STP Logical ports count calculations.

[Alexey Stytsenko]

Good day!

As it stays in Cisco docs STP logical ports number is calculated by formula:

Sum [(Each trunk on the switch) * (Active VLANs on each trunk)] + (Number of
nontrunking interfaces on the switch)

And this number can be checked by issuing command

show spanning-tree summary total

And i didnt find out any remarks that this number calculated differently for RSTP and MST, so i assume that calculations process is the same.

I tried to check this number on my equipment and was a bit surpised with results:

R1#show spanning-tree summary totals
Switch is in mst mode (IEEE Standard)
Root bridge for: none
EtherChannel misconfig guard            is enabled
Extended system ID                      is enabled
Portfast Default                        is disabled
Portfast Edge BPDU Guard Default        is disabled
Portfast Edge BPDU Filter Default       is disabled
Loopguard Default                       is disabled
Platform PVST Simulation                is enabled
PVST Simulation                         is enabled
Bridge Assurance                        is enabled
UplinkFast                              is disabled
BackboneFast                            is disabled
Pathcost method used                    is long

Name                   Blocking Listening Learning Forwarding STP Active
---------------------- -------- --------- -------- ---------- ----------
2 msts                       0         0        0         24         24

Hm.. only 24 logical ports but lets check number of trunks in system and vlans that spans over those trunks:

R1#show interfaces trunk

-----Omitted for brevity----

Port                Vlans allowed and active in management domain
Po30                5,97,99,111,165,202,212-213,401-406,1521-1539,1600-1604
Po31                97,165,401,405-406,1600
Po41                1521-1539
Po52                99,111,202,212-213,401,403-404
Po70                97,165,401,405-406,1600
Po71                97,165,200,401,405-406
Po100               97,1603
Po101               97,165,401,405-406,1600
Po102               97,165,401,405-406,1600

-----Omitted for brevity----

So according to logical port definition only Po41 must have more STP logical ports  than i got in ouput of show spanning-tree summary total command.

Can somebody clarify this?

[paul driver]

Hello

Excluding the logical vlans, I count from you output ( could be wrong)

11 vlans + 9 PC = 20

So If you have you have further 4 access ports enabled in these vlans then that should come to 24 instances?

res
Paul

[Alexey Stytsenko]

Good day.

Sry, but i dint understand what mean "excluding logical vlans".  You mean reserved vlans:

1002 fddi-default                     act/unsup
1003 token-ring-default               act/unsup
1004 fddinet-default                  act/unsup
1005 trnet-default                    act/unsup

?

If yes, than I have 40 vlans except those.

Besides i can get you formula 11 vlans + 9 PC. As i addused the formula according to Cisco docs is:

Sum [(Each trunk on the switch) * (Active VLANs on each trunk)] + (Number of
nontrunking interfaces on the switch)