📘 CCNA 200-301 v1.1
2.5 Interpret basic operations of Rapid PVST+ Spanning Tree Protocol 2.5.c PortFast
2.5.c PortFast
1. What is PortFast?
PortFast is a Cisco feature used with Spanning Tree Protocol (STP) and Rapid PVST+ to speed up port transitions on access ports.
Normally, when a switch port comes up, STP checks for loops before allowing traffic to flow. This process involves port states (like listening and learning), which can take 30 seconds or more before the port starts forwarding user traffic.
PortFast bypasses these waiting states for certain ports where no loops can occur — for example, ports connected to end devices like PCs, printers, or servers, not to other switches.
So, PortFast makes the port go directly from “Blocking” to “Forwarding” state as soon as it comes up.
2. Why PortFast is Needed
Without PortFast:
- When a computer connects to a switch port, STP goes through Listening (15 sec) and Learning (15 sec) before the port forwards frames.
- During this time, the computer might try to get an IP address using DHCP, but the packets are blocked because the port is not yet forwarding.
- As a result, DHCP might fail, or there might be a delay in network access.
With PortFast:
- The port starts forwarding immediately.
- The connected device can send and receive data without waiting for STP to finish its checks.
- Network access is instant, and services like DHCP work smoothly.
3. Important Point — Only Use PortFast on Access Ports
✅ Access ports = Ports connected to end devices (like PCs, printers, IP phones).
❌ Never use PortFast on trunk ports or ports connected to other switches, routers, or bridges — because loops could form, and PortFast would skip the loop-check process.
If PortFast is accidentally enabled on a trunk link between two switches and a loop forms, it can cause a broadcast storm, which may bring down the network.
4. PortFast and Rapid PVST+
PortFast works with Rapid PVST+ (Per-VLAN Spanning Tree Plus) — the Cisco version of RSTP (Rapid Spanning Tree Protocol).
In Rapid PVST+, the protocol is already faster than traditional STP, but PortFast is still needed because:
- It instantly transitions the port to forwarding.
- It helps with end-device connectivity and reduces DHCP delay.
- It marks the port as edge port (in RSTP terminology).
In Rapid PVST+, when a port is configured with PortFast, it is treated as an edge port.
If an edge port detects a BPDU (Bridge Protocol Data Unit), it automatically loses its edge status and goes through the normal STP process to prevent loops.
This is called BPDU Guard interaction (explained below).
5. How PortFast Works Internally
When a port is configured with PortFast:
- The port is immediately placed in the Forwarding state as soon as it comes up.
- The switch does not send or expect BPDUs initially from this port.
- If the port receives a BPDU, it means another switch might be connected.
- In Rapid PVST+, the port stops being a PortFast (edge) port and transitions through normal STP states to prevent loops.
So PortFast does not “disable STP” — it just bypasses the delay when no loop risk exists.
6. PortFast Configuration
You can enable PortFast on a single interface or globally for all access ports.
a. Interface-level configuration:
Switch(config)# interface FastEthernet0/1
Switch(config-if)# spanning-tree portfast
This enables PortFast on one specific port.
b. Global configuration (for all access ports):
Switch(config)# spanning-tree portfast default
This enables PortFast automatically on all access ports, but not on trunk ports.
7. Verify PortFast
To check if PortFast is enabled:
Switch# show spanning-tree interface FastEthernet0/1 portfast
It should show:
PortFast is enabled
8. PortFast + BPDU Guard (Recommended Together)
Cisco strongly recommends using BPDU Guard along with PortFast for safety.
- PortFast makes ports transition to forwarding immediately.
- BPDU Guard protects the network by disabling the port if it ever receives a BPDU (a sign that another switch is connected).
Configuration example:
Switch(config)# interface FastEthernet0/1
Switch(config-if)# spanning-tree portfast
Switch(config-if)# spanning-tree bpduguard enable
Or enable globally:
Switch(config)# spanning-tree portfast default
Switch(config)# spanning-tree bpduguard default
9. Key Points to Remember (Exam Focus)
| Feature | Description |
|---|---|
| Purpose | Makes access ports transition immediately to forwarding state |
| Used on | Access ports (connected to end devices) |
| Do not use on | Trunk ports or switch-to-switch links |
| Prevents delay | Removes the 30-second STP delay for end devices |
| Works with | STP and Rapid PVST+ (acts as edge port) |
| If BPDU received | PortFast disables (in RPVST+) or STP process begins |
| Best practice | Use PortFast with BPDU Guard |
10. Exam Tip
For CCNA:
- Know the command syntax (
spanning-tree portfast,spanning-tree portfast default). - Know that it is used on access ports only.
- Understand it allows ports to go immediately to forwarding.
- Know it should be paired with BPDU Guard for protection.
- In Rapid PVST+, PortFast = edge port.
✅ Summary
- PortFast is a Cisco feature that lets access ports go straight to the forwarding state.
- It’s safe only for ports connected to end devices.
- It’s designed to eliminate delay in connecting users or servers to the network.
- Works with Rapid PVST+ as an edge port.
- Always use with BPDU Guard to prevent loops if misused.
