Question on - Loops

Unanswered Question
Apr 11th, 2010

Hi

I had seen some design with 2 Core and edge-switch is connecting to 2 core.

what configuration is done on Edge-switch to connect to 2 Core, how does it avoids Loop.

Another Scenario would be One Core and multiple of edge-switches. Each edge-sw connects to core and another SFP ports connects to othere Edge-Sw.

To connect Core to Edge-Sw  ( switchport mode trunk , switchport trunk encap dot1q ); what config is needed to avoid loops

Nasr

I have this problem too.
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Jon Marshall Sun, 04/11/2010 - 14:36

nasr.khan wrote:

Hi

I had seen some design with 2 Core and edge-switch is connecting to 2 core.

what configuration is done on Edge-switch to connect to 2 Core, how does it avoids Loop.

Another Scenario would be One Core and multiple of edge-switches. Each edge-sw connects to core and another SFP ports connects to othere Edge-Sw.

To connect Core to Edge-Sw  ( switchport mode trunk , switchport trunk encap dot1q ); what config is needed to avoid loops

Nasr

Nasr

You simply need your switches to run some form of STP (spanning-tree protocol) whether that be PVST+ or rapid-PVST+ or MST. STP will automatically work out a loop free L2 topology within your network. By default PVST+ runs on most switches so unless you want to change this you don't really have to configure anything unless you want to configure additional features such as uplinkfast/backbonefast/portfast.

If you want to run another form of STP you can use the "spanning-tree mode [mst|pvst|rapid-pvst]" command to choose.

Jon


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andrew.butterworth Sun, 04/11/2010 - 14:48

A lot depends on the topology of the network.  Typically in a collapsed Core/Distribution with Access switches then you wouldn't normally create layer-2 loops.  i.e. each core/distribution switch would present the Access Layer VLAN on the downlinks to the access-layer switch but not between the pair of core/distribution switches and therefore there is no loop.  Each core/distribution switch would terminate the SVIs and typically run a 1st-hop redundancy protocol (HSRP, VRRP or GLBP) for the Access layer devices (PCs).

If you present the same VLANs to multiple access switches or on the link between the two core/distribution switches then you create a loop and STP needs to be running and configured correctly so ports will block.  I am not a fan of big layer-2 networks or even layer-2 networks with loops so my preference would be to avoid loops and design the network without them.

Have a read of the Campus design SRND at http://www.cisco.com/go/srnd

Andy

Ganesh Hariharan Sun, 04/11/2010 - 22:39

Hi

I had seen some design with 2 Core and edge-switch is connecting to 2 core.

what configuration is done on Edge-switch to connect to 2 Core, how does it avoids Loop.

Another Scenario would be One Core and multiple of edge-switches. Each edge-sw connects to core and another SFP ports connects to othere Edge-Sw.

To connect Core to Edge-Sw  ( switchport mode trunk , switchport trunk encap dot1q ); what config is needed to avoid loops

Nasr

Hi Nasr,

Avoidence of loops will be taken care by STP which runs default on all cisco switches and with 2 core and 1 edge switch you can have redundacy for outgoing traffic for cirew switch by making HSRP  or VRRP configuration at core level.

Two edge and 1 core with this design STP will take for automatic blocking of ports and you genrally need to configure etherechannel betwee the edge switches with allowing all the vlans or selected vlans.

Check out the below link for configuring the same

http://www.cisco.com/en/US/tech/tk389/tk213/technologies_configuration_example09186a0080094689.shtml

Hope to Help !!

Ganesh.H

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