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BASIC STP Question

visitor68
Level 4
Level 4

Folks - please forgive my stupidity, but it's been a while since I gave STP more than 2 seconds of mind-time. 

So...every, and I mean every, tutorial I read/watch on You Tube shows how STP convergence takes place by showcasing only the connections between the switches and how BPDUs are passed between them. But what does a switch do with the BPDU besides consuming it and converging accordingly? Meaning, does the switch forward/flood that BPDU out ALL ports (besides the port it received it on), including the ports to which endpoints are connected, such as in access switches? It is, after all, a multicast frame.

5 Replies 5

Reza Sharifi
Hall of Fame
Hall of Fame

Sorry, but the answer to my question is not found in the post. I would prefer someone just answering my question instead of having me go on a wild goose chase. 

Martin L
VIP
VIP

 

In STP only Root switch creates Configuration BPDUs to send them out on all ports to neighbors while Topology Change Notification (TCN) BPDU is created by any switch when needed.  The books do not specified how new info is added to Root BPDU before forwarding it over to others.  I think Root BPDU is copied, new info is added (port id, priority, and cost) and then forwarded out of all non-blocking ports.  So, I guess you could say Root BPDU is edited before forwarding, right?

All non-blocking ports are forwarding BPDUs even access ports (u could stop it on access port with filter command). Blocking port is just listing to BPDUs but not creating them and not forwarding them. 

Yes, it is multicast frame with reserved multicast MAC id.  This frame is there to id the frame and fast process it by appropriate process.  There are several reserved MACs for Cisco STP flavor.  

 

Regards, ML
**Please Rate All Helpful Responses **

Joseph W. Doherty
Hall of Fame
Hall of Fame

Yes, STP, by default, floods across all ports.  This is how it avoids loops, i.e. insuring that no link is not analyzed.

That said, it's possible to filter BPDUs, going out ports, but when doing so, if you do create a L2 loop, STP will not detect it.

Interesting, if a Cisco "port fast" configured edge port receives a BPDU, by default, the port will drop out of "port fast" mode, but otherwise work normally.  Only if you also configure BPDUGuard, will a received BPDU error out the port.

Seb Rupik
VIP Alumni
VIP Alumni

Hi there,

As you say a switch will initially transmit its own BPDU to participate in the root bridge selection process. When a switch receives a superior BPDU it takes the contents and begins transmitting this newly configured BPDU to the neighbouring switches.

BPDU are sent out of all switchports as this lets connected devices (especially other switches) know it is connected to an STP/RSTP device. This is desirable as it should/ will lead to a loop free Layer2 topology.

 

cheers,

Seb.

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