Calculation of MST cost - Page 2

rasmus.elmholt · ‎07-03-2025

Hi all,

Could someone please explain to me why I see the things I do with MST?

I have the below setup in a lab, where SW1 is configured with the lowest Cost and is the root bridge.

Everything is working as expect, in regards to STP, and SW4 is Blocking its port Gi1/2 and 1/3 towards SW3.

I found this old post from @Peter Paluch in regards to Common and Internal STP cost, but am not sure it has anything to do in regards to what I am seeing, except that I got confused by the path cost of 0... https://community.cisco.com/t5/switching/mst-is-showing-root-cost-for-mst0-as-zero/m-p/1960068/highlight/true#M219070

But my question is: Why will the shutdown of a port in the LAG between SW2 and SW3 result in the STP topology to change so much that the traffic from SW3 will go through a 100mbit link to SW4->SW2->SW1?

rasmus.elmholt · ‎07-07-2025

Here you go:

SW10#show span mst configuration 
Name      [CiscoSTD]
Revision  1     Instances configured 1

Instance  Vlans mapped
--------  ---------------------------------------------------------------------
0         1-4094
-------------------------------------------------------------------------------

SW2#show span mst configuration 
Name      [CiscoSTD]
Revision  1     Instances configured 1

Instance  Vlans mapped
--------  ---------------------------------------------------------------------
0         1-4094
-------------------------------------------------------------------------------
SW3#show span mst configuration 
Name      [CiscoSTD]
Revision  1     Instances configured 1

Instance  Vlans mapped
--------  ---------------------------------------------------------------------
0         1-4094
-------------------------------------------------------------------------------
SW4#show span mst confi
Name      [CiscoSTD]
Revision  1     Instances configured 1

Instance  Vlans mapped
--------  ---------------------------------------------------------------------
0         1-4094
-------------------------------------------------------------------------------

MHM Cisco World · ‎07-07-2025

Spanning tree mst config

Show current

Show pending

Share above for all SW

MHM

rasmus.elmholt · ‎07-07-2025

I have not attached the configuration for SW10 as it is not relevant here, but SW2, SW3 and SW4 is shown below:

SW2(config)#spanning-tree mst config 
SW2(config-mst)#
SW2(config-mst)#show current 
Current MST configuration
Name      [CiscoSTD]
Revision  1     Instances configured 1

Instance  Vlans mapped
--------  ---------------------------------------------------------------------
0         1-4094
-------------------------------------------------------------------------------
SW2(config-mst)#
SW2(config-mst)#show pending 
Pending MST configuration
Name      [CiscoSTD]
Revision  1     Instances configured 1

Instance  Vlans mapped
--------  ---------------------------------------------------------------------
0         1-4094
-------------------------------------------------------------------------------
SW2(config-mst)#

SW3(config)#spanning-tree mst config 
SW3(config-mst)#
SW3(config-mst)#show current 
Current MST configuration
Name      [CiscoSTD]
Revision  1     Instances configured 1

Instance  Vlans mapped
--------  ---------------------------------------------------------------------
0         1-4094
-------------------------------------------------------------------------------
SW3(config-mst)#
SW3(config-mst)#show pending 
Pending MST configuration
Name      [CiscoSTD]
Revision  1     Instances configured 1

Instance  Vlans mapped
--------  ---------------------------------------------------------------------
0         1-4094
-------------------------------------------------------------------------------
SW3(config-mst)#

SW4(config)#spanning-tree mst config 
SW4(config-mst)#
SW4(config-mst)#show current 
Current MST configuration
Name      [CiscoSTD]
Revision  1     Instances configured 1

Instance  Vlans mapped
--------  ---------------------------------------------------------------------
0         1-4094
-------------------------------------------------------------------------------
SW4(config-mst)#
SW4(config-mst)#show pending 
Pending MST configuration
Name      [CiscoSTD]
Revision  1     Instances configured 1

Instance  Vlans mapped
--------  ---------------------------------------------------------------------
0         1-4094
-------------------------------------------------------------------------------
SW4(config-mst)#

MHM Cisco World · ‎07-07-2025

when you do
show spanning tree interface xxx detail
do you see
port is boundary

check this point

MHM

rasmus.elmholt · ‎07-07-2025

It does not seem to be a boundary port.

