[c-nsp] Problems manipulating rapid spanning tree

[Mourad BERKANE]

Sam Stickland wrote:

> Hi,
> 
> I've got the following network layout
> 
>      +--  A: cisco 6509 --+
>      |                    |
>      FE                   |
>      |                    |
>  B:cisco 3550             GE
>      |                    |
>      FE                   |
>      |                    |
>  C:cisco 2950 ---GE--- D:cisco 2950
> 
> 
> Switch A is the primary root of the spanning tree domain for vlan 218. 
> Switches C and D are access switches and have uplinkfast enabled.
> 
> On switch C, 'sh spanning-tree vlan 218' shows the following:
> 
> Interface        Role Sts Cost      Prio.Nbr Type
> ---------------- ---- --- --------- -------- ----
> Fa0/4            Desg FWD 3019      128.4    P2p
> FE               Root FWD 3019      128.25   P2p
> GE               Altn BLK 3004        0.26   P2p
> 
> (I've changed the interface names to fit in with the speed designators 
> in the diagram).
> 
> Why is switch C choosing to take the FE (100Mbit) paths to the primary 
> root, instead of the the GE paths?
> 
> As you can see I've also tried setting the port priority on switch C's 
> GE port to 0 to try and influence a tie-breaking decision. The path cost 
> is also listed as less (3004 versus 3019).
> 
> For the sake of completeness here's vlan 218's spanning-tree as switches 
> A, B and D see it.
> 
> Switch A (cisco 6509)
> ---------------------
> 
> Interface        Role Sts Cost      Prio.Nbr Type
> ---------------- ---- --- --------- -------- ----
> GE               Desg FWD 4         128.2    P2p
> FE               Desg FWD 19        128.193  P2p
> 
> 
> 
> Switch B (cisco 3550)
> ---------------------
> 
> Interface   Port ID                   Designated                Port ID
> Name        Prio.Nbr    Cost Sts      Cost Bridge ID            Prio.Nbr
> ----------- -------- ------- --- --------- -------------------- --------
> FE(C)       128.18        19 FWD        19 32986 000b.4636.8e80 128.18
> FA(A)       128.21        19 FWD         0 24794 00b0.c23b.7c00 128.193
> 
> 
> 
> 
> Switch D (cisco 2950)
> ---------------------
> 
> Interface        Role Sts Cost      Prio.Nbr Type
> ---------------- ---- --- --------- -------- ----
> GE(A)            Root FWD 3004      128.25   P2p
> GE(C)            Desg FWD 3004      128.26   P2p
> 
> 
> I understand that the network layout here isn't optimial, but I can't 
> understand why spanning-tree isn't taking the GE path.
> 

just looking at your port cost, it's normal:

from C to root A:
via B: metric is 3019 + 19
via D: metric is 3004 + 3004

on your 2950 you have a cost of 3004 for a Gige compared to a port cost 
of 4 on your 6500! just take care of this.