From my understanding, if the interface cost is equal, the interface priority is used to breaker. the higher, the better. I am testing to make sure my understanding is correct. But seems not, I do not know what I missed here.
lab@ex2# run show spanning-tree interface
Spanning tree interface parameters for instance 0
Interface Port ID Designated Designated Port State Role
port ID bridge ID Cost
ae0 128:3 128:3 8192.c042d009bda1 10000 FWD ROOT
ge-0/0/3 128:490 128:490 8192.c042d009bda1 20000 BLK ALT
ge-0/0/4 128:491 128:491 8192.c042d009bda1 20000 BLK ALT
Spanning tree interface parameters for instance 1
Interface Port ID Designated Designated Port State Role
port ID bridge ID Cost
ae0 128:3 128:3 4097.c042d009bda1 10000 BLK ALT
ge-0/0/3 128:490 128:490 4097.c042d009bda1 9999 FWD ROOT
ge-0/0/4 240:491 128:491 4097.c042d009bda1 9999 BLK ALT
Spanning tree interface parameters for instance 2
Interface Port ID Designated Designated Port State Role
port ID bridge ID Cost
ae0 128:3 128:3 4098.c042d009be61 10000 FWD DESG
ge-0/0/3 128:490 128:490 4098.c042d009be61 20000 FWD DESG
ge-0/0/4 128:491 128:491 4098.c042d009be61 20000 FWD DESG
From the above, I expected to see ge-0/0/4 elected to be ROOT as it has a higher priority, but it is not, why ?
thanks for insights !!