Hi Elie, Good point. I don't think I was wrong, but you're definitely more precise in your answer.
I was simply pointing out that in the same way the BID consists of multiple components, a priority value, a VLAN ID, and a MAC address, so does the root port get decided by the Port ID which is actually a composite value that includes the port priority and the number of the port. In Cisco's doc, they actually call it a port index and port priority: http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a00800ae96a.shtml On Fri, Feb 10, 2012 at 3:05 PM, Elie Raad <[email protected]> wrote: > Hello Bob, > > you mentioned "If bridge IDs are equal, use lowest port ID (mixture of > port number and > port priority)" well i would disagree with you concerning this point and > would like to be more precise . it is the port priority > that comes first than port ID. so if you dont configure the port priority( > equal on both by default :128) than the lowest port ID will take over. > > > Best Regards, > > Elie Raad > > ________________________________________ > From: [email protected] [ > [email protected]] on behalf of Bob McCouch [ > [email protected]] > Sent: Friday, February 10, 2012 9:05 PM > To: Rob Tyrrell > Cc: [email protected] > Subject: Re: [OSL | CCIE_RS] Vol1 Lab3 Task3.8 > > 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 > > > _______________________________________________ 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? 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