cancel
Showing results for 
Search instead for 
Did you mean: 
cancel
Announcements

782
Views
0
Helpful
9
Replies
Contributor

few questions on rstp

Hi everybody

Please consider the topology based on rstp:

                                                                                                (root port)

                                                                   sw1-----------------------f1/1 -SW2

                                                                                                        f1/3(Designated port)

                                                                                                          |

                                                                                                          |

                                                                                                          |

                                                                                                        f1/4( alternate)

                                                                                                         SW3

1)Sw1 is the root.  My book says in rstp, bpdu are sent every two seconds . Does it mean sw3 will also send bpdu out of its alternate port to sw2?

2) Do we need to configure port connected to host as edge port in rstp or rstp does that for us?

=================================================================================================

Please consider the following topology based on rstp.

                                                                                                    sw1

                                                                                                      |

                                                                                                     sw2

                                                                                                      |

                                                                                                      |

                                                                                                     f1/2

                                                                                                     Sw3

                                                                                                       |(edge)

                                                                                                       |

                                                                                                    host

3) Sw1 is the root bridge; sw3 f1/2 is the root port. Next we add a link between sw1 and sw3 as shown below:

                                                                                            ---------sw1

                                                                                             |         |

                                                                                             |       sw2

                                                                                                       |

                                                                                             |         |

                                                                                                     f1/2

                                                                                              -------Sw3

                                                                                                       |(edge)

                                                                                                       |

                                                                                                    host   ( sorry for the poor drawing)

  Sw3 will perform synch on its f1/2 because it is receiving better bpdu from sw1 than it does from from sw2. What will happen on sw3 edge port?

thanks and have a great weekend

Everyone's tags (3)
4 ACCEPTED SOLUTIONS

Accepted Solutions
Rising star

Re: few questions on rstp

Hi,

1) in 802.1D after the network converges and Root is selected, only the root bridge sends out BPDUs, it changed in RSTP, every switch sends its own BPDU, but sends it appropriately.. to downstream switches as far as i know.

2) Edge ports are basically those ports configured with the PORTFAST feature, its set by the admin.

3) nothing happenes to the edge port. only the port connected to the new link becomes new ROOT PORT and f1/2 goes into block (back up) state. as long as there is no bpdu received in the edge port, it maintains its status.

Hope it Helps,

Soroush.

Hope it Helps!

Soroush.

Re: few questions on rstp

Hello Sara,

1) In 802.1W the Alternate port and the Backup port are the equals to the blocked port on the 802.1D, you only expect BPDU's on a alternate port, you do not send.

2) You will need to configure the port-fast feature to make it a Edge port ( As long as does not receive any BPDU it will work like this, if it receives one it will go to the point-to-point mode port  or shared port)

3) As soon as you connect the other link the switch will start receiving superior BPDU's ( Lower Root Priority) so this will kick of a reconvergence on the STP topology.  To the edge port nothing will happen.

Please refer to the following links for extra information:

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a0080094cfa.shtml#states

Regards,

Julio

Julio Carvajal
Senior Network Security and Core Specialist
CCIE #42930, 2xCCNP, JNCIP-SEC
Highlighted
Rising star

Re: few questions on rstp

hi Julio,

jcarvaja wrote:

Hello Sara,

1) In 802.1W the Alternate port and the Backup port are the equals to the blocked port on the 802.1D, you only expect BPDU's on a alternate port, you do not send.

if it is correct, should i send out a suggestion to Cisco website to correct below line in the link u sent here?

"A blocked port receives a more useful BPDU than the one it SENDS out on its segment."

Soroush.

Hope it Helps!

Soroush.

Re: few questions on rstp

Hello Soroushm,

I was refering to the 802.1D behavior, on 802.1W is different as you can see on the link

Every port on a switch running 802.1W sends out a BPDU at hello time intervals regardless of wheter BPDUs are received from the root. With this , any switch anywhere in the network can play an active role maintaining the topology....

Regards,

Remember to rate all the helpful posts

Julio

Julio Carvajal
Senior Network Security and Core Specialist
CCIE #42930, 2xCCNP, JNCIP-SEC
9 REPLIES 9
Rising star

Re: few questions on rstp

Hi,

1) in 802.1D after the network converges and Root is selected, only the root bridge sends out BPDUs, it changed in RSTP, every switch sends its own BPDU, but sends it appropriately.. to downstream switches as far as i know.

2) Edge ports are basically those ports configured with the PORTFAST feature, its set by the admin.

