Re: Spanning-tree: changing FWD vs BLK ports

Hello!

Yes, changing the port cost is significant on your local switch
only...changing port priority is something you would do on a remote
"upstream" switch. Take the following example:

Cat1 --- Cat2

Let's say Cat1 is the root bridge and you have dual links between Cat1
and Cat2, we will call them Fa0/19 and Fa0/20. By default Fa0/19 on
Cat2 will be the root port because:
   - The sending BID is the same
   - The cost to the root bridge is the same
   - The port-priority is the same
   - Fa0/19 is lower than Fa0/20

Now, let's say you want Fa0/20 to be the root port instead. Three
things you could do to accomplish this:
   - Change the cost of Fa0/20 on Cat2 to be something lower than
Fa0/19's cost. Notice this is done locally on Cat2
   - Change the bandwidth of Fa0/20 on Cat2 to be something faster
than Fa0/19's bandwidth. This will indirectly change the
spanning-tree cost
   - Change the port-priority to something lower on Fa0/20 of Cat1.
Notice this is done on the remote end. In this case, remember that
the port-priority is send in the BPDU itself...when Cat2 receives the
BPDU it will see the port-priority being sent to it on both Fa0/19 and
Fa0/20. Fa0/20 if lower will win the day and become the root port.

Hope that helps!!!!!

Also, if you are really digging into spanning tree, check out one of
our latest blogs on the topic over at blog.ipexpert.com or
specifically: http://blog.ipexpert.com/spanning-tree-direct-vs-indirect-link-failures/

On Sat, Mar 27, 2010 at 4:35 PM, Muzammil Malick <malickmuz_at_gmail.com> wrote:
> 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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