Home » Cisco » 300-115 » What is the approximate amount of time it takes for a PortFast-enabled port to transition from blocking to forwarding?
What is the approximate amount of time it takes for a PortFast-enabled port to transition from blocking to forwarding?
A. Immediately
B. 15 seconds
C. 20 seconds
D. 30 seconds
E. 50 seconds
Correct Answer: A
Explanation/Reference:
Answer:
Instead of waiting for STP to cycle through the blocking, learning, and listening states, PortFast will place the port in the forwarding state immediately.
When PortFast is enabled on a port, the attached end station can join the network almost immediately rather than waiting up to 50 seconds for spanning tree to converge. This feature is designed to enable the connections to workstations and servers to be put into the forwarding state as soon as possible after a spanning-tree reconvergence.
Bypassing the listening and learning states creates an exposure for spanning-tree loops. The default behavior of a PortFast-enabled port is to put the port immediately into a blocking state if a BPDU is received.
The following command enables PortFast: switch(config-if)# spanning-tree portfast
You should only enable PortFast on a port that connects an end station. Enabling PortFast on a port that connects another switch could create a loop.
Objective:
Layer 2 Technologies Sub-Objective:
Configure and verify spanning tree
References:
Catalyst 3750 Switch Software Configuration Guide, 12.2(40)SE > Configuring Optional Spanning-Tree Features > Understanding Port Fast
