BPDU question

[Stuart Teo]

SwitchA---SwitchB---SwitchC

3 switches are connected as above. SwitchA & SwitchC has STP turned ON. SwitchB has STP turned OFF. Will SwitchA & SwitchC see each other's BPDUs?

[Ralph Bean_2]

Hello Hwee Liang -

The answer depends upon the manufacturer and model of Switch and, sometimes, the firmware version.

In general, I recommend not having STP disabled on switches that are between other switches that have STP enabled.

If you have a compelling reason to disable STP on an intermediate switch in the way you describe, you can test the intermediate switch as follows. Either remove these three switches from your LAN or choose a "down" time when you can afford to have your LAN not working. Configure STP as you have described, with STP disabled in the middle switch. Disconnect these three switches from the rest of your LAN. Make sure that the only connection between the two STP switches is the non-STP switch. Do "show span" on the two switches that have STP enabled and see whether one of them lists the other as root bridge. If so, then the middle switch is forwarding the BPDUs. If the middle switch is not forwarding the BPDUs, then both will be Root.

Regards,
Ralph

[Stuart Teo]

I tested with 3 Procurve 4000m and they passed the BPDUs along. That's why I posted the question. Doesn't the standard say something about this? If it should or should not be passed?

[Ralph Bean_2]

Hello Hwee Liang -

I am not highly familiar with the STP specs (802.1d, 802.1w, 802.1s), but those who are have told me that the specs assume that STP is always turned on.

Regards,
Ralph

[OLARU Dan]

IEEE 802.1D does not require that all bridges / switches in the LAN run STP. So non-STP switches will be transparent to BPDUs.

The problem with STP is that all network printers using powersave feature will be sluggish unless you change their port from "Norm" to "Fast" in STP configuration. With many printers (that may move around) maintaining this can be a hassle. The same problem can be seen in NetWare clients with "Norm" ports.

What I did was to periodically run STP for 2-3 hours after workhours, eliminate possible loops and then suspend STP.

[Kevin Richter_1 · Accepted solution]

STP BPDUs are sent to a reserved multicast address (01-00-5E-80-00-00). Legacy bridges which do not support 802.1D STP will forward these multicast packets to all active interfaces (other than the port on which it was received.) This is also the expected behavior of an 802.1D capable bridge with STP disabled.

Therefore, with SwitchB running with STP off, A and C should see each other's BPDUs - which matches your observed behavior with the HP Procurve 4000M switch.

IEEE specs being typically inscrutable, I would recommend Annex F (labeled as informative) of the 802.1D document (1998 Edition). This may seem "outdated," but the relevant sections on forwarding of PDUs (including STP BPDUs) has not been significantly altered in the 802.1w or 802.1s revisions (2001 and 2002, respectively). The references here speak of filtering. If a bridge does not have a reason to filter (drop, discard or choose not to forward) the packet, then it will be forwarded.

OLARU Dan's comment correctly references another of the exact terms stated in Annex F: the non STP switches (and Legacy bridges) will be "transparent" to the BPDUs. His comments regarding "Norm" or normal versus "Fast" mode STP are useful in deploying and troubleshooting an STP implementation but not directly relevant to the BPDU question.

[Stuart Teo]

Hi Kevin,

Thanks for the answer. It's great to know that you're reading the forums.

[Stuart Teo]

Reviving an old thread. I have the below setup:



5308 e.8.42 RSTP
|
|(vlan2)
|
4000 c.9.16 STP-off
|
|(vlan3)
|
5308 e.8.07 RSTP


It's interesting to note that both the 5308 think that they are root, suggesting that the 4000 isn't passing BPDUs. the VLAN shouldn't matter since the BPDU multicast frames would breach that partitioning, wouldn't it?

[OLARU Dan]

Maybe 5300XLs run per-VLAN spanning tree by default. There should be some commands to make VLANs transparent to BPDUs.

[Stuart Teo]

Nope, the 5308s are not running MSTP. I'm sure of that.