01-01-2012 11:15 AM - edited 03-07-2019 04:07 AM
On a production 6509, I am seeing this:
RTR-01#sh spanning-tree int gi2/3 root
VLAN0001 0
VLAN0010 0
VLAN0011 0
VLAN0012 0
VLAN0013 0
VLAN0014 0
VLAN0015 0
VLAN0016 0
VLAN0017 0
VLAN0018 0
VLAN0019 0
VLAN0020 0
VLAN0021 0
VLAN0022 0
VLAN0023 0
VLAN0024 0
VLAN0025 0
VLAN0026 0
VLAN0111 0
VLAN0112 0
RTR-01#sh spanning-tree vlan 111 int gi2/3
Vlan Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
VLAN0111 Root FWD 4 128.67 P2p
Now I thought the command "sh spanning-tree int gi2/3 root" showed cost to the root bridge. So with everything being zero, its implied this the root, which it is but not for vlan 111 and actually all 1XX Vlans have a different root. Why does vlan111 show its root as out int gi 2/3 but the root cost shows zero?
Issue is we have a issue where a 2950 is acting as root bridge for our wireless vlans, wrong....it should be the 6509, but before I change it over, was wondering about the root port/cost question.
Thanks!
Solved! Go to Solution.
01-02-2012 12:34 PM
Hello Jimmy !
Thanks for the intersting outputs.
I have reproduced with some catalyst switch (and checked the outputs on some live networks) =>
It appears the command «sh spanning-tree int gi2/3 rootcost » indicates the root cost of the BPDU as it arrives on the interface before the cost is incremented by the ingress Gi2/3 port path cost (of 4).
In your specific case, as the 2950 is the root for vlans 1xx, 2950 is sending vl1xx BPDUs with a root cost defined to 0, explaining why we see 0 for vlans 111 – 112 on the root port of the 6500.
Now when adding a third switch (switch 3) to reach this diagram => Cisco2950-----(Gi2/3)_Cisco6500_(Gi2/4)-----(Gi0/1)Switch3.
On switch 3 => "sh spanning-tree int gi0/1 rootcost" gives
…..
VLAN0025 0
VLAN0026 0
VLAN0111 4
VLAN0112 4
It appears the « sh spanning-tree [vlan x] int y rootcost » is 0 for the ROOT switch and for the root port of a «directly connected» switch as the BPDU has not still been processed (incrementation of the port cost).
When changing the spanning-tree port path cost of int Gi2/3 of 6500 to 10.
sh spanning-tree int gi0/1 rootcost on Switch3 now gives
……
VLAN0025 0
VLAN0026 0
VLAN0111 10
VLAN0112 10
=============================
What is funny is if you have a trunk to a another switch connected to your 6500 (like described as below) :
Cisco2950_-----(Gi2/3)_Cisco6500_(Gi2/4)-----(Gi0/1)Switch3
"show spanning-tree int gi2/4 rootcost" should give (Gi2/3 port cost back to default of 4)
…….
VLAN0025 0
VLAN0026 0 => Normal as 6500 is the root for this vlan
VLAN0111 4
VLAN0112 4 => The cost has been incremented per the BPDU processing in ingress.
===============================
The documentation is quite rare for this keyword => From Nexus 5k guides (which should be the same as for Catalysts) => rootcost Displays the path cost to the root for specified interfaces for the specified VLANs.
So I would not have to worry about your outputs.
Best regards.
Karim
01-02-2012 12:34 PM
Hello Jimmy !
Thanks for the intersting outputs.
I have reproduced with some catalyst switch (and checked the outputs on some live networks) =>
It appears the command «sh spanning-tree int gi2/3 rootcost » indicates the root cost of the BPDU as it arrives on the interface before the cost is incremented by the ingress Gi2/3 port path cost (of 4).
In your specific case, as the 2950 is the root for vlans 1xx, 2950 is sending vl1xx BPDUs with a root cost defined to 0, explaining why we see 0 for vlans 111 – 112 on the root port of the 6500.
Now when adding a third switch (switch 3) to reach this diagram => Cisco2950-----(Gi2/3)_Cisco6500_(Gi2/4)-----(Gi0/1)Switch3.
On switch 3 => "sh spanning-tree int gi0/1 rootcost" gives
…..
VLAN0025 0
VLAN0026 0
VLAN0111 4
VLAN0112 4
It appears the « sh spanning-tree [vlan x] int y rootcost » is 0 for the ROOT switch and for the root port of a «directly connected» switch as the BPDU has not still been processed (incrementation of the port cost).
When changing the spanning-tree port path cost of int Gi2/3 of 6500 to 10.
sh spanning-tree int gi0/1 rootcost on Switch3 now gives
……
VLAN0025 0
VLAN0026 0
VLAN0111 10
VLAN0112 10
=============================
What is funny is if you have a trunk to a another switch connected to your 6500 (like described as below) :
Cisco2950_-----(Gi2/3)_Cisco6500_(Gi2/4)-----(Gi0/1)Switch3
"show spanning-tree int gi2/4 rootcost" should give (Gi2/3 port cost back to default of 4)
…….
VLAN0025 0
VLAN0026 0 => Normal as 6500 is the root for this vlan
VLAN0111 4
VLAN0112 4 => The cost has been incremented per the BPDU processing in ingress.
===============================
The documentation is quite rare for this keyword => From Nexus 5k guides (which should be the same as for Catalysts) => rootcost Displays the path cost to the root for specified interfaces for the specified VLANs.
So I would not have to worry about your outputs.
Best regards.
Karim
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