Re: Spanning-tree: changing FWD vs BLK ports

Hi

Changing the port-priority value on the local switch will ensure that the
port on the neighbouring switch receives a better BPDU and so the remote
port is the port
that is influenced. With port cost value you will affect the local port as
you are seeing.

On 27 March 2010 20:04, Jules Eyabane <jep7619_at_gmail.com> wrote:

> hello, everyone:
>
> i am testing spanning-tree behavior and my books seems to indicate that the
> blocking/forwarding state on the root ports are determined in the following
> order:
>
> 1. port with lowest priority
> 2. port cost
> 3. lowest port number, if tie between the above two.
>
> based on the assumption above, i have 2 switches whereby when i change the
> port priority on gig 0/48 to a lower value to force it to become the FWD
> port, i don't get any movement whatsoever. below is my config from both
> routers. although it shows MST, i got the same result when i was running a
> single instance STP:
>
> **********First Switch - ROOT**************
>
> DLS1-3560-Mid#sho span mst
>
> ###### MST00 vlans mapped: 1-4094
> Bridge address 0015.63e3.6b80 priority 32768 (32768 sysid 0)
> Root this switch for CST and IST
> Configured hello time 2, forward delay 15, max age 20, max hops 20
>
> Interface Role Sts Cost Prio.Nbr Type
> ---------------- ---- --- --------- --------
> --------------------------------
> Fa0/1 Desg FWD 200000 128.3 P2p
> Fa0/2 Desg FWD 200000 128.4 P2p
> Fa0/17 Desg FWD 200000 128.19 P2p
> Fa0/18 Desg FWD 200000 128.20 P2p
> Fa0/33 Desg FWD 200000 128.37 P2p
> Fa0/47 Desg FWD 200000 128.51 P2p
> Fa0/48 Desg FWD 200000 128.52 P2p
>
> DLS1-3560-Mid#
>
> *****************Second Switch, NON-ROOT****************
>
> DLS2-3560G-Bottom#sho span m
> 05:01:00: %SYS-5-CONFIG_I: Configured from console by console
> DLS2-3560G-Bottom#sho span mst
>
> ###### MST00 vlans mapped: 1-4094
> Bridge address 0016.9ddd.d700 priority 32768 (32768 sysid 0)
> Root address 0015.63e3.6b80 priority 32768 (32768 sysid 0)
> port Gi0/1 path cost 0
> IST master address 0015.63e3.6b80 priority 32768 (32768 sysid 0)
> path cost 200000 rem hops 19
> Operational hello time 2, forward delay 15, max age 20
> Configured hello time 2, forward delay 15, max age 20, max hops 20
>
> Interface Role Sts Cost Prio.Nbr Type
> ---------------- ---- --- --------- --------
> --------------------------------
> Gi0/1 Root FWD 200000 128.1 P2p
> Gi0/2 Altn BLK 200000 128.2 P2p
> Gi0/17 Altn BLK 200000 128.17 P2p
> Gi0/18 Altn BLK 200000 128.18 P2p
> Gi0/33 Altn BLK 200000 128.33 P2p
> Gi0/47 Altn BLK 200000 128.47 P2p
> Gi0/48 Altn BLK 200000 16.48 P2p
>
> DLS2-3560G-Bottom#
> ***********************************
>
> what am i missing. i know for sure that when i change the cost to a lower
> value, i do get the sought behavior. but i want to make sure i know at
> least
> another way of changing the root port.
>
> thanks in advance
>
> JEP.
>
>
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Received on Sat Mar 27 2010 - 20:35:34 ART