802.1D TCNs

Hello all,

Started studying for the CCNP switch. It has been a while since my CCNA and I have a question about traditional 802.1D STP:

642-813 OCG, Page 143 - "A topology change occurs when a switch moves a port into the Forwarding state or moves a port from the Forwarding or Learning states into the Blocking state. In other words, a port on an actives switch comes up or goes down. The switch sends a TCN BPDU out its root port so that ultimately the root bridge receives news of the change"

642-813 OCG, Page 146 (Indirect Topology Changes) " 1. Catalyst A and C both show a link up condition; data begins to be filtered elsewhere on the link. 2. No link failure is detected, so no TCN messages are sent."

Based on figure 7-7 which can be see here: CCNP SWITCH 642-813 Official Certification Guide - David Hucaby - Google Books

When the max age expires, the port 1/1 on Switch C will lose its root port status. I am confused because the OCG first says TCNs are sent from transitions from Forwarding to Blocking (and blocking to forwarding). But then it says TCNs are not sent because no link failure was detected...despite a link transitioning when the topology changes (the previous blocking port becoming the root port).

I have a number of questions:

Does Switch C lose sync with the topology and think its the root? Is that why a TCN is not sent? If the switch does not lose sycn, shouldn't a TCN BPDU be sent because the blocking port will transition to forwarding when it becomes the new root port? And the previous root port, that would just be a designated port (after all, no BPDUs are being received there, so no need to block) so it would go from a forwarding root to a forwarding DP. I would think a TCN would have to be sent to help the CAM table age out those entries.

If anyone can shed some light on this it would be helpful

Find more posts tagged with

Free for TechExams community: Cybersecurity salary guide

Compare cert salaries and plan your next career move

Button

Comments

AwesomeGarrett

If you look at the topology switch C loses its connection to the root bridge thus it cannot send a TCN BPDU and switch A interface 1/2 does not transition through any STP states (Although I'm unsure if it would send out a configuration BDPU to switch B stating that a change in the topology has occurred). At this point switch C flushes the root bridge BPDU after the max age timer expires. Interface 1/2 on switch C will receive the BDPU's from the root bridge (switch A) propagated by switch B. At this point switch C will accept switch A as the root and finish moving through the STP port states and become a root port.

Once interface 1/2 on switch C transitions to the forwarding state it does not send a TCN BPDU because it was not part of the topology after it was lost. The ports on switch A and B never changes STP states thus no need for TCN BPDU's.

EDIT: Because interface 1/1 on switch C is up and not receiving BPDU's it will become a designated port.

Switch A will not send configuration BDPU's because the STP port state did not change.

Switch C does loose synchronization with the topology because it is not receiving root bridge BPDU's from a root port.

I apologize if it reads like I'm all over the place but I just got done studying myself and thats how my brain is at the moment. Should probably get more sleep.

MikeO5422

Thanks, if the switch loses sync and thinks its the root then it now makes sense to me.