Again, I'm on the same path as you so take my perspective with the caution it deserves!
I consider the 'designated' and 'non-designated' descriptions to relate to the role that a particular port plays i.e. which port on any one link is considered to be the lowest cost to the root (and hence becomes designated) as opposed to those ports (routes) with higher costs (which become non-designated). In the literal sense, 'designated' means 'chosen to do a particular job' or 'given a particular purpose'. On any one link there can only be one designated port (i.e. one end of the link must be, or is chosen to be, closest to the root bridge).
Designated ports (along with root ports) are put into a forwarding state, although remember that this doesn't usually happen immediately - they have to go through the blocking > listening > learning > forwarding state transition steps first. Non-designated ports have no role to play and hence are put into blocking mode.
If it helps, consider root/designated/non-designated as port roles, and fowarding/blocking as port states. Every bridge has a root port, and every link has a designated port.
P.S. Try not to get too frustrated at learning STP as the elegance and effectiveness of it hides the underlying fact of the matter that graph theory (from which spanning trees, and the STP algorithm, comes from) is actually a very complex area of mathematics. Just try and get to grips with the order of events (election of a root bridge, identificiation of root ports on each bridge, identification of designated ports on each link, application of port states) coupled with the correct terminology for the process/parts and leave the underlying magic to do its bit!
One thing to keep in mind is that every collision domain must have a bridge with a designated port. The easy and simple way to look at is like this; The designated port's only responsibility is to advertise BPDU's on that segment and to accept and forward traffic from downstream neighbors.
If you have not yet watched this presentation, I highly suggest it. The presenter is clear and concise with the information being presented.
Think of a designated port as the port that you use to access each collision domain as stated above. For every port on a switch, it "could" be connected to multiple devices, like through a hub, and still connect to another switch. So every link between two switches requires one side to be forwarding at least. This ensures that everywhere in the LAN, there is a path to every device. However, there should only be one side that sends to this area as well otherwise we get duplicate frames.
Now lets assume a scenario where we have two switches connected with 2 ports to each other. You would say, why would I need one forwarding side for each link, I have two links and as long as 1 is sending, it should be fine right? Well not quite. Ethernet is a shared medium standard. So if we block both sides from one link and only use the other link, then if the blocked link is shared with a device, through a hub, or using coax cables physically sharing the same cable, then we would not be able to reach that device since the link to them are blocked.
This is why this forum is so Awesome!!!
Thanks to all the posts in this thread. Matt/Tony what great explanations; and thanks Amine for the link.
Cisco, could you please have one common area where folks can find your "wonderful" flash animations and other QLMs which weren't specifically created for CLN? We here on CLN only seem to come across these presentations when others get lucky finding them and decide to post them when responding to questions.
Just to add a little to this because I saw it was unanswered...
Designated ports exist on every segment if you have 3 switches in a triangle two connections to root would form a v. That last line is the designated connection and the bridge with the lowest BID would win if all other things were equal.
If you have multiple connections then the picture then it needs that last senders PID to determine which of the two becomes designated.
I don't like the statement that designated ports are found mainly on the root bridge. A designated port is on every lan segment so that is kind of a broad statment. If you think of Designated ports as in charge and root ports as subordinate it is a little clearer I find.
Designated ports feed root ports.
Non-Designated ports are ports that block because they are neither root, nor designated.
Hope this helps and I didn't make any mistakes that confuse you.
STP uses the Spanning Tree Algorithm (STA) to determine which switch ports on a network need to be configured for blocking to prevent loops from occurring.
After the root bridge has been determined, the STA calculates the shortest path to the root bridge. Each switch uses the STA to determine which ports to block.
There are four distinct port roles that switch ports are automatically configured for during the spanning-tree process.
1- Root Port
The root port exists on non-root bridges and is the switch port with the best path to the root bridge. Root ports forward traffic toward the root bridge
The designated port exists on root and non-root bridges.
a- For root bridges, all switch ports are designated ports.
b-For non-root bridges, a designated port is the switch port that receives and forwards frames toward the root bridge as needed. Only one designated port is allowed per segment.
The non-designated port is a switch port that is blocked, so it is not forwarding data frames and not populating the MAC address table with source addresses. A non-designated port is not a root port or a designated port.
The disabled port is a switch port that is administratively shut down.
When determining the root port on a switch, the switch compares the path costs on all switch ports participating in the spanning tree. The switch port with the lowest overall path cost to the root is automatically assigned the root port role because it is closest to the root bridge. In a network topology, all switches that are using spanning tree, except for the root bridge, have a single root port defined.
After a switch has determined which of its ports is configured in the root port role, it needs to decide which ports have the designated and non-designated roles.
1-The root bridge automatically configures all of its switch ports in the designated role
2-Other switches in the topology configure their non-root ports as designated or non-designated ports.
A Designated Port is used to forward data or you can say its in a forwarding state, a Root bridge or as i say supervisor switch has all of its ports in this state means Designated state , Non-Root Bridge switches also have designated ports but it can be not always in this state means forwarding, now non-Designated ports means ports that are blocked (they don't forward data ) to avoid loops or a broadcast storm they are redundant path used as a backup path.
Why everyone keeps mentioning that a non-designated port is a blocking port? As I see it, the word designated explains the role of the port (not state). The role for stp can either be designated or root with the latter being a non-DP but also forwarding. To make it worse I saw this question in the ICND 2 modules here in CLN:
21. Regarding STP, what is the state of a nondesignated port?
Before I answered it I knew that according to the writer the right answer would be A. Yet if you think about the states and roles the state of a non-dp can be the state of a root (role) port, which is forwarding. Makes sense?