I have question on Portfast .
i have layer 3 switch that connects to another layer 3 switch.
here is switch 1 port config
description uplink bmanysw02
switchport trunk encapsulation isl
switchport trunk allowed vlan 1,5,9
switchport mode trunk
logging event link-status
udld port aggressive
channel-group 1 mode desirable
spanning-tree portfast disable*******
as seen above port fast is disabled on this uplink port.
i want to know why we do not use port fast on uplink ports or ports which connect to other switches.i know that port fast is normally used on user access port.
A loop is not formed by enabling portfast on trunk ports. But if you do have a loop and your switch interconnects have portfast enabled then the switch will not be able to detect the loop in time.
Enabling portfast on a port does not disable STP but it allows the port to forward immediately.
A loop is formed when you physically connect up the switches in a certain way. There is nothing wrong with having loops, in fact a lot of L2 networks have loops for redundancy but if there is a loop STP needs to block one of the ports to break that loop. If portfast is enabled on a switch interconnect forming a loop then because portfast allows a port to immediately forward it would be too late for STP to try and break the loop because the L2 network would be overwhelmed.