We have following network diagram, but SW3 isn't in place, and I have a question before I plug SW3 into the network.

Sorry, I put in the wrong diagram before. The following diagram is the correct one:

enter image description here


Currently, the following ports are in FWD mode, and 2G is Alt blocking port.

SW1 - 10G interface in FWD - 2G BLK
SW2 - 10G interface in FWD - 2G BLK

Here is the output of SW2 switch. PO1 is an etherchannel interface 2x1G.

SW2#sh spanning-tree int te1/0/2

Vlan                Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
VLAN0010            Root FWD 2         128.52   P2p
VLAN0020            Root FWD 2         128.52   P2p
VLAN0030            Root FWD 2         128.52   P2p

SW2#sh spanning-tree int po1

Vlan                Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
VLAN0010            Altn BLK 3         128.456  P2p
VLAN0020            Altn BLK 3         128.456  P2p
VLAN0030            Altn BLK 3         128.456  P2p


  1. Which path will SW3 take to the Root Bridge?
  2. If I increase the cost of the 2G interface on SW3, then it will take the 10G interface path, but the problem is what will happened when the link goes down between SW1 and SW2?
  3. I want SW3 switch to primarily use the 10G interface, and if the link between SW1 and SW2 is down, then use 2G backup link.
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can provide your own answer and accept it.
    – Ron Maupin
    Commented Aug 6, 2017 at 22:49

2 Answers 2



Based on your new drawing, it make more sense. Switch 2 prefers the path through Switch 1 because it has a cost of 6, but the 2G connection has a cost of 7.

Switch 3 will have a cost of 8 (or 9 in a failure of the link between switches 1 and 2) through Switch 2, but like Switch 2, the cost for Switch 3 through the 2G connection is 7, so it will be the preferred path in either scenario.

This design doesn't follow best practices. Since each switch has two connections, both connections should be to the layer above. A best practice is to never connect access switches to each other. You also want to pay attention to the oversubscription ratio (maximum 20:1 for access to distribution, and maximum 4:1 for distribution to backbone).

  • I am confused with your question, If i plug SW3 today the way i showed you in diagram then which path it will take? 10G or 2G ?
    – Satish
    Commented Mar 28, 2017 at 23:37
  • Based on your values, if every link is up, Switch 3 will use the 2G link, but I think that your Switch 2 is also using its 2G link because you have its cost at 3 while its 10G link has a cost of 4.
    – Ron Maupin
    Commented Mar 28, 2017 at 23:44
  • I have updated question. NO sw2 currently using 10G and its 2G link is in BLK mode. i think i didn't explain that, in short SW1 and SW2 both using 10G for root bridge and 2G is in blocking mode. that is why i am confused how spanning tree calculation cost?
    – Satish
    Commented Mar 28, 2017 at 23:46
  • Are you sure of the path costs? If you have 2+2 for the 10G link, and 3 for the 2G link, then it should be using the 2G link.
    – Ron Maupin
    Commented Mar 28, 2017 at 23:49
  • See my question i have updated with command output.. i am 100000% sure and my monitoring tool showing 10G driving 500mbp data and 2G has only 100kbps so its very clear SW2 using 10G for data.
    – Satish
    Commented Mar 28, 2017 at 23:53

For this topology, you should enable "spanning-tree pathcost method long" on all switches.

Have you correctly configured the root for the spanning trees using "spanning-tree vlan x-y priority xxxx" as I assume you are using some flavor of PVST?

  • Yes we are using PVST, what is spanning-tree pathcost method long ?
    – Satish
    Commented Mar 30, 2017 at 2:23
  • The long method uses a 32-bit metric to calculate cost whereas the default method uses 16 bits. Therefore, you get more granularity when using links with speeds past 1G. You should also be using Rapid PVST+ instead of PVST with this many loops in your topology.
    – Rooster
    Commented Mar 30, 2017 at 2:35

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