Correction: There are 2x1G links between all the CATS…not port-channeled
On Feb 10, 2012, at 2:25 PM, Rob Tyrrell wrote: > 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 >> _______________________________________________ >> For more information regarding industry leading CCIE Lab training, please >> visit www.ipexpert.com >> >> Are you a CCNP or CCIE and looking for a job? Check out >> www.PlatinumPlacement.com >> >> http://onlinestudylist.com/mailman/listinfo/ccie_rs >> > _______________________________________________ For more information regarding industry leading CCIE Lab training, please visit www.ipexpert.com Are you a CCNP or CCIE and looking for a job? Check out www.PlatinumPlacement.com http://onlinestudylist.com/mailman/listinfo/ccie_rs