When I shutdown gi0/1 on SW3 I get the following output on SW2:

SW2#
*Jul  7 12:54:30.457: MST[0]: Po1 state change forwarding -> blocking
*Jul  7 12:54:30.458: MST[0]:<RX- Po1 inferior designated BPDU Prot:0 Vers:3 Type:2
*Jul  7 12:54:30.458: MST[0]:   Role     :Desg Flags[AFL] Age:0 RemHops:20
*Jul  7 12:54:30.459: MST[0]:   CIST_root:32768.5009.0003.0000 Cost   :0
*Jul  7 12:54:30.459: MST[0]:   Reg_root :32768.5009.0003.0000 Cost   :0
*Jul  7 12:54:30.459: MST[0]:   Bridge_ID:32768.5009.0003.0000 Port_ID:32833
*Jul  7 12:54:30.459: MST[0]:   max_age:20 hello:2 fwdelay:15
*Jul  7 12:54:30.460: MST[0]:   V3_len:64 region:CiscoSTD rev:1 Num_mrec: 0
SW2#
SW2#
SW2#show span int po1 det
 Port 65 (Port-channel1) of MST0 is designated blocking (dispute)
   Port path cost 20000, Port priority 128, Port Identifier 128.65.
   Designated root has priority 24576, address 5009.0001.0000
   Designated bridge has priority 32768, address 5009.0002.0000
   Designated port id is 128.65, designated path cost 0
   Timers: message age 0, forward delay 3, hold 0
   Number of transitions to forwarding state: 12
   Link type is point-to-point by default, Internal
   PVST Simulation is enabled by default
   BPDU: sent 346865, received 171534
SW2#
*Jul  7 12:54:45.458: MST[0]: Po1 state change blocking -> learning
*Jul  7 12:54:47.125: MST[0]: Po1 state change learning -> blocking
*Jul  7 12:54:47.125: MST[0]:<RX- Po1 inferior designated BPDU Prot:0 Vers:3 Type:2
*Jul  7 12:54:47.125: MST[0]:   Role     :Desg Flags[ALP] Age:0 RemHops:17
*Jul  7 12:54:47.126: MST[0]:   CIST_root:24576.5009.0001.0000 Cost   :0
*Jul  7 12:54:47.126: MST[0]:   Reg_root :24576.5009.0001.0000 Cost   :250000
*Jul  7 12:54:47.126: MST[0]:   Bridge_ID:32768.5009.0003.0000 Port_ID:32833
*Jul  7 12:54:47.126: MST[0]:   max_age:20 hello:2 fwdelay:15
*Jul  7 12:54:47.126: MST[0]:   V3_len:64 region:CiscoSTD rev:1 Num_mrec: 0

MHM Cisco World · ‎07-07-2025

debug spanning-tree mstp bpdu receive 
debug spanning-tree mstp bpdu transmit

Check this for each SW see if it receive/send bpdu

MHM

rasmus.elmholt · ‎07-07-2025

I guess we are only interested in the BPDUs on po1?

As shown in the previous commands the BPDU counters increment so we must be sending and receiving BPDUs.

I will get back with some debugs.

MHM Cisco World · ‎07-07-2025

SW1(root)-SW2

The interface in SW2 will show only receive bpdu

The interface in SW1(root) will show only send bpdu

Interface show both there is loop sure

Now

Check

1-show port channel summary

Check if both port members of PO is pending

2- check mst digest it must same in all SW

MHM

rasmus.elmholt · ‎07-09-2025

@MHM Cisco World wrote:

SW1(root)-SW2

The interface in SW2 will show only receive bpdu

The interface in SW1(root) will show only send bpdu

Interface show both there is loop sure

MHM

As far as I understand MST will send BPDUs on both ends of a link internally in the region:

"Unlike regular converged spanning tree topology, both ends of a link can send and receive BPDUs simultaneously."

-https://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/24248-147.html

@MHM Cisco World wrote:

Check

1-show port channel summary

Check if both port members of PO is pending

2- check mst digest it must same in all SW

MHM

SW2#show span mst configuration digest 
Name      [CiscoSTD]
Revision  1     Instances configured 1
Digest          0xAC36177F50283CD4B83821D8AB26DE62
Pre-std Digest  0xBB3B6C15EF8D089BB55ED10D24DF44DE

SW2#show etherchannel summary | be Group
Group  Port-channel  Protocol    Ports
------+-------------+-----------+-----------------------------------------------
1      Po1(SU)         LACP      Gi0/1(s)    Gi0/2(P)    
2      Po2(SU)         LACP      Gi0/3(P)    Gi1/0(P)

The digest is the same, so they are all in the same internal region. and the LAGs are configured as they should be. But the LAG is in MST P2P Dispute when one of the interface in Po1(SW2 <-> SW3) is shutdown

MHM Cisco World · ‎07-09-2025

But one of port show "s"

Meaning it standalone not bundle with PO

Make double check this interface' make sure you add PO config under correct port

MHM

rasmus.elmholt · ‎07-09-2025

The BPDUs are only sent from the RB towards the leafs of the STP tree.

SW2#show span int gi 0/0 detail | inc BPDU
   BPDU: sent 398, received 940
SW2#show span int gi 0/0 detail | inc BPDU
   BPDU: sent 398, received 946

The Root port on SW2 is only receiving BPDUs.