Cisco – Spanning Tree cost calculation

ciscoNetworknetwork-corespanning tree

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

EDIT

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

Questions:

  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.

Best Answer

Edit:

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).

Related Topic