that it's advertising on Gi1/1. SW2 will thus select Gi1/0 as the root port. But suppose that you wanted Gi1/1 to be the root port instead. Although there are a few ways to do this, I recommend one of the following:
Decrease the port cost of Gi1/1 on SW2
Decrease the port priority of Gi1/1 on SW4
Alternatively, you could increase the port cost or priority, but it's best not to. If you were to add another connection between the switches later—say using a lower port number like Gi0/3—it would then become the root port. By decreasing the port cost or priority, you can ensure that your desired port always remains the root port.
Modifying Port Cost
To get SW2 to use Gi1/1 as the root port, you can decrease the port cost from 4 to 2:
SW2(config)#int gigabitEthernet 1/1 SW2(config-if)#spanning-tree vlan 1 cost ? <1-200000000> Change an interface's per VLAN spanning tree path cost SW2(config-if)#spanning-tree vlan 1 cost 2 SW2(config-if)#do show spanning-tree vlan 1 VLAN0001 Spanning tree enabled protocol rstp Root ID Priority 32769 Address 0015.fa83.e900 Cost 6 Port 6 (GigabitEthernet1/1) Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Bridge ID Priority 32769 (priority 32768 sys-id-ext 1) Address 0015.fa88.4e80 Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Aging Time 300 sec Interface Role Sts Cost Prio.Nbr Type ------------------- ---- --- --------- -------- -------------------------------- Gi0/0 Altn BLK 4 128.1 P2p Gi0/1 Altn BLK 4 128.2 P2p Gi0/2 Desg BLK 4 128.3 P2p Gi0/3 Desg BLK 4 128.4 P2p Gi1/0 Altn BLK 4 128.5 P2p Gi1/1 Root FWD 2 128.6 P2p ! Output truncated
Gi1/1 is now the root port. Let's change it back to 4:
SW2(config-if)#spanning-tree vlan 1 cost 4 SW2(config-if)#do show spanning-tree vlan 1 VLAN0001 Spanning tree enabled protocol rstp Root ID Priority 32769 Address 0015.fa83.e900 Cost 8 Port 5 (GigabitEthernet1/0) Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Bridge ID Priority 32769 (priority 32768 sys-id-ext 1) Address 0015.fa88.4e80 Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Aging Time 300 sec Interface Role Sts Cost Prio.Nbr Type ------------------- ---- --- --------- -------- -------------------------------- Gi0/0 Altn BLK 4 128.1 P2p Gi0/1 Altn BLK 4 128.2 P2p Gi0/2 Desg BLK 4 128.3 P2p Gi0/3 Desg BLK 4 128.4 P2p Gi1/0 Root FWD 4 128.5 P2p Gi1/1 Altn BLK 4 128.6 P2p ! Output truncated
Gi1/0 is once again the root port.
Modifying Port Priority
You can change the port priority for Gi1/1 on SW4 as follows:
SW4(config)#int gigabitEthernet 1/1 SW4(config-if)#spanning-tree vlan 1 port-priority ? <0-224> port priority in increments of 32 SW4(config-if)#spanning-tree vlan 1 port-priority 64
On SW2, you can see the designated port ID for Gi1/1 is now 64.6, and it's the root port:
SW2#show spanning-tree vlan 1 interface gi1/1 detail Port 6 (GigabitEthernet1/1) of VLAN0001 is root forwarding Port path cost 4, Port priority 128, Port Identifier 128.6. Designated root has priority 32769, address 0015.fa83.e900 Designated bridge has priority 32769, address 001d.45cf.e800 Designated port id is 64.6, designated path cost 4 Timers: message age 15, forward delay 0, hold 0 Number of transitions to forwarding state: 3 Link type is shared by default BPDU: sent 43, received 1851
The converged topology is shown in Figure 2.6.
Figure 2.6 Converged VLAN 1 topology with SW2 Gi1/0 blocking
Calculating Blocked Ports
Now we come to perhaps the easiest part of Spanning Tree: determining which remaining ports to block. Consider the connections between SW1 and SW4. Both are directly connected to the root, SW3, and have no need of a link to each other. Because SW1 has the higher bridge priority, it will block both its ports, like so:
SW1#show spanning-tree vlan 1 | i Gi2/0|Gi2/1 Gi2/0 Altn BLK 4 128.9 P2p Gi2/1 Altn BLK 4 128.10 P2p
Blocked ports don't forward traffic, but they still receive and process BPDUs. This ensures that every bridge can learn about changes to the Spanning Tree topology. SW4 has a lower bridge priority and has both its ports forwarding:
SW4#show spanning-tree vlan 1 | i Gi2/0|Gi2/1 Gi2/0 Desg FWD 4 128.9 P2p Gi2/1 Desg FWD 4 128.10 P2p
Any non-root ports that aren't blocking are called designated ports. The job of designated ports is to forward BPDUs from the root bridge so that Spanning Tree can reconverge in the event of a topology change.
Port States
In RSTP, a port can have one of three states: learning, forwarding, or discarding. A port eventually settles into either a forwarding or discarding state. The learning state is transitory and only occurs when RSTP is in the process of determining which ports to block.
Discarding—The port processes incoming BPDUs but doesn't send them. IOS lists a port in the discarding state as BLK or blocking. This is the initial state for all ports.
Learning—The port sends and receives BPDUs but doesn't forward data traffic. The switch looks at ingressing Ethernet frames and adds the source MAC addresses to its MAC address table.
Forwarding—Sends and receives BPDUs, and it also passes normal user and control plane traffic.
Port Roles
In addition to the root and designated port roles, RSTP has two additional port roles you need to know: alternate and backup.
Alternate—An alternate port provides an alternate path to the root. If a root port fails, the switch will place the best (lowest cost) alternate port into a forwarding state. This lets the topology reconverge in a matter of milliseconds.
Backup—You're unlikely to ever see a backup port role, except on an exam. Imagine that two ports are connected to a hub and hence are on the same segment. The port with the lowest cost—or if the costs are equal, the port with the lowest designated port priority—will be the designated port. The other port connected to the segment will be the backup.
Link Types
One way that RSTP achieves a fast convergence time is by avoiding putting some ports through the three-step process of discarding, learning, and forwarding. The idea is that by predefining ports that are directly connected to another switch or to an edge device, RSTP can more quickly figure out which ports to block and which to transition to the forwarding state. RSTP defines three link types:
Point-to-point (P2P)
P2P Edge
Shared
Point-to-Point (P2P)
The P2P link type
