portfast enabled port behavior in STP root change

[davidxiayuan]

Hi Folks

Could someone please let me know would those switch ports that having portfast enabled still undergoing blocking and forwarding status change (downtime involved) in the case of STP root switch changing event or they will stay out of the disruptions ?

Thank you

[Mark Malone]

Hi portfast decreases the timers so the listening and learning stages are basically removed from the stp calculation for that interface and the port goes from blocking straight to forwarding when portfast is enabled , it doesn't completely remove it from stp , but stp is more aware of what the port is set as so the port is moved straight to forwarding,stp is still running on a fwd/blk port as it still needs to detect loops , PF should only be used on access ports , pcs/phones printers never between switches on trunk , always use it with bpduguard

if you want a port removed from any stp calculation you use the stp bpdufilter command , it will disable bpdus for that port basically removing it from the stp calculation , only use in certain circumstances maybe connecting to a device that does not understand stp

[davidxiayuan]

Thanks for your reply Mark. So for instance, if switch port 1-10 that configured in Vlan 300 with portfast enabled and the switch is running in PVST mode. When the STP vlan 300 root bridge changing from current root bridge to another root bridge, are those port from 1-10 in vlan 300 will undergoing blocking and forwarding process or they will stay out of the disruption?

Correct me if I was wrong, as my original understanding is if a port is in the normal STP status, when the root bridge changes, the STP will block the port first until the new root bridge is elected and then put that port into forwarding mode again regardless if you running MSTP/RST/PVST? And during the process, it will be less than a minute disruptions to the whatever devices that connects to those ports.

Thanks

[Mark Malone]

Hi

they will go through the process but much quicker , once a port is part of stp it will always go through some form of calculation as its still part of the layer 2 stp topology but it will be minimal with portfast as it decreases the waiting timers, the only way I think to stop it being part of stp is to use the filter command as that removes it from calculation as no bpdus are sent when that's enabled at the local port so its removed from overall calculation

You can see this if you you look at an interface in stp detail

sh spanning-tree interface t1/1/3 detail

 Port 5770 (Port-channel122) of VLAN0001 is designated forwarding
   Port path cost 1, Port priority 128, Port Identifier 128.5770.
   Designated root has priority 4097, address 0008.e3ff.fd90
   Designated bridge has priority 4097, address 0008.e3ff.fd90
   Designated port id is 128.5770, designated path cost 0
   Timers: message age 0, forward delay 0, hold 0
   Number of transitions to forwarding state: 1
   Link type is point-to-point by default
   BPDU: sent 3730030, received 5

Then 2 seconds later run it again see increase in bpdus

Port 5770 (Port-channel122) of VLAN0001 is designated forwarding
   Port path cost 1, Port priority 128, Port Identifier 128.5770.
   Designated root has priority 4097, address 0008.e3ff.fd90
   Designated bridge has priority 4097, address 0008.e3ff.fd90
   Designated port id is 128.5770, designated path cost 0
   Timers: message age 0, forward delay 0, hold 0
   Number of transitions to forwarding state: 1
   Link type is point-to-point by default
   BPDU: sent 3730053, received 5

But if you use filter these basically stop incrementing