Thanks, Bob! This is extremely helpful. Especially the part about "2. If root
cost through two bridges are equal, use lowest bridge ID."
Cat4 is actually the root in this topology:
Cat1-------------Cat2
| - - |
| - |
| - - |
Cat3--------------Cat4
There are 2g port-channels between each Cat, in a full mesh. The lab has you
shutdown the links between Cat1 and Cat4, so that you have to either go via
Cat2 or Cat3. It has you lower the cost on one of the links from Cat1 to Cat3,
so STP flows through Cat 3, naturally. Then it has you shutdown the link you
lowered the cost on, and the DSG says you SHOULD now be traversing Cat2, as
that is where you enabled a lower port-priority.
My take: Based on your info, it will never get to this decision, as Cat3 has a
lower BID (MAC, since priorities are both default). So possibly the DSG is not
accurate based on the hardware in my rack. :(
Thanks also for the info on where to verify port-priority!! :)
Cheers,
Rob
On Feb 10, 2012, at 2:05 PM, Bob McCouch wrote:
> Hi Rob,
>
> Remember that port priority is only evaluated if you have multiple ports that
> connect to the same "upstream" bridge. Root port selection uses the following
> preference:
>
> 1. Lowest root cost
> 2. If root cost through two bridges are equal, use lowest bridge ID
> 3. If bridge IDs are equal, use lowest port ID (mixture of port number and
> port priority)
>
> In your output, it sounds like Cat3 is the root. In that case, Cat1 won't
> care about the priorities advertised by Cat2. If you want Cat2 to be the
> root, you need to give it a lower bridge ID (MSTP priority) on the desired
> VLANs. If your goal was to make Cat1 go through Cat2 to reach the root Cat3,
> you could use port cost, but that would have to be done on Cat1.
>
> Also note that the "Prio" value shown in "sh spann" does NOT display the port
> priority of the incoming BPDUs. It displays the local priority shown in BPDUs
> sent OUT from the bridge that displays that output. If you look on Cat2, you
> should see the desired priority in "sh spann," or from Cat1 you can do a "sh
> spann interface fa0/24 detail" (I think... going from memory) and if BPDUs
> are being received on this port you should see the advertised port priority.
>
> Hope that helps,
> Bob
>
>
>
> On Fri, Feb 10, 2012 at 12:04 PM, Rob Tyrrell <[email protected]> wrote:
> I am having an issue with port-priority influencing the root cost of its
> downstream neighbor. I have configured the port-priority according to the
> DSG on Cat2 Fa0/24:
>
> Cat2-MST(config-if)#do sh run int f0/24
> Building configuration...
>
> Current configuration : 107 bytes
> !
> interface FastEthernet0/24
> switchport mode dynamic desirable
> spanning-tree mst 0 port-priority 64
> End
>
> But Cat 1 still shows the secondary link to Cat3 as its root port, and does
> not appear to have learned the preferred priority to Cat2.
>
>
> Cat1-MST(config-if)#do sh span
>
> MST0
> Spanning tree enabled protocol mstp
> Root ID Priority 24576
> Address 0018.baf8.5a80
> Cost 0
> Port 21 (FastEthernet0/21)
> Hello Time 3 sec Max Age 15 sec Forward Delay 15 sec
>
> Bridge ID Priority 32768 (priority 32768 sys-id-ext 0)
> Address 000b.be96.d800
> Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
>
> Interface Role Sts Cost Prio.Nbr Type
> ------------------- ---- --- --------- --------
> --------------------------------
> Fa0/1 Desg FWD 200000 128.1 P2p
> Fa0/4 Desg FWD 200000 128.4 P2p
> Fa0/5 Desg FWD 200000 128.5 P2p
> Fa0/11 Desg FWD 2000000 128.11 Shr
> Fa0/21 Root FWD 200000 128.21 P2p
> Fa0/23 Altn BLK 200000 128.23 P2p
>
> Interface Role Sts Cost Prio.Nbr Type
> ------------------- ---- --- --------- --------
> --------------------------------
>
> Fa0/24 Altn BLK 200000 128.24 P2p
>
> I have shut/no shut all relevant interfaces, but still no change. I have
> even removed and re-added the port-priority config on Cat2.
>
> Any ideas what I may be missing?
>
> Thanks,
>
> Rob
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_______________________________________________
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www.ipexpert.com
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www.PlatinumPlacement.com
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