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.![STP Topology Ethernet Square.jpg]()
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!
