Loop

Unanswered Question
Jul 19th, 2010

Hi

I got a scenario where SW1 connects to SW2  ; sw2 connects to Sw3  ; Sw3 connects to Sw1

Sw1 = 3560

Sw2 = 2960

Sw3 = LinkSys

Sw1-------------------Trunk port-------------------Sw2  ( Fiber )

Sw2--------------------trunk port-------------------sw3  ( GigaEthernet )

Sw3--------------------Trunk port------------------sw1  ( GigaEthernet )

Spanning-tree mode PVST ( this is the default on 3560 )

Will there be a loop in this scenario

I have this problem too.
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Hi

In sw2 there is pc1
Pc1 send broadcast packet , sw2 will send the packet out of all port except the was received the packet (PC1 port)
That means sw2 will send it to sw1 and 3 ,, then sw3 will send it to sw1 , sw1 will send the packet was received from sw2 to sw3 , And the packet was received from sw2 to sw3
Then sw3 will send the packet was received from sw1 to sw2 ............etc

Hope this help

Ganesh Hariharan Mon, 07/19/2010 - 01:33

Hi

I got a scenario where SW1 connects to SW2  ; sw2 connects to Sw3  ; Sw3 connects to Sw1

Sw1 = 3560

Sw2 = 2960

Sw3 = LinkSys

Sw1-------------------Trunk port-------------------Sw2  ( Fiber )

Sw2--------------------trunk port-------------------sw3  ( GigaEthernet )

Sw3--------------------Trunk port------------------sw1  ( GigaEthernet )

Spanning-tree mode PVST ( this is the default on 3560 )

Will there be a loop in this scenario

Hi,

As per STP only one path will be open for data communication and redudant will be blocked, say from sw1 to sw 2 to sw 3 open and sw 3 to sw 1 will be in  blocked state.

Hope to help !!

Ganesh.H

Ganesh Hariharan Mon, 07/19/2010 - 02:28

Hi Ganesh,

Is it possible to influence which link to prefer.

You can by making spanning tree port cost, By calculating and assigning the port cost of the  switch ports, you can ensure that the shortest (lowest cost) distance to  the root switch is used to transmit data. You can calculate and assign  lower path cost values (port costs) to higher bandwidth ports by using  either the short method (which is the default) or the long method.

http://www.cisco.com/en/US/docs/switches/lan/catalyst4000/7.1/configuration/guide/spantree.html#wp1158533

Hope to help !!

Ganesh.H

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