STP Designated Port

[Smack2k]

Hello,

Passed my ICDN1 and working on my ICDN2.  Have a question concerning STP and Designated Port

Seems like in almost every example I have seen / read so far in books, the Designated Port in a LAN Segment that is in a forwarding state is connected on the other end to a Switch with its port in Blocking State.  IF this is the case (which I assume it isnt and I am just looking at it wrong), anything sent from the DP would just be blocked by the port on the other end, so what would the point of the DP be?

Struggling with understanding fully of the role of the Designated Port.  I know how it gets chosen and I know it deals with traffic going away from the Root Bridge, but outside that, its actual functionality I am a little confused on as I keep seeing it connected to Blocking Ports.....

Thanks for the assistance

[Reza Sharifi · Accepted solution]

Have a look at this post and the explanation given by Jon.

https://community.cisco.com/t5/switching/stp-root-port-vs-designated-port/td-p/1517842

HTH

[Smack2k]

Thanks for the links...very very helpful....I think I have a good understanding of it now...

[balaji.bandi]

There is good video i suggest to view adding to other post suggestions.

https://www.youtube.com/watch?v=RwcC_Dcf3BE&list=PLd4Eu31e6RINRPblO8Gm03cp69vjXUMJ0

[paul driver]

Hello

My understanding is that an stp DP role is to forward frames on a converge stp connected segment, It continually sends out bpdu's on this DP port advertising its superior path cost it has calculated towards the root switch to its directly connected switch.

sw1  sw3
sw2  sw4

Lets say sw 1 is the root switch so all its port a designated forwarding.
sw 2 computed rpc to root sw1 is 40
sw 2 advertises rpc towards sw4 as 40

sw 4 computed rpc towards sw1 is 20 (via sw3)
sw 4 advertises rpc towards sw2 is 20

sw4 is advertising bpdu towards sw2 with a preferred lower rpc and as such its port will be chosen as the DP for the segment between sw2-sw4, So sw2 will set its port connected to sw4 as a root port, sw2 port towards SW1 will be blocked