In this video series, I'll cover Advanced Spanning Tree Features - specifically PortFast, BPDU Guard, and BPDU Filter. This will be based on a blog post that I wrote 5 years and it is still published on the VIP Perspectives blog on Cisco Learning Network. (See: https://learningnetwork.cisco.com/s/blogs/a0D3i000002SKQwEAO/advanced-stp-features-portfast-bpdu-guard-and-bpdu-filter)
Let's start with PortFast!
The PortFast feature allows the port to enter the forwarding state immediately, bypassing the listening and learning states. PortFast is used to minimize the time a port must wait for STP to converge. This is the first and most obvious improvement that PortFast brings to legacy STP operation. Second, The switch never generates a TCN when the link status of a PortFast enabled port changes.
There is much misinformation about the operational details of PortFast circulating throughout the Internet. One of the most common misconceptions is that PortFast effectively disables STP, and BPDUs are no longer sent or received on the port. Everything about this statement is wrong. Not only does a PortFast-enabled port transmit BPDUs, but the operational PortFast status is actually dependent on incoming BPDUs. If the port receives BPDUs, the PortFast feature is disabled.
It is important to understand the difference between the administrative and operational state of PortFast. The administrative state refers to what is configured on the device, and the operational state defines whether the feature is actually enabled or disabled.
There are basically two ways to enable PortFast: globally (spanning-tree portfast default) or per interface (spanning-tree portfast). Both commands enable PortFast on operational access ports.
There is an additional interface-level command (spanning-tree portfast trunk) which enables PortFast on trunk ports. NOTE: This command enables PortFast on *both* access and trunk ports. There is no configuration command to enable PortFast *only* on operational trunk ports.
As a summary, here are the most important take-aways from this video:
- PortFast immediately moves the port into the forwarding state, bypassing listening
- and learning states.
- Enabling PortFast does NOT disable STP.
- A PortFast enabled port will continue to transmit BPDUs.
- If a BPDU is received, PortFast is disabled.
- The switch never generates a TCN when a PortFast enabled port goes up or down.
- There is a difference between administrative and operational state.
- The spanning-tree portfast trunk command enables PortFast on both access and trunk ports.