Trunk backup link

Unanswered Question
Jun 2nd, 2010

Hi all,

We have to configure a catalyst 4507 in a new building. This Catalyst will have a 10Gb link to another building (another Catalyst 4500). This link will be configured as trunk.

But, we have to configure a wifi backup link in case the optical fiber stops working. We will configure the wifi bridges in order to transmit all VLANs. But, how can we make sure that, in normal conditions, all traffic goes through the trunk link using the optical fiber (10gb) and not through the trunk link using the wifi bridge?

Our solution is based on STP:

     We should make sure that the Catalyst of the old building is the Root Bridge. Afterwards, we should increase the port priority of the port using the wifi link in that Root Bridge, so the new Catalyst will block the port connected to the wifi bridge. We think this should be enough, shouldn't it?

     Or, no configuration should be needed since the wifi will be connected to a GigabitEthernet and the optical fiber to a 10GibabitEthernet interface? I mean, the 10 GigabitEthernet will have a lower cost. That is, the STP default configuration should be enough. Isn't this right?

What would you recommend?

Thanks in advance!!

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Giuseppe Larosa Wed, 06/02/2010 - 02:48

Hello Inaki,

the 10G link will have a lower spanning-tree cost for all Vlans and that it is enough.

In order to make sure the backup link is not  used unless primary link is failed you can use

spanning-tree cost 3000

on interface connecting  to radio link on both sides

Hope to help

Giuseppe

Calin Chiorean Wed, 06/02/2010 - 02:50

You are right in both cases, cost and priority. It should work with cost only, but because I like the things to be pretty straight, I would manipulate the priority so that the fiber optic link is always preferred.

Ganesh Hariharan Wed, 06/02/2010 - 10:11

Hi all,

We have to configure a catalyst 4507 in a new building. This Catalyst will have a 10Gb link to another building (another Catalyst 4500). This link will be configured as trunk.

But, we have to configure a wifi backup link in case the optical fiber stops working. We will configure the wifi bridges in order to transmit all VLANs. But, how can we make sure that, in normal conditions, all traffic goes through the trunk link using the optical fiber (10gb) and not through the trunk link using the wifi bridge?

Our solution is based on STP:

     We should make sure that the Catalyst of the old building is the Root Bridge. Afterwards, we should increase the port priority of the port using the wifi link in that Root Bridge, so the new Catalyst will block the port connected to the wifi bridge. We think this should be enough, shouldn't it?

     Or, no configuration should be needed since the wifi will be connected to a GigabitEthernet and the optical fiber to a 10GibabitEthernet interface? I mean, the 10 GigabitEthernet will have a lower cost. That is, the STP default configuration should be enough. Isn't this right?

What would you recommend?

Thanks in advance!!

Hi ,

STP Cost fo 10 Gbps links is 2 so to reach to vlan over the trunk to other end of the switch 10 gbps link will be prefred and as safer side as suggested by configure the interface cost of radio link to higher to act as back up link.

Hope to Help !!

Ganesh.H

Inaki Kortazar Mon, 06/07/2010 - 00:51

Hi,

Today I have received the installation plans and realized that the wifi bridge is not directly connected to the new chasis. It is connected to a 3750 stack situated in the first floor. This stack and the chasis are connected via optical fiber (2Gb etherchannel). So the scenario is:


----------------------                     --------------------
|                     |                    |                    |
|                     |                    |                    |

|                      | 10Gb fiber   |                    |
|  Old Chasis   |--------------------|  New Chasis |
|                     |                    |                    |
|                     |                    |                    |
----------------------                     --------------------
       |                                         |
       |                                         |
       |                                         | Fiber (2Gb etherchannel)
       |                                         |
       |                                  ------------------
       | Wifi Bridge (Backup) |     New        |
       ---------------------------------|  3750 stack |
                                        |                   |
                                         -------------------

We think that this does not change too much. If in the old chasis and in the 3750 stack we define a higher STP cost for the ports where the wifi bridge are connected, the wifi link will work as backup, won't it? Or, the Root Bridge ID has something to do here?

We discarded Flex-Links since we read "If the switch is running STP, Flex Links is not necessary because STP already provides link-level redundancy or backup." in the link. But thank you for your help mbroberson1, learning new things is always fine. We didn't know about Flex Links.

Thank you very much!!

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