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Home > Archive > CCNA > December 2000 > Blocking States & Redundant Links
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Blocking States & Redundant Links
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| Hey all.
I'm studying up on Spanning Tree Protocol...I'm not understanding why redundant links would exist if only one switch port is the designated forwarding port.
Thanks! | |
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Originally posted by firechicken:
Hey all.
I'm studying up on Spanning Tree Protocol...I'm not understanding why redundant links would exist if only one switch port is the designated forwarding port.
Thanks!
Redundant links may be introduced into the network design by purpose for fail-over safety. As you probably allready understand from your reading, redundant links causes tremendous trouble. That's where STP comes into play to help us. It makes sure only one of the redundant links are operational at a time. Make sure you understand how the algorithm selects which one to be operational (forwarding). If a switch or port fails, STP will discover this and try to build a new spanning tree using previously blocked port(s) as forwarding ports.
Did that help?
Terje | |
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| Don't confuse "redundant" with "load sharing". Think of redundant as a backup.
Try this: Draw two boxes representing switches with two lines connecting the two boxes. See how traffic could circle thru the two switches? This would be a loop, so Spanning tree shuts down one of those lines breaking the loop. In the event the active line fails, SPT will recalculate and activate the previously blocked port bringing your network back up, thus the term redundant.
Hope that helps,
Yankee | |
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| Hi,
Not too sure if I get the switching portiong right.
To mine understanding, that there is only one designated root port per segment. So if 1 segment is linked to a switch with 2 ports, only 1 is designated root and another would be in blocking state.
In the event that the designated root port fails (e.g fibre optic problems) and the "blocked" port does not hear the BPDU for a period, it will start to go into the listening state and learning and forwarding, providing that there is no contention for for other ports to be designated root port.
Debarros | |
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| Thankyou for clearing up my misconceptions! I read this post and went back to take a look in my books.
The key point that I missed in my textbooks was that a port which is in blocking state still listens for BPDU's.
The way I was thinking was that once a port goes into blocking state, it would be blocking everything.
Thanks again for your time! |
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