i emulated your setup in the lab and got the following results Firstly SW3 is equivalent to your Switch C. SW3# sh spanning-tree vlan 30 VLAN0030 Spanning tree enabled protocol ieee Root ID Priority 24606 Address 0017.e066.0480 Cost 23 Port 23 (FastEthernet0/21) Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Bridge ID Priority 32798 (priority 32768 sys-id-ext 30) Address 000a.b8b3.1580 Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Aging Time 300 Interface Role Sts Cost Prio.Nbr Type ---------------- ---- --- --------- -------- -------------------------------- Fa0/21 Root FWD 19 128.23 P2p Fa0/23 Altn BLK 1000 128.25 P2p fa0/21 is the RP which connects to SW2 ( your equivalent is Switch B) fa0/23 is the blocked port that connects to SW1 (your Switch A) which is the root bridge. fa0/19 is the port that connects to SW4 (your switch D) and is in a shutdown state. I had to manipulate some of the port costs to get the right links to block etc. so don't worry too much about the costs in the outputs. Then i brought up int fa0/19 on SW3. 3w4d: %LINK-3-UPDOWN: Interface FastEthernet0/19, changed state to up sh spanning-tree vlan 30 VLAN0030 Spanning tree enabled protocol ieee Root ID Priority 24606 Address 0017.e066.0480 Cost 23 Port 23 (FastEthernet0/21) Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Bridge ID Priority 32798 (priority 32768 sys-id-ext 30) Address 000a.b8b3.1580 Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Aging Time 300 Interface Role Sts Cost 3w4d: set portid: VLAN0001 Fa0/19: new port id 8015 3w4d: STP: VLAN0001 Fa0/19 -> listening 3w4d: set portid: VLAN00 Prio.Nbr Type ---------------- ---- --- --------- -------- -------------------------------- Fa0/19 Desg LIS 4 128.21 P2p Fa0/21 Root FWD 19 128.23 P2p Fa0/23 Altn BLK 1000 128.25 P2p SW3#10 Fa0/19: new port id 8015 3w4d: STP: VLAN0010 Fa0/19 -> listening 3w4d: set portid: VLAN0011 Fa0/19: new port id 8015 3w4d: STP: VLAN0011 Fa0/19 -> listening 3w4d: set portid: VLAN0020 Fa0/19: new port id 8015 3w4d: STP: VLAN0020 Fa0/19 -> listening 3w4d: set portid: VLAN0021 Fa0/19: new port id 8015 3w4d: STP: VLAN0021 Fa0/19 -> listening 3w4d: set portid: VLAN0030 Fa0/19: new port id 8015 3w4d: STP: VLAN0030 Fa0/19 -> listening 3w4d: set portid: VLAN0031 Fa0/19: new port id 8015 3w4d: STP: VLAN0031 Fa0/19 -> listening 3w4d: set portid: VLAN0999 Fa0/19: new port id 8015 3w4d: STP: VLAN0999 Fa0/19 -> listening As you can see fa0/19 starts listening and the RP is still forwarding - however almost immediately 3w4d: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/19, changed state to up 3w4d: STP: VLAN0001 new root port Fa0/19, cost 8 3w4d: STP: VLAN0001 sent Topology Change Notice on Fa0/19 3w4d: STP: VLAN0001 Fa0/21 -> blocking 3w4d: STP: VLAN0010 new root port Fa0/19, cost 8 3w4d: STP: VLAN0010 sent Topology Change Notice on Fa0/19 3w4d: STP: VLAN0010 Fa0/21 -> blockinsh spanning-tree vlan 30 g 3w4d: STP: VLAN0011 new root port Fa0/19, cost 8 3w4d: STP: VLAN0011 sent Topology Change Notice on Fa0/19 3w4d: STP: VLAN0011 Fa0/21 -> blocking 3w4d: STP: VLAN0020 new root port Fa0/19, cost 8 3w4d: STP: VLAN0020 sent Topology Change Notice on Fa0/19 3w4d: STP: VLAN0020 Fa0/21 -> blocking 3w4d: STP: VLAN0021 new root port Fa0/19, cost 8 3w4d: STP: VLAN0021 sent Topology Change Notice on Fa0/19 3w4d: STP: VLAN0021 Fa0/21 -> blocking 3w4d: STP: VLAN0030 new root port Fa0/19, cost 8 As soon as the TCN's are sent the fa0/21 interface which was the root port is blocked. sh spanning-tree vlan 30 VLAN0030 Spanning tree enabled protocol ieee Root ID Priority 24606 Address 0017.e066.0480 Cost 8 Port 21 (FastEthernet0/19) Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Bridge ID Priority 32798 (priority 32768 sys-id-ext 30) Address 000a.b8b3.1580 Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Aging Time 15 Interface Role Sts Cost Prio.Nbr Type ---------------- ---- --- --------- -------- -------------------------------- Fa0/19 Root LIS 4 128.21 P2p Fa0/21 Altn BLK 19 128.23 P2p Fa0/23 Altn BLK 1000 128.25 P2p Port fa0/19 then goes into learining mode SW3# 3w4d: STP: VLAN0001 Fa0/19 -> learning 3w4d: STP: VLAN0010 Fa0/19 -> learning 3w4d: STP: VLAN0011 Fa0/19 -> learning 3w4d: STP: VLAN0020 Fa0/19 -> learning 3w4d: STP: VLAN0021 Fa0/19 -> learning 3w4d: STP: VLAN0030 Fa0/19 -> learning 3w4d: STP: VLAN0031 Fa0/19 -> learning 3w4d: STP: VLAN0999 Fa0/19 -> learning sh spanning-tree vlan 30 VLAN0030 Spanning tree enabled protocol ieee Root ID Priority 24606 Address 0017.e066.0480 Cost 8 Port 21 (FastEthernet0/19) Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Bridge ID Priority 32798 (priority 32768 sys-id-ext 30) Address 000a.b8b3.1580 Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Aging Time 15 Interface Role Sts Cost Prio.Nbr Type ---------------- ---- --- --------- -------- -------------------------------- Fa0/19 Root LRN 4 128.21 P2p Fa0/21 Altn BLK 19 128.23 P2p Fa0/23 Altn BLK 1000 128.25 P2p And finally it goes into forwarding 3w4d: STP: VLAN0001 sent Topology Change Notice on Fa0/19 3w4d: STP: VLAN0001 Fa0/19 -> forwarding 3w4d: STP: VLAN0010 Fa0/19 -> forwarding 3w4d: STP: VLAN0011 Fa0/19 -> forwarding 3w4d: STP: VLAN0020 Fa0/19 -> forwarding 3w4d: STP: VLAN0021 Fa0/19 -> forwarding 3w4d: STP: VLAN0030 Fa0/19 -> forwarding 3w4d: STP: VLAN0031 Fa0/19 -> forwarding 3w4d: STP: VLAN0999 Fa0/19 -> forwarding sh spanning-tree vlan 30 VLAN0030 Spanning tree enabled protocol ieee Root ID Priority 24606 Address 0017.e066.0480 Cost 8 Port 21 (FastEthernet0/19) Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Bridge ID Priority 32798 (priority 32768 sys-id-ext 30) Address 000a.b8b3.1580 Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Aging Time 15 Interface Role Sts Cost Prio.Nbr Type ---------------- ---- --- --------- -------- -------------------------------- Fa0/19 Root FWD 4 128.21 P2p Fa0/21 Altn BLK 19 128.23 P2p Fa0/23 Altn BLK 1000 128.25 P2p So your gut feeling is correct :), the RP does get blocked on SwitchC while the 1/3 port goes through the listening/learning/forwarding process. HTH Jon