SWTCH 300-115 1.6 Configure and verify spanning tree

1.6.a PVST+, RPVST+, MST

We understand that the sys-id-ext field in spanning-tree is represented by 12 bits of the total 2 byte priority field. The other 4 bits represent the priority in multiples of 4096. We also understand that 2 to the 12th gives us 4096 possible vlans (0-4095 is 4096 vlaues total, 4094 of which are actually configurable), and the determined vlan (if not the default Vlan 1) plus the priority (if adminstratively configured other than the default 32768) completes the priority calculation:

SW2#sh spann | i Br
Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)

We have 4 bits for priority. gmail sign in guide 4 bits gives us only 16 possible priority values. (2 to the 4th, feel free to do the binary)

65536/16=4096

This is also why the root primary/secondary macro is decremented by 4096; there is no choice.

SW2(config)#spann vlan 1 prio 32767
% Bridge Priority must be in increments of 4096.
% Allowed values are:
0     4096  8192  12288 16384 20480 24576 28672
32768 36864 40960 45056 49152 53248 57344 61440

 

sys-id-ext