STP: Root port vs. designated port

[Kevin Melton]

Forum

I am having difficulty understanding the difference between the root port and a designated port.  Please read the following definitions from Todd Lammle's CCNA study guide...

Root port - The root port is always the link connected to the root bridge, or the shortest path to the root bridge.  If more than one link connects to the root bridge, then a port cost is determined by checking the bandwidth of each link.  The lowest cost port becomes the root port.  if multiple links have the same cost, the bridge withe the lower advertising Bridge ID is used.  Since multiple links can be from the same device, the lowest port number will be used.

Designated Port - a designated port is one that has been determined as having the best (lowest)cost.  A designated port will be a forwarding port.

I am having some difficulty understanding this, and wanted the perspective of someone who may be able to expain the difference between the two port types.  To me it sounds like the Root Port and the Designated Port are the same...

Why are they not the same, and what are the differences between each?

Thanks

Kevin

[Grayson Wells]

Well, it waits to see if it receives a superior BPDU, yes. But a switch will expect to receive BPDU's on a designated port because it knows it is connected to a switch. If it receives BPDU's on a normal edge port in forwarding mode then it will shut the port down with BPDU Guard or filter it if you are using filter. The difference there is that forwarding interfaces are edge ports and designated ports are not edge ports. The concept of an edge port is important because in other versions of code (NX-OS for example), you actually say spanning-tree port type edge instead of using the portfast command. Also, the vast majority of time that a switchport shutdowns due to bpduguard it does so because some local tech decided to loop the port so that the switch is plugged into itself. It is rare, in my experience, for someone to plug in a rogue switch other than a residential switch that doesn't understand spanning-tree.

[opinder_malhotra]

Hi echastoESL : Thanks for clarifying as this makes sense and confirms to what I was thinking. The way I understand this is :

Root port from the access switch connects to a designated port on the distribution switch.

All access ports other than uplinks are designated ports because they are forwarding the traffic downlinking to the hosts

I hope my understanding is correct.

[CiscoLadder]

Another way to remember is;
> Root Ports face towards to Root Switch

> Designated Ports face away from the Root Switch

> If one end of a segment is a root port, then the other end is a designated port. In other words: (1) Root Port and (1) Designated Port ‘per segment’...

Additionally, all ports in the Root Switch are Designated Ports...

[Jon Marshall]

k-melton wrote:
Jon
Is it correct then to assume that Access ports cannot be Designated Ports?... 
Thanks for your detailed response as it helps me undertand this better.
Kevin

Kevin

Robert has summed it up nicely.

Just to clarify, it depends what you mean by access port. If you mean a port configured as "switchport mode access" as opposed to "switchport mode trunk" then an access port could be a DP if it was connected to another switch on that port.

If you mean access in terms of end devices then see Roberts response.

Jon

[Kevin Melton]

Jon

I am unsure why I do not see or cannot view Roberts response.

At my clients, I always use portfast on access ports.  But I do hear what you are saying.  So we could have a DP on a switch that is an access port connected to another switch.  Alas, but wouldnt we want to trunk at that point??  Perhaps there are examples why we would not trunk in that situation.

Thx again

Kevin

[Jon Marshall]

Kevin

I am unsure why I do not see or cannot view Roberts response.

That's weird because it has been marked with a star as answering the question and only you can do that so i assumed you had seen it ?

In answer to your point, you should use portfast on ports connected to PCs etc. i just wanted to point out that a port configured as an access port ie. only in one vlan could still be connected to a switch.

Edit - your point about trunks. You are right, it is far more common to use trunks to connect switches. If you have a separate switch management vlan then you have to use a trunk as you have that vlan + at least one user vlan. But if you only had one vlan shared between the switches, you could if you wanted only use an access port.

Anyway here is Robert's response -

Access ports are probably going to be in the forwarding state, and in reality you should probably have the spanning-tree portfast command on those ports so that they do not cycle through the stages of STP, better for them to go straight to forwarding. The designated port should never be an access port. It is a port that is basically a downlink to another switch.

If you are looking at the switch that is acting as the root bridge for the VLAN then it will show all the ports as designated ports because they are the downlinks to the rest of the network. If you are in a switch further down the spanning tree it will have a port called the root port, which is the pathway to the root bridge. So if you have a port in the root port state, it is understood that it is connected to a port that is in a designated port state. This is the case because the root port is the uplink towards the root, and the designated port is the downlink to a lower portion of the spanning tree. Hope that clears things up.

Jon

[mohammedafroz303]

Hello Sir,

Please clear my doubt ?

[CiscoLadder]

I know this is really old, but does anyone recall who made the video ? attached by ‘LAMAV’ ?

I love this guys teaching style. I’m studying for my CCENT>>>CCNA and I’d love to follow this guy !

The filename that was attached is:
77267-BCMSN05 - STP Part I.wmv.zip
77267-BCMSN05 - STP Part I.wmv

https://community.cisco.com/kxiwq67737/attachments/kxiwq67737/6016-discussions-lan-switching-routing/149449/1/77267-BCMSN05%20-%20STP%20Part%20I.wmv.zip

Here’s a link to the original post...
https://community.cisco.com/t5/switching/stp-root-port-vs-designated-port/td-p/1517842

I can be reached here’s as:
ciscoladder
or preferably direct to:
ciscoladder@gmail.com

Man, if you could help me find out who this is, I’ll owe you !

Best Regards,
Dave

[ADWAITANANDADAS]

Hi Folks, I am a late comer. But I believe this cane be a better explanation;

Spanning-Tree is a structural algorithm that does NOT have a cycle/LOOP. It draws a conclusion that connects every bridge with MINIMUM possible number of uplinks.
To achieve such connections between the bridges we can draw several spanning-tree. For Example with 3 switches: A-B-C, B-C-A and C-A-B are the three possible spanning-trees.(Follow the diagram below)

So Fundamentally, Root Bridge is elected to arrange the spanning-Tree with a particular order.
Root Port election ensure the minimum connection and Designated port election ensure the a loop free structure. (One of the DPs in a Segment has to be blocked). Rest as the theory says both election follow the same criteria.

All of you are welcome to correct where I am wrong.


Cheers
Adwaita