Designated and Root Port

Unanswered Question
Oct 29th, 2007

I am confuse with the term designated and root port.

"The designated port is the port that is the single interface to forward traffic to the root bridge"

"The root port represent a switch's lowest-cost path to the root bridge"

1) Can i say all ports from root bridge port are root ports since they are the lowest-cost path to the root bridge?

2) Say Switch A is the root bridge and Switch B and C is connected through port 1 to Switch A. Can i say port 1 of Switch B and C is a designated port since it is the single interface to forward to the root bridge?

3) Can the root bridge have designated port?

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Jon Marshall Mon, 10/29/2007 - 08:28


Designated port is a port on the switch that leads AWAY from the root bridge.

Root port is a port on the switch that leads TO the root bridge.

1) All ports on root bridge are designated ports and in forwarding state

2) No - port 1 of switch B & C will be root ports.

3) Yes, all ports on root bridge are designated ports.

Edit - just to clarify. There is one designated port per segment so it does not necessarily follow that each switch will have a designated port.



kian_hong2000 Mon, 10/29/2007 - 08:47

Say Switch A is the root bridge with the lowest mac address. Switch B and C is connected to Switch A through Port 1. Switch B has the second lowest mac address. And both Switch B and C is connected through port 2.

1) Port 2 of Switch C is disable because of loop. But why Port 2 of Switch B is also call a designated port. I thought designated port only apply to root bridge only?

2) You mention "Root port is a port on the switch that leads TO the root bridge" Can i say root bridge can never have root port?

3) You mention that "there is only one designated port per segment" if that is the case, can i say that even root brdge should have only one designated port? But you say all ports on root bridge are designated ports?

Jon Marshall Mon, 10/29/2007 - 08:57

1) Designated port on a switch is the port responsible for forwarding traffic onto that segment. So You have a segment between B & C. Therefore port 2 on switch B is designated port for that segment.

2) Yes you can

3) Depends on how many segments you have. Think of it in terms of vlans. So a root bridge can be responsible for multiple vlans and will have a designated port on each vlan.



bvsnarayana03 Mon, 10/29/2007 - 11:52

Jon explained nicely.

I Remember this way:

Root port-> sends bpdu's towards root.

Designated port -> sends bpdu's away from root bridge.

Root bridge is king of topology. If it has to send messages to rest of topolgy, it'll use designated ports (send bpdu away frm root).

When non-root bridge need to communicate to root,it'll use root port(send bpdu twrds root)

This shud clarify ur 1st point.

2. ur very true about ur 2nd point

3. Since root needs to send msg to rest of topolgy, it shud have all ports as designated unless changed to portfast or disabled.

Francois Tallet Mon, 10/29/2007 - 12:16

Before I give yet another version, and because it is often said that a root bridge only has designated ports, I would like to forward this quiz;-)

A designated port is the port that sends the best BPDU on a LAN.

BPDUs can always be compared, and their fields are used in sequence to do the comparison. From the most important to the least, here are the fields included in a BPDU:

-1- Root ID

-2- Cost to the Root

-3- Sender Bridge ID

-4- Sender Port ID.

Knowing that the Root bridge has the best ID in the network, and that the cost to the root from the root is obviously 0, who could beat the BPDU sent by the root bridge?



Jon Marshall Mon, 10/29/2007 - 12:52

Hi Francois

To be honest i don't really know. If 2 ports on the root bridge are connected to each other then one would not become designated as far as i can tell but that's all i can think of.


Francois Tallet Mon, 10/29/2007 - 16:50

Hi Jon,

You are right!

I think the following defintions can help sort out *any* STP topology question, regardless of the STP mode. I tried to make them as short as possible, let me know if they still make sense:

Bridges advertise the best information (i.e. the best BPDU) they know of. This information can be collected from BPDUs received or it can be the bridge own's information.

*Root Bridge:

The bridge that has the best information in the network. The information of the root bridge cannot be beaten by any other bridge (root ID is the single most important parameter).

*Root port:

The port on which a given bridge receives the best information (the cost received in the BPDU is incremented by the port cost configured on the interface that received it for the purpose of the root port election).

*Designated port:

The port that sends the best BPDU on a given LAN segment.

*Alternate port:

A port that receiving a better information (BPDU) from another bridge *but* that is not the root port.

*Backup port:

A port that is not receivng better information from another port on the same bridge.

So, to come back to the "quiz". How could port X on the root R be not designated? It would mean that it receives better information than the one it would send out. Port X of R sends the following information:

-root ID: R, this value is the best (by definition)

-root path cost: 0, this value is the best

-sender bridge ID: R, this value is the best

-port ID: X

In the best case, a bridge can only match the first three parameters, and this unique bridge is the Root! So if the X is not designated, it can only be the result of a BPDU sent by a port Y on the root, with Y being a better port ID than X. According to the above definition, X will be a backup port. So basically, as you guessed, a Root bridge can have only two kinds of ports: designated or backup.

Note that backup is a blocking state. For some reason, there is a rule floating around that suggest that a root bridge can never block a port...




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