STP Max Age on Access Ports

CertifiedMonkeyCertifiedMonkey Posts: 172Member ■■□□□□□□□□
Hi all,

After going over STP material on Cisco Learning Network, I was under the impression that STP puts a port in the blocking state for Max_Age time when a PC is connected. Then, After the Max_Age timer expires, the port goes forwarding as a designated port because it's the only port sending BPDUs onto the segment.

However, while labbing, this wasn't the case. The port instantly transitioned from blocking to listening as soon as the port came up:

***Cable Connected***

*Mar 1 01:21:15.911: setting bridge id (which=3) prio 20481 prio cfg 20480 sysid 1 (on) id 5001.b4a4.e351.bb00
*Mar 1 01:21:15.911: set portid: VLAN0001 Gi0/1: new port id 8019
*Mar 1 01:21:15.911: STP SW: Gi0/1 new blocking req for 1 vlans
*Mar 1 01:21:15.911: STP: VLAN0001 Gi0/1 -> listening
*Mar 1 01:21:17.908: %LINK-3-UPDOWN: Interface GigabitEthernet0/1, changed state to up
*Mar 1 01:21:18.914: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/1, changed state to up
*Mar 1 01:21:30.919: STP SW: Gi0/1 new learning req for 1 vlans
*Mar 1 01:21:30.919: STP: VLAN0001 Gi0/1 -> learning
*Mar 1 01:21:45.926: STP SW: Gi0/1 new forwarding req for 1 vlans
*Mar 1 01:21:45.926: STP[1]: Generating TC trap for port GigabitEthernet0/1
*Mar 1 01:21:45.926: STP: VLAN0001 Gi0/1 -> forwarding

My questions are:
Was the Max_age timer ever used on access ports that connected to a PC?

Have you ever ran into a scenario where you had to wait for the Max_Age timer to expire?

TIA

Comments

  • WastedHatWastedHat Posts: 132Member ■■■□□□□□□□
    I think you may be confusing the Max Age timer and the Forward Delay timer.


    The Max Age timer is the amount of time the switch will wait for a Hello BPDU before it considers another switch or link to be down, requiring a change to the STP topology. It defaults to 10x the Hello with STP and 3x the Hello with RSTP.

    The Forward Delay timer is used when a port has to move from a Blocking state to a Forwarding state. To prevent loops it will move the interface into a Listening state then a Learning state for the amount of time specified by the Forward Delay timer.


    It defaults to 15s for each state so your output is perfectly normal:

    *Mar 1 01:21:15.911: STP: VLAN0001 Gi0/1 -> listening

    *Mar 1 01:21:30.919: STP: VLAN0001 Gi0/1 -> learning

    *Mar 1 01:21:45.926: STP: VLAN0001 Gi0/1 -> forwarding


    The Listening/Learning state is only necessary on links that connect to other switches. If you know there will be a PC connected then you can enable PortFast on the interface to bypass the 30s wait and go straight from Blocking to Forwarding. Hope this helps.
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