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## Root port vs Designated port?

To my understanding, the designated port becomes the forwarding port, which means that its forwards frames. Root port has the lowest bridge ID and it also forwards frames...does this mean that the root port is the designated port? what is the difference? When is there a root port and a designated port on the same switch?

3 REPLIES
VIP Purple

## Re: Root port vs Designated port?

Hello,

it might be easiest to understand the difference when you look at it from the perspective of the root switch: the designated port is the port leading away from the root bridge (that is why all forwarding ports on the root are designated ports), while the root port is the forwarding port on the non-root switch pointing towards the root.

Does that make sense ? Here is an extended explanation from KnowledgeNet:

Q:What is the difference between the root port and the designated port on a non-root bridge router. I know how the designated port is determined, but just what does it do?

A:To understand the differences, you must understand the concepts of the root bridge and non-root bridges. Remember that STP creates a logical layer 2 tree of your bridged network. At the bottom of the tree is the root bridge and every other bridge has only one (active) path to the root bridge. To create this tree, STP places ports in one of two states: blocking or forwarding. Determining which port should be blocking and which port should be forwarding can be done automatically by the STP algorithm or you can give it a little help my modifying some attributes such as bridge priority, port cost, port priority, etc. Assuming the root bridge has been selected, each port on the root bridge becomes a designated port and therefore a forwarding port. All other bridges are now considered non-root bridges. To create the tree throughout the topology, STP will go through two different types of elections: root port elections and designated port elections. Every non-root bridge will select its best path to the root bridge. The best path port is called the root port. To stop loops, the third election (called the designated port election) comes into play. Here, each segment in the bridged network elects one designated port and places it in forwarding mode. This designated port is the branch leading away from the root. The idea is, if you only have one port handling traffic on a segment, no loops can be formed.

Have a look at this link as well:

Spanning-Tree - At A Glance

http://www.cisco.com/application/pdf/en/us/guest/netsol/ns24/c643/cdccont_0900aecd800d813a.pdf

Regards,

GP

Community Member

## Re: Root port vs Designated port?

Hey Gp,

I get the whole selection process and the designated port vs the root port. Just clarify my own confusion, root port is the port pointing towards the bridge (forwards and receives info from root bridge) right? and designated port is the port pointing towards a non-root bridge which fowards and receives info from non-root bridge??

VIP Purple

## Re: Root port vs Designated port?

Hello,

exactly. I am trying to draw it schematic:

RP (Root Port)

DP (Designated Port)

RS (Root Switch)

NRS (Non-Root Switch)

RS - DP --> RP - NRS - DP

Regards,

GP

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