STP, LAN segments, designated switch and ports

rjbarlow

Hi all, I'm preparating for the CCNA exam. I need help with Spanning Tree Protocol, I have some problems particularly to recognize designated switch and ports on a switched LAN.



According to the graphic that I have created, it represent a Spanning Tree after the STP algorithm have finished to work, I concluded that root port are:

fa 0/1 for switch2 and switch3

fa 0/2 for switch4-5 and 6

fa 0/1 for switch7

Based on the fact that all these ports are that go toward the root switch (switch1).

I have confused also because the word "segment" is often used in ambiguous way when it is spoken about STP, one time to refer as a single segment within two switch (therefore as a single collision domain), and one time to refer as a LAN link with multiple switch/bridges attached to. For example, one manual says that:

-"Many bridges can attach to the same Ethernet segment. The bridge with the lowest
administrative cost from itself to the root bridge, as compared with the other bridges
attached to the same segment, is placed in forwarding state. The lowest-cost bridge on
each segment is called the designated bridge, and that bridge’s interface, attached to that
segment, is called the designated port.

And also:

- "A designated switch for each LAN segment is selected. The designated switch is the one closest to the root switch through which frames are forwarded to the root."

Also:

-"The root bridge is always the designated bridge on all connected segments."

According to these definitions, for You, what are the designated switch and ports for segments that go:
from switch6 to switch1;
from switch7 to switch1;
and
from switch3 to switch1?

It is a rule for recognize designated switchs and ports on a switched LAN?

Thank You.

MrfixitRight

"A designated switch for each LAN segment is selected. The designated switch is the one closest to the root switch through which frames are forwarded to the root."

Do you mean "designated port"? As in-

A designated port is one that is active and forwarding traffic, but doesn’t lead to the root. If there are two switches that have equal-cost paths to get to the root and are connected to each other, there must be some way of resolving the topological loop that exists. The switches simply examine the bridge IDs, and whichever device has the lower bridge ID is the one that will be responsible for forwarding traffic from that segment.

And there is this:

There can be only one root bridge in any given network. The root bridge ports are called designated ports, and designated ports operate in what is called forwarding state. Forwarding state ports send and receive traffic.

If you have other switches in your network, they are called non-root bridges. However, the port that has the lowest cumulative cost to the root bridge is called a root port, and it sends and receives traffic. The cost is determined by the bandwidth of a link.


Jeeze! Now I'm confused! (From 2 diffenent CCNP books)

rjbarlow

Hi MrfixitRightl, first thanks for reply.
I'm sure that a root port is for each switch in the LAN and it is the closer interface to the root switch (selected however on bandwith)...

About designated switch and ports, the fact is that I am just uncertain on the exact meant of the term "segment", because it is used in various contexts in talking about STP....
However I think I have understand that exist a designated switch for each one segment, intended as collision domain, therefore if we consider path from switch6 to switch1, the designated switch for switch6 is switch5 and for switch5 is switch4 and for switch7 is switch5. But I am NOT sure about that. If someone can clear this argument it will be many appreciate.

ignign0kt

I think you mean designated port, not designated switch. You're right, it's better to split the image to show segments, and in this case, each area between a switch is a segment.

So: (excuse my amazing mspaint skills)


You already understand the Root Bridge election, and which switches are Non-Root bridges, and which ports are not Root Ports. So now we come to designated ports. There can only be 1 designated port per segment. So the designated port for segment 3, would be port 2 of switch 2, because that's how segment 3 would receive BPDU's from the root bridge. To make things clear: The designated port for segment 4 would be port 1 of switch 4. The designated port for segment 5 would be port 1 of switch 5.
So yeah.. the answer to your original question is that each area between 2 switches is a segment, and there are 6 segments in the image. It's just weird to look at because usually there would be other devices between the switches.
I would also like to note that STP isn't needed in this situation because there are no redundant paths.
Hope this helps

rjbarlow

Yep, I meant just "designated switch", like some manuals say:

- "Many bridges can attach to the same Ethernet segment. The bridge with the lowest administrative cost from itself to the root bridge, as compared with the other bridges attached to the same segment, is placed in forwarding state. The lowest-cost bridge on each segment is called the designated bridge (switch) and that bridge’s interface, attached to that segment, is called the designated port."

Also:

- "A designated switch for each LAN segment is selected. The designated switch is the one closest to the root switch through which frames are forwarded to the root."

Howevwer I'm happy because I am understanding the stp, then I had said that the graph represents a spanning tree after stp algorithm already has worked therefore redundant links are not shown.

Thanks man.

Cheers