RSTP: root bridge election and convergence

Answered Question
May 2nd, 2009

Hello

I have 3 questions about RSTP:

1. In STP durring max-age time (20s) ports are in blocking state. Durring that time root-bridge election occurs (for diameter 7 it will take at most 14s), then root/designated/non-designated port are choosen.

And how does the root-bridge election looks in RSTP ? How long does it take ? Do we also need 20s for that ?

(i've read http://cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a0080094cfa.shtml but there is nothing about root-bridge election)

2. RSTP convergention: when will it end ?

Let's suppose we have S1-S2-S3-S4-S5. S3 is root bridge. Now let's connect S1 with S5.

Do i understeand correctly that synchronisation will be only between S1 and S5 ? After setting designated port from one side and alternate port from the other synchronisation will end ? Or maybe it will spread across whole network ?

3. In stable topology hello BPDU are send only to designated(forwarding) ports. Durring root election process they are send to all (blocking) ports. When is this logic changed ? When switch knows that is should start sending BPDU hello only on designated ports ? (in RSTP) ?

Thanx

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Correct Answer by Giuseppe Larosa about 7 years 7 months ago

Hello Maciej,

1) OK

2) I made an error in writing I wanted to mean if there is no change in root port in S1 and S5 then no other switches are involved.

I should have written "if the answer is false to first answer then ..."

Sorry for confusion

Hope to help

Giuseppe

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Giuseppe Larosa Sat, 05/02/2009 - 02:03

Hello Maciej,

1) max-age is used only on half-duplex link or if a legacy 802.1D neighbor is detected on a port.

if the campus network has only RSTP enabled switches with point-to-point links between them and all edge ports are correctly classified (this is important it is not only a matter of clarity) using spanning-tree portfast there is no need for using max-age timers.

2) when connecting S1 to S5 convergence hands after the two have performed synchronization on the new link so it can take as less as 100 msec the time to exchange BDPUs.

3) before election of a root bridge it is not possible to decide which ports have to be blocked everything is defined after choice or the root bridge.

However, ports are blocked for used traffic they can receive BPDUs.

The moment to decide that election is ended is when all received BPDUs agree on the root bridge bridge-id.

Hope to help

Giuseppe

mlopacinski Sat, 05/02/2009 - 04:01

Thanx for the reply

1. If there is no need for using max-age in RSTP how do switches elect root-bridge ? They have to do it somehow ? Or maybe there is no election of root-bridge ?

2. Does it mean that after S1 synchronizes to S5 network is stable and no other switches need to perform synchronisation ?

3. this point is clear for me now :)

Thanx

Giuseppe Larosa Sat, 05/02/2009 - 04:18

Hello Maciej,

1) election of root bridge for each 802.1W Rapid STP instance is the first event that happens in a switched network.

I'm not sure here: if the BPDUs propagate using hello interval up to max-age is needed to elect the root bridge, if they can use the handshake and sync mechanism on each point-to-point link it is a sequence of handshakes and it takes less time to elect the root bridge.

Using the two seconds hello interval can be more secure. After all, the root bridge election shouldn't be a frequent event.

This behavior is more probable because there is actually no description of how the root bridge is elected in RSTP.

This would open the question of when the handshaking capabilities are started.

2) yes, they just need to see the following:

does S1 or S5 need to change its root port ?

who between S1 and S5 is the DP on new segment ?

if the first answer is true there is no change in topology and the other switches are not involved.

Hope to help

Giuseppe

mlopacinski Sun, 05/03/2009 - 06:29

1. Ok, thanx

2. You said that if first answer "does S1 or S5 need to change its root port ?" is true there is no change in topology. I fought that it's exatly opposite: if S1,S5 DO NOT need to change root port there is no topology change and other switches are not involved. Please clarify.

Correct Answer
Giuseppe Larosa Sun, 05/03/2009 - 11:19

Hello Maciej,

1) OK

2) I made an error in writing I wanted to mean if there is no change in root port in S1 and S5 then no other switches are involved.

I should have written "if the answer is false to first answer then ..."

Sorry for confusion

Hope to help

Giuseppe

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