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.

      Port Roles

      In addition to the root and designated port roles, RSTP has two additional port roles you need to know: alternate and 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)

      The P2P link type indicates that the link is a link between only two switches. Thus, each switch knows that it needs to negotiate with only one switch on the other end of the link. RSTP will automatically set the P2P link type on any full-duplex link. You can manually set the P2P link type using the interface command spanning-tree link-type point-to-point.

P2P Edge

      This type is reserved for edge devices that don't run Spanning Tree, such as computers, servers, and phones. An edge port skips over the learning state and transitions instantly to a forwarding state. RSTP still sends BPDUs on edge ports just in case there's a switch on the other end. But if it receives a BPDU, it will change the port type to a normal Spanning Tree port. If you're familiar with PortFast, this is essentially the same thing. In fact, IOS uses the portfast keyword to configure an edge port, as follows:

      SW2(config)#int gi1/3 SW2(config-if)#spanning-tree portfast edge ? trunk Enable portfast edge on the interface even in trunk mode <cr> SW2(config-if)#spanning-tree portfast edge trunk %Warning: portfast should only be enabled on ports connected to a single host. Connecting hubs, concentrators, switches, bridges, etc... to this interface when portfast is enabled, can cause temporary bridging loops. Use with CAUTION

      Keep in mind that an edge port can be an access port or a trunk port that's connected to multiple devices. If the port is a single-VLAN access port, you should use the spanning-tree portfast edge interface command. But if it's a trunked interface—such as one connected to a server running virtual machines or a computer plugged in behind an IP phone on a separate voice VLAN—you must use the spanning-tree portfast edge trunk interface command.

      Shared

      The Shared port type is one you shouldn't expect to see very often. It's for a port that may be connected to multiple devices, usually via a hub. RSTP considers any half-duplex link to be shared. In this case, RSTP falls back to timer-based 802.1D behavior and may take up to 50 seconds to converge.