The command spanning-tree vlan forward-time is used to configure the Forward Delay timer for a specific VLAN. In Spanning Tree Protocol (STP) and its variants like Rapid PVST+, the Forward Delay timer dictates the duration a port spends in the listening state and subsequently in the learning state before it can transition to the forwarding state. Each of these states (listening and learning) lasts for the period defined by the forward-time value. Therefore, this command allows an administrator to set the specific time period for these states.

Cisco IOS LAN Switching Command Reference - Spanning-Tree Commands: spanning-tree

[vlan vlan-list] forward-time seconds

"Use the spanning-tree forward-time command in global configuration mode or the

spanning-tree vlan forward-time command in interface configuration mode to set the time

that a Layer 2 port waits before it changes from the listening and learning states to the

forwarding state."

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Source: Cisco. (Specific document version varies, e.g., Catalyst 9300 Series Switches, IOS

XE Bengaluru 17.6.x). A general reference can be found in Cisco IOS command references

for switches. Example: Cisco Catalyst 3750 Switch Software Configuration Guide,

12.2(25)SEE - "Configuring STP" section, "STP Timers".

CCNA 200-301 Official Cert Guide, Volume 1 by Wendell Odom (Cisco Press):

Chapter 3: "Spanning Tree Protocol Concepts" discusses STP timers. "Forward Delay: The

number of seconds an interface stays in the listening state, and then the number of seconds

an interface stays in the learning state. Default: 15 seconds."

Source: Odom, W. (2019). CCNA 200-301 Official Cert Guide, Volume 1. Cisco Press.

(Chapter on STP).

Cisco documentation on "Understanding and Configuring Spanning Tree Protocol (STP) on

Catalyst Switches":

This document typically explains the STP timers: "Forward DelayThe time that is spent in

the listening and learning state. The default is 15 seconds."

Source: Cisco Systems, "Understanding and Configuring Spanning Tree Protocol (STP) on

Catalyst Switches" (available on Cisco.com, specific document ID may vary by switch

model and IOS version).