Intro
It is well known that RSTP uses synchronization to speed up convergence in
switched networks. Not many articles or books give the full picture how this
process really works. The synchronization process is often oversimplified
and readers are left with the IEEE standard if they want to understand all
of the details. This post will give you a better understanding of how the
RSTP synchronization really works.
Initial synchronization
In regular 802.1D when switches first boot up ports are brought online
the switch claims to be root because it has not yet heard any better BPDUs.
This is no different in RSTP or RPVST+ which is Ciscos implementation.
Take a look at the following topology.
The goal here is to make SW1 the root bridge. But until better BPDUs have
been heard all switches will claim root. That is how STP works, it stores
the best BPDU received in on a port. To emulate a network coming online
to begin we will have all ports shutdown and then try to bring them up
at the same time. Debugs and captures will be run to show how the synchronization
process works. The following debugs have been enabled.
SW1#sh debug Spanning Tree: Spanning Tree event debugging is on Spanning Tree state sync support debugging is on
So we start at looking at the debugs from each switch in order.
SW1
setting bridge id (which=3) prio 16385 prio cfg 16384 sysid 1 (on) id 4001.aabb.cc00.0100 RSTP(1): initializing port Et0/0 RSTP(1): Et0/0 is now designated %LINEPROTO-5-UPDOWN: Line protocol on Interface Ethernet0/0, changed state to up RSTP(1): transmitting a proposal on Et0/0 RSTP(1): received an agreement on Et0/0 STP[1]: Generating TC trap for port Ethernet0/0
SW1 assumes its port is designated and sends out a proposal. SW2 will agree to this
proposal.
SW2
%LINK-3-UPDOWN: Interface Ethernet0/0, changed state to up %LINK-3-UPDOWN: Interface Ethernet0/1, changed state to up setting bridge id (which=3) prio 32769 prio cfg 32768 sysid 1 (on) id 8001.aabb.cc00.0200 RSTP(1): initializing port Et0/0 RSTP(1): Et0/0 is now designated RSTP(1): initializing port Et0/1 RSTP(1): Et0/1 is now designated %LINEPROTO-5-UPDOWN: Line protocol on Interface Ethernet0/1, changed state to up RSTP(1): transmitting a proposal on Et0/0 RSTP(1): transmitting a proposal on Et0/1 %LINEPROTO-5-UPDOWN: Line protocol on Interface Ethernet0/0, changed state to up RSTP(1): transmitting a proposal on Et0/0 RSTP(1): transmitting a proposal on Et0/1 RSTP(1): received an agreement on Et0/1 STP[1]: Generating TC trap for port Ethernet0/1 RSTP(1): transmitting a proposal on Et0/0 RSTP(1): transmitting a proposal on Et0/0 RSTP(1): updt roles, received superior bpdu on Et0/0 RSTP(1): Et0/0 is now root port RSTP(1): syncing port Et0/1 RSTP(1): synced Et0/0 STP[1]: Generating TC trap for port Ethernet0/0 RSTP(1): transmitting an agreement on Et0/0 as a response to a proposal RSTP(1): transmitting a proposal on Et0/1 RSTP(1): received an agreement on Et0/1
SW2 initializes all ports as designated and starts sending out proposals. It
then receives a better BPDU from SW1 so it has to sync its downstream ports (Et0/1).
