagram of what I am trying to accomplish. Any suggestions would be great.


mchoo2005 Wed, 07/25/2007 - 18:21
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Strange problem... Try these:

1. Configure the native VLAN for the trunk link between Edge Router and Edge Switch to VLAN 107

2. On Edge Switch: Configure default gateway to point to Vlan107 interface on Edge Router

I might be stating the obvious here, but... What's the Spanning Tree mode that the Edge Router is running? It should be running either "rapid-pvst" or "pvst", not mst.

BTW, you don't need "ip default-gateway" on Edge Router, since it's Layer-3. You only need this command for Layer-2 switches.

lamav Wed, 07/25/2007 - 18:53
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On the edge switch, have you done a "sh trunk" to see if the trunk is up and if vlan 402 is being allowed on that trunk?

I see that the trunk interface on the edge switch that is connected to the edge router is not configured for dot1q encapsulation.


interface GigabitEthernet0/1

description UPLINK-TO-4507 switchport trunk allowed vlan 107-112,402

switchport mode trunk

lamav Wed, 07/25/2007 - 19:00
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Sorry, I just realized its an IOS 4507 switch, so the command to view the trunk information is:

show interfaces GigabitEthernet0/1 trunk

sri.chalasani Thu, 07/26/2007 - 06:59
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I have moved the trunk port on the switch and edge router to native vlan 107 to no avail. I also changed the default gateway to the default of vlan 107 but that also did not help.

The encapsulation is not specified because the IOS version on the 2950 only supports dot1q so you can't change it.

With the 'sh trunk int' command it shows VLAN 402 as being trunked.

lamav Thu, 07/26/2007 - 16:28
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OK, so let me get this straight:

1.) The trunk between the edge router and edge switch is UP.

2.) Vlan 402 is allowed on that vlan.


So, what exactly do you mean when you say the " Port attempting to access VLAN 402 without success"? Do you mean that you connect a device to that port and the port wont come up?

if so, what kind of device is it? Is it a PC? is it set for DHCP? If so, is it getting an ip address within vlan 402's IP scope? If it is a PC and it is hard-coded, is the IP address annd mask configured correctly?

sri.chalasani Fri, 08/03/2007 - 07:40
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Problem solved. Had to specify vtp trunk mode on the middle switch.


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