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Forum Replies

  1. Wow. Excellent lesson. Very clear and to the point. I am definetly enjoying this site and learning. Excellent refresher cource !
    Please keep up the great work. BTW…Yes I am recommonding this site.

  2. Hi Srini,

    When a non-edge or root port moves into the forwarding state then this triggers as topology change in RSTP. When this happens it will start the TC while timer which 2x hello (4 seconds in total) and it will flush the MAC addresses that were learned on all non-edge interfaces.

    When the TC while timer is active, all BPDUs that the switch will send will have the TC bit set. Other switches that receive this BPDU will start their own TC while timer and they will flush all MAC addresses except the interface where they received the BPDU with the TC bit.

    I hope this helps.


  3. Hi John,

    That’s correct, RSTP has features that are similar to uplinkfast and backbonefast, there’s no need to activate these.


  4. Hello Azeem! I will attempt to answer your questions one by one:

    Question 1.1: In RSTP, I powered on 3 Switches, now all the 3 switches trying to select the Root bridge,In this process will they send proposal bit set in their BPDUs or not?

    Selection of the root bridge in RSTP IS similar to STP. RSTP still uses the best bridge ID (Priority + MAC address) to determine the root bridge. And yes, the proposal bit will be set in their BPDUs. The purpose of the proposal bit however, is not to find the root bridge, but to allow for the sync process, that is, to verify that all ports of the switch are in sync with the fact that the root bridge has been determined.

    Question 1.2. In RSTP, After Root bridge is elected, next in Root port selection, will they use (proposal/Agreement) negotiations or not?

    Yes, proposal/agreement negotiations will occur after the root bridge is elected. Notice, that once the root port is selected, it remains in a blocked state until the rest of the ports go through the sync process. Once those ports complete the sync process, then the root port can be unblocked and a BPDU with the agreement bit set can be sent. (A port is considered in sync if it is either in the blocking state or if it is an edge port.) For more info on the sync process, check this link out: http://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/24062-146.html#agree

    Question 2: In the above figure, the link between SW-A and SW-C is failed, now SW-C has two alternate ports Fa 0/16 and Fa 0/17.which port will choose as Root port? Ans: I am thinking lowest port no will chose as Root Port.( Fa 0/16). Please correct me.

    First of all, I assume you are referring to the diagram in Rene’s lesson with the HUB connected in between switches B and C. If two or more ports on a switch may become root ports, such as in the scenario you describe in your question, they go through the following process to determine which will be the root port:

    1. The total cost to the root bridge. In the example you are describing, this is the same.
    2. Which port receives the lowest bridge ID from the neighbour it is connected to. In this case, both ports are connected to Switch B so both receive the same bridge ID.
    3. Finally, the RECEIVED port priority is used, that is the port priority that is being received in the ports on switch B. Just to clarify, this is not the port priority of the local ports, but of the ports to which each port is connected.

    Having said that, if the network topology didn’t have the hub, and the Fa0/16 port of switch C was directly connected to the Fa0/16 port of Switch B and the same was true for the Fa0/17 ports, then, yes, the Fa0/16 would be chosen due to a default received port priority of 128.16. So, the answer to your question would be YES.

    n my RSTP topology, I have three Switches up and running. SW_A->SW_B->SW_c->SW_A (triangle shape). SW_A is Root Bridge.
    Question 3.1 SW_A (Root Bridge) send BPDUs for every 2 sec. SW-B will receive BPDUs from SW_A. Now SW_B will forward this BPDUs to SW_C or not ?

    No, BPDUs from the root bridge are not relayed by the other bridges. This is why there is the proposal/agreement mechanism. In essence, when switch B for example is connected to switch A, they exchange BPDUs and determine that switch A is the root bridge. The sync process occurs on the rest of the ports on switch B and any switches connected to these ports will go through the same process. This creates a cascading effect away from the root bridge where each designated bridge proposes to its neighbours to determine if it can make a rapid transition. However, if a legacy STP BPDU is detected, RSTP will revert to legacy STP on that interface.

    Question 3.2.If All Switches generate BPDUs every two seconds (hello time) ,SW_C have one Root port and one Altrnate Port, will SW_C send out BPDUs from (Altrnate port and Root Port) for every two seconds?

    In RTP, Alternate ports are in a discarding state and root ports are in a forwarding state. Discarding state in RTP will not send BPDUs but it listens for BPDUs, while a port in forwarding state will transmit and receive BPDUs.

    Question 4 TO clarify my understanding on (POrtFast / EdgePort). when RSTP is configured, by default EdgePort is enabled.
    When a Host is connected to (RSTP configured Switch), the port will jump from blocking to forwarding and send 10 BPDUs to confirm the no BPDUs are receiving.
    When a Host is connected to (RSTP configured Switch) and PortFast command is configured, the port will jump from blocking to forwarding. No BPDUs are sent out.

    Almost! A port on a switch will be considered an edge port by RSTP under the following conditions:

    1. When portfast is enabled. If you enable portfast on a port, it is automatically an edge port.
    2. If the connection is full duplex, the switch will automatically assume it is an edge port.

    The port will no longer be an edge port if one of the following occurs:

    1. If portfast is not configured and the port receives a BPDU. Under these circumstances, the port will no longer be considered an edge port
    2. If portfast is not configured and the link type is half duplex.

    Question 5: If Switch receives BPDUs with TC bit set, It clears the MAC addresses learned on all its ports, except the one that receives the topology change.why it will not clear the MAC address entries on the received port ?

    If a topology change is detected, a switch will relearn all of the MAC addresses from scratch in order to regain a fresh understanding of which MAC addresses can be found where. It does not however remove the MAC address of the device that sent the BPDU with the TC bit set to 1. This is because, since this BPDU was successfully received, the MAC address is still valid and can be kept.

    I hope this has been helpful for you!!!


  5. Hello florian

    No problem! Keep the questions coming, that’s why we’re here! :slight_smile:

    Such a switch will send BPDUs out of all STP enabled ports except blocking ports. Blocking ports will only receive BPDUs.

    I hope this has been helpful!


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