spanning-tree portfast on interface enabled [do you agree?]

- spanning-tree portfast on interface enabled
portfast--->immiedietely jump from blocking to forwarding state [omnit listening
            and learning]
portfast--->don't block bpdu frames!
sw1
interface FastEthernet0/3
 switchport access vlan 3
 switchport mode access
 spanning-tree portfast
---
general topology
            <------------------------bpdu
            <------------------------data

sw3[fa0/1]----------[fa0/2]sw1[fa0/1]----------[fa0/1]sw2

         bpdu------------------------->
          data------------------------->
---
option 1
 sw1 don't receive bpdu on fa0/1 and than:
 > keep portfast behaviore
 > all the time sw1 will send bpdu out of fa0/1 port to sw2
 > accept received bpdu couse stp rebuild, after rebuild we will have
   to options, which can touch sw1 fa0/1 port:
   1 port will be still in forward state and:
     > bpdu send and receive
     > data send and receive
---
                                        X didn't receive bpdu
            <------------------------data

sw3[fa0/1]----------[fa0/2]sw1[fa0/1]----------[fa0/1]sw2

         bpdu------------------------->
          data------------------------->
---
   2 port will be blocked, because stp decide this and:
     > bpdu send and receive
     > data blocked
---
            <------------------------bpdu
                             X<---------data

sw3[fa0/1]----------[fa0/2]sw1[fa0/1]----------[fa0/1]sw2

         bpdu------------------------->
          data--------->X
---
option 2
 sw1 receive bpdu on fa0/1 and than:
 > keep portfast behaviore
 > all the time sw1 will send bpdu out of fa0/1 port to sw2
 > accept received bpdu couse stp rebuild, after rebuild we will
   have to options, which can touch sw1 fa0/1 port:
   1 port will be still in forward state and:
     > bpdu send and receive
     > data send and receive
---
            <------------------------bpdu
            <------------------------data

sw3[fa0/1]----------[fa0/2]sw1[fa0/1]----------[fa0/1]sw2

         bpdu------------------------->
          data------------------------->
---
   2 port will be blocked, because stp decide this and:
     > bpdu send and receive
     > data blocked
---
            <------------------------bpdu
                             X<---------data

sw3[fa0/1]----------[fa0/2]sw1[fa0/1]----------[fa0/1]sw2

         bpdu------------------------->
          data--------->X
---

Comments

  • Hi,

      If BPDU is received inbound on  port, it looses its PORTFAST state (doens't matter if portfast was enabled globally or at the port level). On every network segment , after STP converged and unless you configure STP Bridge Assurance, BPDU's flow one-way only, which is in the downstream direction (from root bridge to non-root bridges), so BPDU's are always sent on a segment by the designated port of the segment which basically represent the shortest path to root bridge.

    Regards,

    Cristian.

  •  

    - spanning-tree portfast on interface enabled
    portfast--->immiedietely jump from blocking to forwarding state [omnit listening
                and learning]
    portfast--->don't block bpdu frames!
    sw1
    interface FastEthernet0/3
     switchport access vlan 3
     switchport mode access
     spanning-tree portfast
    ---
    general topology
                                             <------------------------bpdu
                                             <------------------------data

    sw3[fa0/1]----------[fa0/2]sw1[fa0/1]----------[fa0/1]sw2

             bpdu------------------------->
              data------------------------->
    ---
    option 1
     sw1 don't receive bpdu on fa0/1 and than:
     > keep portfast behaviore
     > all the time sw1 will send bpdu out of fa0/1 port to sw2
     > accept received bpdu couse stp rebuild, after rebuild we will have
       to options, which can touch sw1 fa0/1 port:
       1 port will be still in forward state and:
         > bpdu send and receive
         > data send and receive
    ---
                                            X didn't receive bpdu
                <------------------------data

    sw3[fa0/1]----------[fa0/2]sw1[fa0/1]----------[fa0/1]sw2

             bpdu------------------------->
              data------------------------->
    ---
       2 port will be blocked, because stp decide this and:
         > bpdu send and receive
         > data blocked
    ---
                <------------------------bpdu
                                 X<---------data

    sw3[fa0/1]----------[fa0/2]sw1[fa0/1]----------[fa0/1]sw2

             bpdu------------------------->
              data--------->X
    ---
    option 2
     sw1 receive bpdu on fa0/1 and than:
     > lost portfast behaviore
     > all the time sw1 will send bpdu out of fa0/1 port to sw2
     > accept received bpdu couse stp rebuild, after rebuild we will
       have to options, which can touch sw1 fa0/1 port:
       1 port will be still in forward state and:
         > bpdu send and receive
         > data send and receive
    ---
                <------------------------bpdu
                <------------------------data

    sw3[fa0/1]----------[fa0/2]sw1[fa0/1]----------[fa0/1]sw2

             bpdu------------------------->
              data------------------------->
    ---
       2 port will be blocked, because stp decide this and:
         > bpdu send and receive
         > data blocked
    ---
                <------------------------bpdu
                                 X<---------data

    sw3[fa0/1]----------[fa0/2]sw1[fa0/1]----------[fa0/1]sw2

             bpdu------------------------->
              data--------->X
    ---

    If BPDU is received inbound on  port, it looses its PORTFAST state (doens't matter if portfast was enabled globally or at the port level).

    corrected

    On every network segment , after STP converged and unless you configure STP Bridge Assurance, BPDU's flow one-way only, which is in the downstream direction (from root bridge to non-root bridges), so BPDU's are always sent on a segment by the designated port of the segment which basically represent the shortest path to root bridge.

    ---

     > accept received bpdu couse stp rebuild, after rebuild we will
       have to options, which can touch sw1 fa0/1 port:

    1 port will be still in forward state and:

    2 port will be blocked, because stp decide this and:

    ---

    about this comment, if e.g. we receive better bpdu [better=lower cost to rb] on port which have portfast enabled tha switch must change status of this port, from non-designated to root or blocked

  • On every network segment , after STP converged and unless you configure STP Bridge Assurance, BPDU's flow one-way only, which is in the downstream direction (from root bridge to non-root bridges), so BPDU's are always sent on a segment by the designated port of the segment which basically represent the shortest path to root bridge.

     

    thank you for this suggestion, I read http://packetmagnifier.wordpress.com/2013/09/09/understanding-stp-max-age/

    to understand your info

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