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Trunking problem

I'm having problem with the trunk to trunk. My setup is as follows..


- Management VLAN 192.168.99.X for all devices

-switch6a also has a serial port to receive an external link


I can ping from switch6a all the way to router6b (all ports), but i can not ping anything else past router6b, 

If i try from switch6d i can only ping switch6c and nothing else.


I have attached the config files(.txt) for all my switches and router (into 1 text file)


thank you in advance for all the help.


VIP Advisor

Re: Trunking problem

Do you have high level diagram how these device connected and what port


as per the below is this connected like daisy chain ?




what is the modesl of the Switches and routers, what is version of IOS it is running. ( i know you mentioned 2960 switch).



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Hall of Fame Expert

Re: Trunking problem

Hello Andre,

you describe your network in the following manner:




If between router6a and router6b there is only the serial interface with IP subnet you have not an end to end path in Vlan 99 but your Vlan 99 is partitioned in two broadcast domains interconnected by the two routers using the serial interface.


If so this explains your connectivity issues. Devices on the right part of the network (after routerb) should use a different IP subnet in vlan 99.

Edit; you are already using two different subnets on left side and on right side. This is correct.


In addition, the command switchport trunk enc dot1q may be unsupported because some modern switches do not support ISL option anymore, so the command is not needed. Only 802.1Q trunks are supported on them.


Even if you have an end to end ethernet based path there can be issues because you have the following:


switch switch6b:


interface FastEthernet 0/2
switchport trunk encapsulation dot1q


this interface should be configured as a fas0/1 if it is used to connect to the router router6a because routers do not support DTP . It can work if connected to switch6a by relying on DTP (but depends  on DTP state at least one side should desirable this was the old default, some new switches have a different default see below)


The same happens on switch6c you have configured only one port as trunk.

There is no sign of another interface configured

if switch6c fas0/1 connects to router6b fas0/0 you are fine, if at least one other interface connects to switch6d and it is in DTP mode desirable.


Be aware that there are some switches in newer releases that do not form trunk via DTP by default , because their DTP default settings is not desirable.

It would be better to configure all trunk ports with switchport mode trunk to be sure that all trunks are correctly formed.

You can check the status of trunk interfaces with

show interface trunk


Hope to help




Re: Trunking problem

I'm using the following equipment


2 x 2800 Router

4 x 3700 switch

network setup.jpg

Do not take in account the model router/switches used in packet tracer, have no idea on how to add new device to packet.


Hall of Fame Expert

Re: Trunking problem

Hello Andre.

if I have correctly understood your post you have used packet tracer only to provide a network diagram but you have real devices and the switches are Catalyst 3750.

The topology is the same I have supposed to be.

There is not an end to end LAN path and the routers are connected with a serial interface between them.


What I would like to point out is that interfaces with explicit trunk configuration (switchport mode trunk) are needed on switches for ports connecting to routers, because routers do not support DTP (Dynamic Trunking Protocol).

Ports between switches could use default configuration if their default is dynamic desirable.

C3750 are likely to behave in this way.

You can use

show interface gi1/0/1 switchport

to see the administrative and operational state of each port.


C3750 switches should use a three indexes 1/0/x interface numbering convention as they support stacking and they can host additional uplink modules.


Again I advice to configure each port with switchport mode trunk.


You can also use show interface trunk.



you need to use two different management Vlans because the serial interface breaks the vlans propagation.

your configuration used Vlan 99 on both parts of the network with two different subnets and this is correct.


Hope to help



Re: Trunking problem/solved

Thanks for all the help....

Got it to work.

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