11-10-2010 11:42 PM - edited 03-06-2019 01:59 PM
I want to know whether a port-fast enabled port can get blocked by spanning tree?
11-10-2010 11:49 PM
No, It either is Disabled or Forwarding. Does not transition into any other state, due to which converges quickly.
11-11-2010 12:10 AM
Well I was a bit surprised when I saw this :
r2#sh spanning-tree vlan 10
VLAN10 is executing the ieee compatible Spanning Tree protocol
Bridge Identifier has priority 32768, address aaaa.bbbb.cccc
Configured hello time 2, max age 20, forward delay 15
Current root has priority 32768, address eeee.ffff.gggg
Root port is 1 (FastEthernet0/0), cost of root path is 19
Topology change flag not set, detected flag not set
Number of topology changes 3 last change occurred 00:23:38 ago
Times: hold 1, topology change 35, notification 2
hello 2, max age 20, forward delay 15
Timers: hello 0, topology change 0, notification 0, aging 300
Port 1 (FastEthernet0/0) of VLAN10 is forwarding
Port path cost 19, Port priority 128, Port Identifier 128.1.
Designated root has priority 32768, address eeee.ffff.gggg
Designated bridge has priority 32768, address eeee.ffff.gggg
Designated port id is 128.1, designated path cost 0
Timers: message age 2, forward delay 0, hold 0
Number of transitions to forwarding state: 1
BPDU: sent 0, received 655
The port is in the portfast mode
Port 2 (FastEthernet0/1) of VLAN10 is blocking
Port path cost 19, Port priority 128, Port Identifier 128.2.
Designated root has priority 32768, address eeee.ffff.gggg
Designated bridge has priority 32768, address eeee.ffff.gggg
Designated port id is 128.2, designated path cost 0
Timers: message age 2, forward delay 0, hold 0
Number of transitions to forwarding state: 3
BPDU: sent 37, received 1037
The port is in the portfast mode
r2#
I tested this on a cisco 3640 router with a 16 port esw module connected with another 3640 with 16 esw.
11-11-2010 12:19 AM
Port 2 (FastEthernet0/1) of VLAN10 is blocking
Port path cost 19, Port priority 128, Port Identifier 128.2.
Designated root has priority 32768, address eeee.ffff.gggg
Designated bridge has priority 32768, address eeee.ffff.gggg
Designated port id is 128.2, designated path cost 0
Timers: message age 2, forward delay 0, hold 0
Number of transitions to forwarding state: 3
BPDU: sent 37, received 1037
The port is in the portfast mode
It is expected, as we are seeing BPDU's coming. Portfast role is lost as soon as we see BPDU's, so it becomes a regular STP port. Why would be enable portfast where we expect BPDU's. If it is not expected, enable BPDUGuard.
11-11-2010 05:40 AM
deyadav wrote:
No, It either is Disabled or Forwarding. Does not transition into any other state, due to which converges quickly.
Maybe this is just a question of semantics but enabling portfast does not disable STP on that port. So there is no reason why a portfast port cannot then be blocked.
I guess what you are saying is that if a portfast port sees a BPDU it falls back to a regular STP port which it does but it is still to all intents and purposes configured as a portfast port hence the reason for the output saying it is a portfast port.
I guess it's just another way of looking at the same thing
Jon
11-11-2010 08:33 PM
Hi Jon,
I agree with you, that the portfast enabled port would always show up as portfast in command outputs. However, this feature of faster convergence is lost, if we receive BPDU, and then it acts like a regular STP port, going through listening/learning/blocking port states. So, operationally it no longer serves purpose of portfast.
In this setup, we have a back to back connection between two Switching modules. So, in my view, no point configuring it as portfast.
Regards,
Deepak
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