Don't RSTP and MST use the same algorithm?

My understanding is that both RSTP and MST use the same synchronization process through proposal and agreements and that regardless of who is the root, every switch sends its own BPDUs because these are now used as keep-alives instead.

After running some tests, I decided to look at sent/received BPDUs between SW3 and SW4 running RSTP and between SW1 and SW2 running MST. What I found out is that in the RSTP pair, only the root is sending BPDUs. How come?

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RSTP

SW3----E1/0----SW4

SW4(config)#do sh spann

VLAN0001
Spanning tree enabled protocol rstp
Root ID Priority 32769
Address aabb.cc00.0100
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address aabb.cc00.0100
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec

Interface Role Sts Cost Prio.Nbr Type


Et1/0 Desg FWD 100 128.5 P2p

SW4(config)#do sh spann vlan 1 int e1/0 det | i BPDU
BPDU: sent 42, received 91
SW4(config)#do sh spann vlan 1 int e1/0 det | i BPDU
BPDU: sent 55, received 91
SW4(config)#do sh spann vlan 1 int e1/0 det | i BPDU
BPDU: sent 56, received 91
SW4(config)#do sh spann vlan 1 int e1/0 det | i BPDU
BPDU: sent 57, received 91
SW4(config)#do sh spann vlan 1 int e1/0 det | i BPDU
BPDU: sent 58, received 91

SW3(config)#do sh spann

VLAN0001
Spanning tree enabled protocol rstp
Root ID Priority 32769
Address aabb.cc00.0100
Cost 100
Port 5 (Ethernet1/0)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address aabb.cc00.0200
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec

Interface Role Sts Cost Prio.Nbr Type


Et1/0 Root FWD 100 128.5 P2p

SW3(config)#do sh spann vlan 1 int e1/0 det | i BPDU
BPDU: sent 9, received 45
SW3(config)#
SW3(config)#
SW3(config)#do sh spann vlan 1 int e1/0 det | i BPDU
BPDU: sent 9, received 46
SW3(config)#
SW3(config)#
SW3(config)#do sh spann vlan 1 int e1/0 det | i BPDU
BPDU: sent 9, received 47
SW3(config)#
SW3(config)#
SW3(config)#do sh spann vlan 1 int e1/0 det | i BPDU
BPDU: sent 9, received 48
SW3(config)#
SW3(config)#
SW3(config)#do sh spann vlan 1 int e1/0 det | i BPDU
BPDU: sent 9, received 51
SW3(config)#

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MST

SW1----e1/0----SW2

SW1(config)#do sh spann mst 0

MST0 vlans mapped: 1-1500

Bridge address aabb.cc00.0300 priority 4096 (4096 sysid 0)
Root this switch for the CIST
Operational hello time 2 , forward delay 15, max age 20, txholdcount 6
Configured hello time 2 , forward delay 15, max age 20, max hops 20

Interface Role Sts Cost Prio.Nbr Type


Et1/0 Desg FWD 2000000 128.5 P2p

SW1(config)#do sh spann mst 0 int e1/0 det | i Bpdu
Bpdus sent 49, received 48
SW1(config)#do sh spann mst 0 int e1/0 det | i Bpdu
Bpdus sent 52, received 51
SW1(config)#do sh spann mst 0 int e1/0 det | i Bpdu
Bpdus sent 54, received 53

SW2(config)#do sh spann mst 0

MST0 vlans mapped: 1-1500

Bridge address aabb.cc00.0400 priority 32768 (32768 sysid 0)
Root address aabb.cc00.0300 priority 4096 (4096 sysid 0)
port Et1/0 path cost 0
Regional Root address aabb.cc00.0300 priority 4096 (4096 sysid 0)
internal cost 2000000 rem hops 19
Operational hello time 2 , forward delay 15, max age 20, txholdcount 6
Configured hello time 2 , forward delay 15, max age 20, max hops 20

Interface Role Sts Cost Prio.Nbr Type


Et1/0 Root FWD 2000000 128.5 P2p

SW2(config)#do sh spann mst 0 int e1/0 det | i Bpdu
Bpdus sent 1057, received 1163
SW2(config)#do sh spann mst 0 int e1/0 det | i Bpdu
Bpdus sent 1058, received 1165
SW2(config)#do sh spann mst 0 int e1/0 det | i Bpdu
Bpdus sent 1059, received 1166

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