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New Member

Does port-fast enabled port get blocked?

I want to know whether a port-fast enabled port can get blocked by spanning tree?

5 REPLIES
Cisco Employee

Re: Does port-fast enabled port get blocked?

No, It either is Disabled or Forwarding. Does not transition into any other state, due to which converges quickly.

New Member

Re: Does port-fast enabled port get blocked?

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.

Cisco Employee

Re: Does port-fast enabled port get blocked?

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.

Hall of Fame Super Blue

Re: Does port-fast enabled port get blocked?

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

Cisco Employee

Re: Does port-fast enabled port get blocked?

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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