BPDU

eng_ahmedas

In STP, Does every switch sends BPDU or only the root bridge???

In cisco material, the following 2 statements regarding BPDUs are stated:

a) In legacy STP, BPDUs were only generated by the root and propagated throughout the spanning tree domain.

b)STP communicates Layer 2 information between adjacent switches by exchanging bridge protocol data unit (BPDU) messages.STP sends BPDUs out of every port of the bridge.As a result, when a switch did not receive a configuration BPDU, it did not know where the failure occurred

I am too confused, can anyone help ????

Thanks in advance

Webmaster

In STP only root bridges 'originate' Configuration BPDUs. I'm saying root bridges because initially every switch is a root, that's what they assume. Non-root bridges do not really forward BPDU's like a frame is normally forwarded on a switch (or a packet on a router). However, when a non-root bridge receives a BPDU it reads the information (possibly changing it's known rootID + root path cost + root port, if the path to the root is 'cheaper') and than sends BPDUs forward 'the information' (not the exact BPDU, ie. own bridge ID) out of it's ports. As soon as it stops receive BPDUs on the root port, it will assume itself as being the root again and start originating BPDUs.

If one of the ports of a non-root bridge changes (ie. from forwarding to blocking or down) it can send a special type of BPDU, called Topology Change Notification (TCN) BPDU (up to the root path to the root itself). This TCN BDPU basically indicates the topology has changed. When the root receives this it will propagate the information to the rest of the network.

eng_ahmedas wrote:

In legacy STP, BPDUs were only generated by the root and propagated throughout the spanning tree domain.

Yes, the above applies to regular STP (basically from the 802.1D IEEE standard from 1998. However, that standard has been updated several times and in the latest, the 2004 version, is clearly written STP has now been superseded by the Rapid Spanning Tree Protocol (RSTP). I don't know a lot about RSTP but I think that part you quoted is in the context of RSTP which then does send BPDUs from non-root bridges 'regardless' of whether it hears BPDUs from the root.