3) nothing happenes to the edge port. only the port connected to the new link becomes new ROOT PORT and f1/2 goes into block (back up) state. as long as there is no bpdu received in the edge port, it maintains its status.

Hope it Helps,

Soroush.

Hope it Helps!

Soroush.
Contributor

Re: few questions on rstp

Thanks Soroushm and Julio

Please consider the following example.

             ------------------------------sw1-----------------

            |                                                                    |

            |                                                                    |

            |                                                                    |

          f2(R)                                                               f3(R)

          SW2-f4(A)-------------------------------- -----5(D)SW3

Sw1 is the root bridge; network is converged as shown above. Let say Sw3 f3 goes down, What will happen next?

Will sw3 set propsal bit in bpdu and send it to sw2?

What will sw2 do when it receives a that bpdu with proposal bit set from sw3? Will sw2 perform synch operation on its f2 port even though the bpdu received from sw3 is inferior?

thanks.

Rising star

Re: few questions on rstp

next, sw3 puts the designated port into forwarding state, sends out bpdu with the topology change (tc) bit set to the neighboring switch sw2. sw2 then flushes its MAC table entries and starts the TC timer, sends out bpdu with TC bit out all its designated and root port. it happenes through out the entire network and rapidly everyone know abt the topology change, network converges with the new topology.

Hope it Helps,

Soroush.

Hope it Helps!

Soroush.
Contributor

Re: few questions on rstp

Thanks Soroushm.

My book says in rstp bpdu with tc bit set is only when a port goes to forwarding state.So if this is true then sw3 should not generate bpdu with tc bit set upon losing f3 root port.

2bd thing sw3's f5 port is already in forwarding state .

The question I have is:  Will sw3 and sw2 perform hand shake before changing their port states when sw3 loses its f3 root port? ( i.e will sw3 and sw2 with ports f5 and f4 respectively, perform hand shake before sw2 put its alternate port f4 in forwarding state?

thanks

Rising star

Re: few questions on rstp

Thats exactly right sarah, my bad, i mixed up the Alternate port between sw3 and sw2 ! tc is for when port starts forwarding.

because sw2 still knows the root is alive and well, upon receiving inferior bpdu from sw3, it immediately sends a BPDU to sw3 that contains information about the root bridge. sw3 does not send its own BPDUs and accepts the port that leads to sw2 as the new root port.

Hope it Helps,

Soroush.

Hope it Helps!

Soroush.
Contributor

Re: few questions on rstp

Thanks Soroushm.

I understand  how sw2 and sw3 will exchange bpdu and finally decide the state of their respective ports. But when exchanging those bpdu, do they still use hand shake process i,e setting proposal bit, agreement bit in bpdu?

thanks

Re: few questions on rstp

Hello Sara,

1) In 802.1W the Alternate port and the Backup port are the equals to the blocked port on the 802.1D, you only expect BPDU's on a alternate port, you do not send.

2) You will need to configure the port-fast feature to make it a Edge port ( As long as does not receive any BPDU it will work like this, if it receives one it will go to the point-to-point mode port  or shared port)

3) As soon as you connect the other link the switch will start receiving superior BPDU's ( Lower Root Priority) so this will kick of a reconvergence on the STP topology.  To the edge port nothing will happen.

Please refer to the following links for extra information:

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a0080094cfa.shtml#states

Regards,

Julio

Julio Carvajal
Senior Network Security and Core Specialist
CCIE #42930, 2xCCNP, JNCIP-SEC
Highlighted
Rising star

Re: few questions on rstp

hi Julio,

jcarvaja wrote:

Hello Sara,

1) In 802.1W the Alternate port and the Backup port are the equals to the blocked port on the 802.1D, you only expect BPDU's on a alternate port, you do not send.

if it is correct, should i send out a suggestion to Cisco website to correct below line in the link u sent here?

"A blocked port receives a more useful BPDU than the one it SENDS out on its segment."

Soroush.

Hope it Helps!

Soroush.

Re: few questions on rstp

Hello Soroushm,

I was refering to the 802.1D behavior, on 802.1W is different as you can see on the link

Every port on a switch running 802.1W sends out a BPDU at hello time intervals regardless of wheter BPDUs are received from the root. With this , any switch anywhere in the network can play an active role maintaining the topology....

Regards,

Remember to rate all the helpful posts

Julio

Julio Carvajal
Senior Network Security and Core Specialist
CCIE #42930, 2xCCNP, JNCIP-SEC
CreatePlease to create content
Content for Community-Ad
July's Community Spotlight Awards