SW3
%LINK-3-UPDOWN: Interface Ethernet0/0, changed state to up %LINK-3-UPDOWN: Interface Ethernet0/1, changed state to up %LINEPROTO-5-UPDOWN: Line protocol on Interface Ethernet0/0, changed state to up %LINEPROTO-5-UPDOWN: Line protocol on Interface Ethernet0/1, changed state to up setting bridge id (which=3) prio 32769 prio cfg 32768 sysid 1 (on) id 8001.aabb.cc00.0300 RSTP(1): initializing port Et0/0 RSTP(1): Et0/0 is now designated RSTP(1): initializing port Et0/1 RSTP(1): Et0/1 is now designated RSTP(1): transmitting a proposal on Et0/0 RSTP(1): transmitting a proposal on Et0/1 RSTP(1): updt roles, received superior bpdu on Et0/0 RSTP(1): Et0/0 is now root port RSTP(1): syncing port Et0/1 RSTP(1): synced Et0/0 STP[1]: Generating TC trap for port Ethernet0/0 RSTP(1): transmitting an agreement on Et0/0 as a response to a proposal RSTP(1): transmitting a proposal on Et0/1 RSTP(1): transmitting a proposal on Et0/1 RSTP(1): received an agreement on Et0/1 STP[1]: Generating TC trap for port Ethernet0/1 RSTP(1): updt roles, received superior bpdu on Et0/0 RSTP(1): syncing port Et0/1 RSTP(1): synced Et0/0 RSTP(1): transmitting an agreement on Et0/0 as a response to a proposal RSTP(1): transmitting a proposal on Et0/1 RSTP(1): received an agreement on Et0/1
SW3 goes through the same process. Claims root at first, then hears a better BPDU
and must sync its downstream port.
SW4
%LINK-3-UPDOWN: Interface Ethernet0/0, changed state to up setting bridge id (which=3) prio 32769 prio cfg 32768 sysid 1 (on) id 8001.aabb.cc00.0400 RSTP(1): initializing port Et0/0 RSTP(1): Et0/0 is now designated %LINEPROTO-5-UPDOWN: Line protocol on Interface Ethernet0/0, changed state to up RSTP(1): transmitting a proposal on Et0/0 RSTP(1): updt roles, received superior bpdu on Et0/0 RSTP(1): Et0/0 is now root port RSTP(1): synced Et0/0 STP[1]: Generating TC trap for port Ethernet0/0 RSTP(1): transmitting an agreement on Et0/0 as a response to a proposal RSTP(1): updt roles, received superior bpdu on Et0/0 RSTP(1): synced Et0/0 RSTP(1): transmitting an agreement on Et0/0 as a response to a proposal
SW4 also claims root, hears a better BPDU. It has not downstream ports to
synchronize so the process ends there.
To visualize the process this is what happens at time 0.
This can be seen in the BPDUs as well. This is the BPDU that SW1 sends out.
The BPDU is a proposal and the designated bit is set. It’s not yet trying to learn or
forward on the port.
SW2 sends out the following BPDU.
When SW1 has received agreement BPDU it can start forwarding on its designated port.
SW2 can forward on its root port as soon as it decides that it is root. When SW2
has learned better root information it must synchronize downstream ports so the port
to SW3 is still blocking.
This is the agreement BPDU that SW2 sends to SW1.
The learning and forwarding bits are set and the role is root. The agreement
bit is also set as well as TC so that MAC address tables can be updated. The
TC bit is set for 2x the hello time and is called TcWhile.
The next segment to be synchronized is the one between SW2 and SW3.
At first SW3 claims to be root.
Then SW2 sends out a better BPDU.
SW2 sends a BPDU with TC set because for a brief period of time SW2 was believed to
be root before SW2 heard a better BPDU from SW1. Then SW3 sends agreement BPDU.
After SW2 has received the agreement BPDU it can bring its downstream port (Et0/1)
to forwarding making the topology look like this.
Finally the segment between SW3 and SW4 is synchronized. SW3 sends out the BPDU
and then SW4 agrees to it. TC is set because for a brief period SW3 as believed
to be root.
The final topology is then that all links are forwarding because we have no physical
loop in this topology.
Receiving better root information
So far we had no physical loop in the topology. This is not a very realistic
scenario and to see how RSTP works when receiving better root information we
will add a link between SW1 and SW4 meaning that SW4 has a direct path to the
root like this.
Before we look at what happens when bringing up the port between SW1 and SW4
let us assign port roles to all the ports on the drawing. This is good practice
to understand how STP works and you should be able to do this manually if you
fully understand STP. We are expecting the topology to converge like this.
After SW4 receives better root information, which ports do we need to synchronize
to converge the topology? SW1 does not receive better information, it is the root.
SW4 has a designated port towards SW3 so it needs to synchronize that segment.
SW3 has no designated ports so we except the synchronization process to stop
there. Lets look at debugs and I’ll do a play by play with the drawings.
