I've been trying to understand Message Age timer. Some documentations say "it is similar to hop count"
From Cisco Online Documentation here http://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/19120-122.html
"The message age is not a fixed value. The message age contains the length of time that has passed since the root bridge initially originated the BPDU. The root bridge sends all its BPDUs with a message age value of 0, and all subsequent switches add 1 to this value. Effectively, this value contains the information on how far you are from the root bridge when you receive a BPDU."
I am using the topology shown. SW1 has been elected root. SW4's E1/0 is Blocked by STP. All devices are running in PVST mode.
The message age number is not conforming to the definition outlined in the Cisco Online documentation, as far as my understanding goes. Should I even be seeing a Message Age of 3 while working on this topology?
Furthermore, on SW4 the message age bounces from 3 to 2 then back to 3 over and over again. I suspect it may have something to do with the message age increment overestimate (msg_overestimate) parameter but I don't understand much beyond suspicion on what the purpose of "overestimation" is in the first place and what is happening in the topology.
Also why would I ever see message age of 2 on SW2's E0/0 and SW3's E0/0 which are both directly connected to the root switch and are elected root ports?
SW1#sh spanning-tree vlan 1 detail | i ^ Port | Timers
Timers: hello 1, topology change 0, notification 0, aging 300
Port 1 (Ethernet0/0) of VLAN0001 is designated forwarding
Timers: message age 0, forward delay 0, hold 0
Port 3 (Ethernet0/2) of VLAN0001 is designated forwarding
Timers: message age 0, forward delay 0, hold 0
SW1 is the root of the spanning-tree topology therefore message age is zero on both interface Ethernet0/0 and Ethernet0/2.
SW2#sh spanning-tree vlan 1 detail | i ^ Port | Timers
Timers: hello 0, topology change 0, notification 0, aging 300
Port 1 (Ethernet0/0) of VLAN0001 is root forwarding
Timers: message age 2, forward delay 0, hold 0
Port 5 (Ethernet1/0) of VLAN0001 is designated forwarding
Timers: message age 0, forward delay 0, hold 0
SW3#sh spanning-tree vlan 1 detail | i ^ Port | Timers
Timers: hello 0, topology change 0, notification 0, aging 300
Port 1 (Ethernet0/0) of VLAN0001 is root forwarding
Timers: message age 2, forward delay 0, hold 0
Port 5 (Ethernet1/0) of VLAN0001 is designated forwarding
Timers: message age 0, forward delay 0, hold 0
SW4# sh spanning-tree vlan 1 detail | i ^ Port | Timers
Timers: hello 0, topology change 0, notification 0, aging 300
Port 3 (Ethernet0/2) of VLAN0001 is root forwarding
Timers: message age 3, forward delay 0, hold 0
Port 5 (Ethernet1/0) of VLAN0001 is alternate blocking
Timers: message age 3, forward delay 0, hold 0
Then if I wait a few moments and issue same command on SW4 again it shows message age is back to 2.
SW4# sh spanning-tree vlan 1 detail | i ^ Port | Timers
Timers: hello 0, topology change 0, notification 0, aging 300
Port 3 (Ethernet0/2) of VLAN0001 is root forwarding
Timers: message age 2, forward delay 0, hold 0
Port 5 (Ethernet1/0) of VLAN0001 is alternate blocking
Timers: message age 2, forward delay 0, hold 0
Please help shed some light on this vexing thing.
Thanks!