Cisco Support Community
cancel
Showing results for 
Search instead for 
Did you mean: 
Announcements

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. If you'd prefer to explore, try our test area to get started. And see here for current known issues.

New Member

STP: forward-delay-time

Hello

In STP for the first 20s (max-age) root bridge elections occur. In the same moment in which switch receives last BPDU which allow him to decide who is root bridge it also knows which are designated/non-designated ports. In this moment whole topology is converged. For all this swith have 20s. Why do we need additional 2*forward delay time(2*15s) to start forwarding packets ? Why do we need "listening phase" ? Why switch could not start forwarding/learning phase directly after topology converge ?

Thanx

  • LAN Switching and Routing
1 ACCEPTED SOLUTION

Accepted Solutions
Hall of Fame Super Silver

Re: STP: forward-delay-time

Hello Maciej,

each phase needs to stay for at least the propagation time in a 7 hops network.

Because in STP 802.1D BPDUs are sent every hello interval 2 seconds it comes 15 seconds.

7*2 +1

The objective is to prevent bridging loop.

So the two phases:

listening

learning

and the port moves from one state to another after 15 seconds.

Two switches could take a wrong decision in a lan segment because not aware of another change/decision taken in another lan segment.

Hope to help

Giuseppe

3 REPLIES
Blue
New Member

Re: STP: forward-delay-time

Thanx, but there is no answer for my question.

If topology is created durring blocking phase (or maybe i am wrong and it is not created durring blocking phase?) .

Why do we need to wait 30s more ? I now understeand that changing many port's status might create loops but why to wait 30 seconds more, why this value ? - why not 5s or 180s ?

Hall of Fame Super Silver

Re: STP: forward-delay-time

Hello Maciej,

each phase needs to stay for at least the propagation time in a 7 hops network.

Because in STP 802.1D BPDUs are sent every hello interval 2 seconds it comes 15 seconds.

7*2 +1

The objective is to prevent bridging loop.

So the two phases:

listening

learning

and the port moves from one state to another after 15 seconds.

Two switches could take a wrong decision in a lan segment because not aware of another change/decision taken in another lan segment.

Hope to help

Giuseppe

2701
Views
0
Helpful
3
Replies
This widget could not be displayed.