SW1
%LINEPROTO-5-UPDOWN: Line protocol on Interface Ethernet0/1, changed state to up RSTP(1): initializing port Et0/1 RSTP(1): Et0/1 is now designated RSTP(1): transmitting a proposal on Et0/1 RSTP(1): transmitting a proposal on Et0/1 RSTP(1): received an agreement on Et0/1 STP[1]: Generating TC trap for port Ethernet0/1
SW1 initializes the port and waits for agreement BPDU from SW4 before it can
bring the port into forwarding.
SW4
%LINEPROTO-5-UPDOWN: Line protocol on Interface Ethernet0/1, changed state to up RSTP(1): initializing port Et0/1 RSTP(1): Et0/1 is now designated RSTP(1): transmitting a proposal on Et0/1 RSTP(1): updt roles, received superior bpdu on Et0/1 RSTP(1): Et0/1 is now root port RSTP(1): Et0/0 blocked by re-root RSTP(1): synced Et0/1 RSTP(1): Et0/0 is now designated STP[1]: Generating TC trap for port Ethernet0/1 RSTP(1): transmitting an agreement on Et0/1 as a response to a proposal RSTP(1): transmitting a proposal on Et0/0 RSTP(1): received an agreement on Et0/0 STP[1]: Generating TC trap for port Ethernet0/0
SW4 initializes the port but then receives a better BPDU. Et0/0 is then blocked by
reroot because Et0/1 is now the root port. Et0/0 must be synchronized because it
is now a designated (downstream) port. SW3 then sends an agreement. So looking at
the topology this is what has happened so far.
Then SW3 sends agreement so that SW4 can bring Et0/0 into forwarding.
So now the question is, what happens at SW3?
RSTP(1): updt roles, received superior bpdu on Et0/1 RSTP(1): Et0/1 is now alternate
SW3 did not receive any better root information and it has no designated ports.
This means that the synchronization process can stop. Making the final topology
look like this.
What happens when synchronization fails?
RSTP synchronization is dependant on that all links in the topology are
point to point. This is decided by if the link is running in full duplex or not.
It is possible to force a link to point to point but if you are running your
interfaces in half duplex STP is not your biggest problem!
In the case that the proposal and agreement process fails RSTP has to fall
back on relying on the old timers used in regular STP. There is a timer called
FdWhile which is the same as the forward delay, defaulting to 15s. After trying to
send proposals for 15s it will start to bring the port through discarding, learning
and then to forwarding. I simulated this scenario below by blocking BPDUs between
SW3 and SW4.
20:02:23.338: RSTP(1): Et0/1 is now root port 20:02:23.338: RSTP(1): Et0/0 blocked by re-root 20:02:23.338: RSTP(1): Et0/0 is now designated 20:02:23.338: STP[1]: Generating TC trap for port Ethernet0/1 20:02:23.339: RSTP(1): transmitting a proposal on Et0/0 20:02:23.509: RSTP(1): transmitting a proposal on Et0/0 20:02:25.509: RSTP(1): transmitting a proposal on Et0/0 20:02:27.509: RSTP(1): transmitting a proposal on Et0/0 20:02:29.517: RSTP(1): transmitting a proposal on Et0/0 20:02:31.517: RSTP(1): transmitting a proposal on Et0/0 20:02:33.517: RSTP(1): transmitting a proposal on Et0/0 20:02:35.517: RSTP(1): transmitting a proposal on Et0/0 20:02:37.521: RSTP(1): transmitting a proposal on Et0/0 20:02:38.338: RSTP(1): Et0/0 fdwhile Expired
Every 2 seconds it tries to send a proposal but gets no agreement back.
After 15 seconds the timer expires and RSTP has to go through the regular
phases instead of immediately bringing the port online.
Conclusion
RSTP is a rapid protocol that works by synchronizing the topology. This process
is often overlooked in books on switching and spanning tree. This post describes
in detail how the synchronization process actually works. RSTP is a distance vector
protocol since the cost is learned by listening to BPDUs from other switches. In
some cases this can lead to issues like counting to infinity. For detail on this
refer to INE STP convergence PDF by Petr Lapukhov.
RSTP converges fast as long as the synchronization process works. This process relies
on all links running in full duplex and all switches are running in the same STP mode.
