1

From a recent packet capture, after the sync process in RSTP, only the root bridge sends RSTP BPDUs every 'hello' interval. Why is it so? Shouldn't all switches send BPDUs every 'hello' interval since that's the only way of knowing if a link is down?

enter image description here

From the above picture, only the root (as is evident from the cost) is sending BPDUs.

3
  • Please, never use an image for text. Simply copy the text, paste it into the question, and use the Preformatted-text option ({}). You can obfuscate any text prior to posting the question.
    – Ron Maupin
    Sep 27 at 12:32
  • possibly a stupid question - is this capture from all links on all interfaces? and any chance the topology is a star?
    – Effie
    Sep 27 at 15:59
  • Yes, this is a capture from all links. I did get proposal/agreement from both ports. After sync, only one port sent BPDUs. The topology is just two switches connected to each other. Sep 28 at 4:31
5

according to comments in the question, the actual topology is two bridges:

B1 (root) --- B2.

According to the specification (IEEE Std 802.1w-2001, sec 17.7)

Configuration Messages are transmitted if the information to be transmitted by a Designated Port changes. In addition, Designated Ports transmit Configuration Messages at regular intervals to guard against loss and to assist in the detection of failed components (LANs, Bridges, or Bridge Ports).

That means that periodic hellos are only transmitted on designated ports, not all ports. In particular, hellos are not transmitted on root ports.

The idea is somewhat following. Both STP and RSTP track the liveliness of the "parent" node in the spanning tree (which is the root port). This is important, because if parent node fails, STP/RSTP needs to take actions to repair the tree.

Tracking liveliness of child nodes (bridges attached to designated ports) are not really important. Parent node is not doing anything if the child fails. Note, that the port is not necessary attached to a point-to-point link, and end-systems may still be reachable if the child fails. So RSTP can't turn port off and/or stop sending payload frames there.

So, in RSTP (actually similar to STP) liveliness messages are only sent from parents to children along spanning tree. In STP root sends messages and other bridges relay messages. This is because STP cannot do anything to repair topology without participation of the root node, i.e., until the topology converges with new tree build from root again. With RSTP, bridges can repair topology "more locally" - e.g., switch to alternative ports. Thus each bridge sends the liveliness individually to its "children".

5
  • 1
    Exactly - as a rule of thumb, BPDUs are only transmitted 'away' from the root.
    – Zac67
    Sep 28 at 11:31
  • If only designated ports send BPDUs, then what could be the reason for BPDU flags to contain root port, alternate port and backup port encoding in port role? Sep 29 at 9:25
  • no. only designated ports send periodic "hellos". i am too lazy to go through the spec right now, but AFAIK there is only one single BPDU format for all cases when BPDUs need to be sent. Switches, that need to send hellos, send the same BPDU that they would have sent on this port if the BPDU needed to be sent for other reasons.
    – Effie
    Sep 29 at 15:11
  • Sorry I don't think you got my question. In the RSTP BPDU structure, there is a 8 bits field called BPDU flags. 2 bits in the BPDU flags are reserved for communicating port roles. They have encoded '10' for root port, '11' for designated port and '01' for alternate/backup port. If only designated ports send BPDUs, then what is the reason for this encoding to exist, since no alternate port will ever send a BPDU declaring himself as alternate port? Am I missing something? Will alternate ports send agreement to the designated port? Sep 30 at 9:35
  • I think I did. Only periodic BPDUs are send only on designated ports. On topology changes, BPDUs are transmitted based on different rules (this is, the first sentence in the quote). For these BPDUs, all these fields are necessary. However, there is no special format for periodic BPDUs, so periodic BPDUs have the same information as the BPDUs that would otherwise be sent on topology changes. I think they are even processed the same. So, it is simpler this way. You need to have more than 2 bridges in your topology, to see something interesting happening.
    – Effie
    Sep 30 at 10:36

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.