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Community Member

Why the Spanning tree does not work

Hi,

If I put the hub in between the switches (four or five ethernet switches), the spanning tree will not work. The loop would be created. Grateful if you could enlighten me.

rdgs

4 REPLIES
Hall of Fame Super Blue

Re: Why the Spanning tree does not work

anitachoi3 wrote:

Hi,

If I put the hub in between the switches (four or five ethernet switches), the spanning tree will not work. The loop would be created. Grateful if you could enlighten me.

rdgs

Anita

Depends how you connect it up ie.


sw1 -> sw2 -> hub -> sw3 -> sw4

there is no layer 2 loop there so you will be fine. So using a hub does not necessarily create a L2 loop. However a hub simply forwards a packet out of all it's interfaces except the one it received it on. So yes you could create a loop with a hub but you could also create a loop with L2 switches.

However most hubs don't run STP so yes it is easier to create a loop and this loop will not be blocked on the hub.

Jon

Hall of Fame Super Silver

Re: Why the Spanning tree does not work

Hello Anita,

if you connect 4 or 5 switches to hub ports please check the duplex and speed settings on switch ports:

an hub can only work in half duplex mode and cannot accomodate different speeds: that is user at 100 Mbps see only other user at 100 Mbps and users at 10 Mbps can only see other 10 Mbps users (at least this is what I see in my home hub)

if duplex is harcoded to full on switch ports it is a problem.

Besides this, each switch should see the BDPU sent by other ones and only one port should be elected as designated port in segment and all the other ones should be in blocking.

Verify also that the device is really an hub and not a switch: you can check this using STP bdpu guard on a Cisco switch.

Verify also that your switch ports are not using the dangerous STP bpdu filter enabled at port level this would lead to a loop.

Verify also if STP portfast is enabled on switch ports this can enough to cause the loop.

Using ports without portfast and no bpdu filter and with no duplex/speed issues you should be able to avoid the loop (not guaranteed but likely)

Hope to help

Giuseppe

Community Member

Re: Why the Spanning tree does not work

Hi,

I check the BCMSN reference book, it mentions that

"What happens if a switch doesn’t receive BPDUs in a timely manner or when it doesn’t receive

any? The switch can view that condition as acceptable—perhaps an upstream switch or an

upstream link is dead. In that case, the topology must have changed, so blocked ports eventually

can be unblocked again."

Chapter 10, page 250.

Can Hub forward the BPDU frame?

rdgs

Hall of Fame Super Blue

Re: Why the Spanning tree does not work

anitachoi3 wrote:

Hi,

I check the BCMSN reference book, it mentions that

"What happens if a switch doesn’t receive BPDUs in a timely manner or when it doesn’t receive

any? The switch can view that condition as acceptable—perhaps an upstream switch or an

upstream link is dead. In that case, the topology must have changed, so blocked ports eventually

can be unblocked again."

Chapter 10, page 250.

Can Hub forward the BPDU frame?

rdgs

Anita

Yes, a hub can forward a BPDU. To a hub a BPDU is just another frame to be sent out.

Jon

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