rstp scenaio

[sarahr202]

Hi everybody

  sw1f1/1------------------------------------f1/2sw2

We just power on our two switches above.Sw2 happens to send first bpdu.

Let suppose sw2 sends a bpdu claiming itself to be root,  with proposal bit set, port role as " designated" port state as " forwarding.

Sw1 has lower mac address and lower priority. Sw1 therefore does not agree with sw2 as being a root. How does sw1 communicate this to sw2?

because i do not see any " disagreement" bit  ( my guess sw1 will simply not set" Agreement" bit which will indicate sw1 does not agree , 1 did not have the proper gear to verify this)

Scenario # 2

              sw2-----------sw1-----------------sw3

Above all our switches are running rstp. We just power on our switches. Let suppose our sw1 happens to send first bpdu.

My question will sw1 perform " SYNCH" operation with both sw3 and sw2 at the same time? for example sw1 performs " synch" operation first with sw3 and then sw2 or sw1 performs " SYNCH" operation with both switches sw2 and sw3 at the same time.?

thanks and have a great weekend.

[Abzal · Accepted solution]

Hi,

Scenario #1

Sw1 rejects a Proposal as it has better root bridge information. In this case, it  sends back alternate proposal message with its own root bridge information. Then Sw2 sends back agreement flag set and puts port into forwarding state.

Scenario #2

First root bridge election occurs. Say sw1 sends BPDU proposal bit flag set first claiming as being root. Suppose sw1 has lowest BPDU priority. Sw2 and sw3 send back agreements and put ports into forwarding state.

Now say sw2 has lowest priority and sends a Proposal first. Once sw1 receives the Proposal sync process occurs. Sw1 puts into blocking state downstream ports connected to Sw3. And sends back BPDU agreement bit set when Sw2 receives this packet it puts port into forwarding state. After that Sw1 Proposal to downstream Sw3 and as soon it receives Agreement port is in Forwarding state.

Hope it will help.

Best regards,
Abzal