Spanning-tree Implementation changes
Posted on 2011-10-20
We are collapsing all our smaller switches (2960s and 3750s) into 2 6509s. One of the 3750s is the root bridge for all our vlans, the other 3750 is the secondary root bridge for all our vlans. We would like to have the 6509s replace all our other switches first, and then do the root bridges last.
For example, for vlan 1, the Primary root bridge switch has that vlan with a root bridge of 24577 (when you plug in a switch with no configuration, it defaults the vlan to 24576, and then every vlan gets incremented by 1, unless someone manually configures the bridge ID). Then, when the secondary root bridge was plugged in, it has a bridge ID of 28673, which is greater than 28673, so it becomes a backup root bridge. The root bridge of the spanning tree is the bridge with the smallest (lowest) bridge ID. So the smaller the number, the closer you are to being the root bridge. When we go to plug in these new switches, we must give them all bridge IDs greater than the root bridge IDs. Once we fold in the 3750 switches that are the root bridges, we must redo this, giving them smaller numbers than the current primary root bridges before yanking them.
Now, according to my research: Interestingly, with spanning tree, the cmd is spanning-tree vlan X priority X. But with the priority keyword, the range is 0 to 61440 in increments of 4096, assigning the first number of the interval to a vlan on the switch, and then that switch can only use IDs in that range (ie, the primary root bridge switch can only use 24576 to 28671). The default spanning tree value with a switch is 32768. Only the following number can be used as a priority value.
0, 4096, 8192, 12288, 16384, 20480, 24576, 28672, 32768, 36864, 40960, 45056, 49152 ,53248, 57344, 61440.
Is this correct ?
We cannot use 24576, because it is being used by the spanning tree root bridge, so we can only use numbers bigger than that. Cannot use 28672 because it is being used by secondary root bridge switch. Cannot use 32768 because that is on a 3rd switch.
1. Is the thought process correct ? We give these vlans the worst possible spanning tree priority (61440 and beyond) on the new 6509s, and then we fold in all other switches. When we do the primary root bridge and secondary root bridge, we redo the vlan priority again, but give the vlans priorities lower that the current root bridge and secondary root bridge?
2. I checked another switch (4th 3750) and it has a number of vlans with numbers 32960, 32856, and 32772, which would conflict with the interval for the 3rd switch mentioned above. How is this working? Or is the Cisco document about the intervals no longer valid ?
3. The cmd string per vlan is :
spanning-tree vlan x priority x
does this sound correct ?
4. Anything else I am missing ?
thanks in advance