Port Fast & RSTP

Unanswered Question
Jun 11th, 2010
User Badges:
  • Silver, 250 points or more

Hi


I am confuse with below question.


1] Spanning Tree  Port Fast will enable automatically when i am using RSTP, Whether it will treat same as Edge P2P ?

2] Can we use Port Fast with STP or PVST & that port treated as P2P or Edge P2P?

3] What are the feature that will enable automatically with RSTP ?


Regards

Chetan Kumar

  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 0 (0 ratings)
Loading.
Giuseppe Larosa Sat, 06/12/2010 - 02:37
User Badges:
  • Super Silver, 17500 points or more
  • Hall of Fame,

    Founding Member

Hello Chetan,


it is very important, needed, in order to achieve fast convergence to configure all user ports as edge ports.


In cisco IOS switches this is achieved by using the command spanning-tree portfast that was already there for Cisco proprietary extensions to STP.


The reason of this need, is that RSTP requires hand shakes of negotiation on all point-to-point links between switches.

This is not done on user ports that can be safely added or removed from the tree without even triggering any STP action.


To get fast convergence by means of so called synchronization wave (propagation of handshake one link per time) it is very important to declare all user ports so that they are excluded by this activity.


So while in legacy STP spanning-tree portfast is an optional part that helps with DHCP, in RSTP is a need


2) yes spanning-tree portfast with legacy STP works the same: no TCN BDPU sent when link state changes on port, no need to wait for listening and learning states to complete.


3) RSTP includes standard based mechanims that are equivalent to uplinkfast and backbonefast, these last two don't need to be configured anymore with Rapid PVST or MST


Hope to help

Giuseppe

Chetan Kumar Ress Mon, 06/14/2010 - 06:28
User Badges:
  • Silver, 250 points or more

Hi Giuslar


Thanks for you explanation.


I was also sure that Port fast is an optional feature that need to enable by administrator but some friend made me confuse So Please correct if i am wrong ...


1] So Port Fast is optional feature that can enable as per requriment, Not automatically enable when we run RSTP.


2] Port Fast can be used with STP or PVST.


Regards

Chetan Kumar

Chetan Kumar Ress Mon, 06/14/2010 - 06:49
User Badges:
  • Silver, 250 points or more

Hi Giuslar


If possible can you provide any document for same or Cisco Link where i can get the detail .


Regards

Chetan Kumar

Chetan Kumar Ress Mon, 06/14/2010 - 07:49
User Badges:
  • Silver, 250 points or more

Hi Giuslar


Thanks for document.


But still i am having confusion that if i run RSTP & i had not configured user connected port as a Port - Fast then how much time it will take that port to go in forwarding state.


And in RSTP if i not configured Port fast then in output of RSTP it will show as P2P or Edge P2P.


Regards

Chetan Kumar

Giuseppe Larosa Tue, 06/15/2010 - 12:12
User Badges:
  • Super Silver, 17500 points or more
  • Hall of Fame,

    Founding Member

Hello Chetan,

>> And in RSTP if i not configured Port fast then in output of RSTP it will show as P2P or Edge P2P.

if you don't configure spanning-tree portfast the port is considered P2P not edge P2P


see



sh spanning-tree vlan 401


VLAN0401

  Spanning tree enabled protocol rstp

  Root ID    Priority    8192

             Address     0005.741b.1f11

             Cost        2

             Port        657 (Port-channel17)

             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec


  Bridge ID  Priority    33169  (priority 32768 sys-id-ext 401)

             Address     8843.e108.9340

             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

             Aging Time 300


Interface           Role Sts Cost      Prio.Nbr Type

------------------- ---- --- --------- -------- --------------------------------

Gi1/42              Desg FWD 4         128.42   P2p

Gi1/46              Desg FWD 4         128.46   P2p

Po17                Root FWD 1         128.657  P2p


These ports gi1/42 and gi/46 are trunks to vmware hosts not to other switches
no BPDUs are received but ports are not classified as Edge P2P
sh spanning-tree int g1/46 det | inc received
   BPDU: sent 16421, received 0
   BPDU: sent 16421, received 0
   BPDU: sent 16421, received 0
   BPDU: sent 16421, received 0
   BPDU: sent 16421, received 0
   BPDU: sent 17024, received 0
   BPDU: sent 16425, received 0
   BPDU: sent 16421, received 0
   BPDU: sent 16425, received 0
Hope to help
Giuseppe
Chetan Kumar Ress Tue, 06/15/2010 - 12:20
User Badges:
  • Silver, 250 points or more

Hi Giuslar


Thanks for you explanation with helpful output.


So if I remove the cable from G1/42 & Connect after few Seconds then how much time it will take to reach FWD state.


If i am not using PortFast feature .


Regards

Chetan Kumar

Ganesh Hariharan Sun, 06/13/2010 - 23:50
User Badges:
  • Purple, 4500 points or more
  • Community Spotlight Award,

    Member's Choice, February 2016

Hi


I am confuse with below question.


1] Spanning Tree  Port Fast will enable automatically when i am using RSTP, Whether it will treat same as Edge P2P ?

2] Can we use Port Fast with STP or PVST & that port treated as P2P or Edge P2P?

3] What are the feature that will enable automatically with RSTP ?


Regards

Chetan Kumar

Hi Chetan Kumar,


RSTP defines the concept of an edge port. An edge port is a designated port that connects to a collision domain with no other switches present. That would include any port that directly attaches to an end-station, a router, or a server.


These ports are automatically made part of the spanning tree and never cycle through the learning mode regardless of what happens on the other ports. A switch can automatically detect an edge port by noting the absence of configuration BPDUs from any attached system, or it can be manually configured that way by the network administrator.


Ports that are not edge ports are referred to as link ports, and they participate in the spanning tree process. If they are full duplex (e.g., point-to-point) a new, faster recovery mechanism is defined. If they are half duplex (e.g., connected to a hub) they fall back to the original STP model.


Spanning-tree auto-edge-port: Supports the automatic identification of edge ports. The port will look for BPDUs for 3 seconds; if there are none it begins forwarding packets.


Hope to Help !!


Ganesh.H


Remember to rate the helpful post

Chetan Kumar Ress Tue, 06/15/2010 - 11:39
User Badges:
  • Silver, 250 points or more

Hi Ganesh


Thanks for you explanaiton , But that was not my question.


1] I would like to ask that if i am using RSTP then whether all end user connected ports will be consider as a Edge Port ? ( Not enabled port fast)


If Yes , then whether it will be directly go in  FWD State

If No , Then  whether it will pass all state that come under RSTP.


And what will be time taken to go in FWD State : 30 Sec or 1 Sec or XX Sec ?


2]  Without using Portfast Feature the End User Port will be known as P2P or Edge P2P. ? ( In Show spanning-tree output)


An edge port that receives a BPDU immediately loses edge port status and becomes a normal spanning tree port. At this point, there is a user-configured value and an operational value for the edge port state


The Mention statement is from Cisco.


I would like to know that wether The Statement is True when Portfast is enabled or when port fast is not Enabled.


Neither edge ports or PortFast enabled ports generate topology changes when the link toggles.


The Statement is from Cisco & I would like to know that What is difference between Portfast Port  & Edge port. ?



Regards

Chetan Kumar

Actions

This Discussion