Cisco Support Community
cancel
Showing results for 
Search instead for 
Did you mean: 
Announcements

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. And see here for current known issues.

Bronze

rstp, synch process at start up?

Hi everybody

Let say we have three switches configured with rstp. We just power them up

Do switches use proposal /agreement processes before transitioning their ports in forwarding state even when they just power up?  Or do they perform  "synch"  process only when there is topology change?

In STP ,for example  when the switches just power up,  root election is held, once the root bridge is elected, each switch determines its port role.Timers such as MAX age ,forward delay do not play any role. These times play their role if there is changes in topology.  Is my understanding correct?

thanks and have a great weekend

2 ACCEPTED SOLUTIONS

Accepted Solutions
New Member

Re: rstp, synch process at start up?

RSTP, for the most part, acts the same as STP only faster. The same election process occurs on startup before ports go to a forwarding state. RSTP also has different port state names, but they basically do the same things. The same BPDUs are sent (topology change and normal BPDUs). The big difference is that RSTP remembers the backup path should its designated port go down.

This is just a a quick rundown. Grab a book or pull up the Cisco docs for more detailed info.

Sent from Cisco Technical Support iPhone App

New Member

Re: rstp, synch process at start up?

I think I understand the scenario you're proposing. If SW1 is connected then the both ports on SW1 and SW2 will be discarding in a designated blocking state until it receives a BPDU from the opposing switch. When a port is in a designated blocking state, each BPDU that goes out of that port has the proposal bit set. Once SW2 receives a proposal BPDU with a superior Bridge ID in a designated port, then it will transition it's current root port to designated and the port to SW1 would be its new root port. After that, when it goes into a proposal/agreement process, it will transition all edge and designated ports to the discarding state and send out the same BPDU with the agreement bit on. Once SW1 gets that same BPDU back except with the agreement bit on, it will quickly transition that port to forwarding. This process continued happening on the switches outwardly from the originating point. RSTP doesn't use shorter timers, it just has acknowledgements instead.

I was a bit misinformed on the BPDUs, as well, in my previous post. The TC BPDU is not used in RSPT. A BPDU with the TC bit set is sent out instead, as opposed to a completely different type of frame.

Check out this video: http://www.youtube.com/watch?v=Pbv2wLQ2xyY

It is a scenario like what you described that walks through the proposal/agreement process.

3 REPLIES
New Member

Re: rstp, synch process at start up?

RSTP, for the most part, acts the same as STP only faster. The same election process occurs on startup before ports go to a forwarding state. RSTP also has different port state names, but they basically do the same things. The same BPDUs are sent (topology change and normal BPDUs). The big difference is that RSTP remembers the backup path should its designated port go down.

This is just a a quick rundown. Grab a book or pull up the Cisco docs for more detailed info.

Sent from Cisco Technical Support iPhone App

Bronze

rstp, synch process at start up?

Thanks Ryan

I found this great link ; however  I still have some confusion with regard to " synch" process.

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a0080094cfa.shtml

For example.

Please consider the following example of rstp

sw2--rootport----------------Designated port SW3---------------(switched network)

Let say  someone connected a new sw1 with lower priority than the current root bride to sw2  i.e

sw1-------------------sw2-rootport-----------------designated port SW3-------------(switched)

My question  is upon receiving  superior bpdu with proposal bit set from sw1, will sw2 perform " synch" on its current root port? ( because the link and my book say switch performs " synch" on its non-edge designated port; it does not mention root port)

thanks and have a great weekend

New Member

Re: rstp, synch process at start up?

I think I understand the scenario you're proposing. If SW1 is connected then the both ports on SW1 and SW2 will be discarding in a designated blocking state until it receives a BPDU from the opposing switch. When a port is in a designated blocking state, each BPDU that goes out of that port has the proposal bit set. Once SW2 receives a proposal BPDU with a superior Bridge ID in a designated port, then it will transition it's current root port to designated and the port to SW1 would be its new root port. After that, when it goes into a proposal/agreement process, it will transition all edge and designated ports to the discarding state and send out the same BPDU with the agreement bit on. Once SW1 gets that same BPDU back except with the agreement bit on, it will quickly transition that port to forwarding. This process continued happening on the switches outwardly from the originating point. RSTP doesn't use shorter timers, it just has acknowledgements instead.

I was a bit misinformed on the BPDUs, as well, in my previous post. The TC BPDU is not used in RSPT. A BPDU with the TC bit set is sent out instead, as opposed to a completely different type of frame.

Check out this video: http://www.youtube.com/watch?v=Pbv2wLQ2xyY

It is a scenario like what you described that walks through the proposal/agreement process.

367
Views
0
Helpful
3
Replies