I'll try to explain this:
Think of a root port as the closest port to the root bridge on a given
BRIDGE. Think of a designated port as the closest port to the root bridge
on a given SEGMENT. This is the port used by all bridges on a given segment
to get to the ROOT. Consider the following basic diagram to explain this
further with 1 root bridge, 3 non-root bridges and 3 segments:
|ROOT|--segment 1--|A|--segment 2--|B|--segment 3--|C|
The root port on bridge A is the closet int to ROOT - the int on the left.
The designated port on segment 1 is actually the int on ROOT that's in
segment 1. The root port on Bridge B is the closest int to ROOT - the int
on the left. The designated port on segment 2 is the closet interface to
ROOT in segment 2 - the int on the right side of bridge A. The root port on
bridge C is the the closest int to ROOT - the int on the left. The
designated port on segment 3 is the port closest to ROOT - the int on the
right side of bridge B.
So, you wind up with something like a consistent and logical topology:
ROOT(DP)--(RP)A(DP)--(RP)B(DP)--(RP)C
The real distinction is knowing that a root port is a designation specific
to a switch and a designated port is specific to a segment. To show this,
we can make the following modification to the above topology:
ROOT(DP)--(RP)A(DP)--(RP)B(DP)--(RP)C
|
|--(RP)D(DP)--(RP)E
In this case, there are 2 root ports in segment 2 but there will always be
ONLY 1 designated port per segment. This is one of the foundational
concepts of STP. Also, the ROOT will never have a root port, all non-root
bridges will have ONLY 1 root port (per VLAN) and there will ONLY be 1
designated port per segment (per VLAN). Root ports send BPDUs and
designated ports receive BPDUs.
Hope this helps,
Rik
-----Original Message-----
From: Lomker, Michael [mailto:[EMAIL PROTECTED]]
Sent: Thursday, March 28, 2002 2:44 PM
To: [EMAIL PROTECTED]
Subject: RE: Designated Port/Switch and Root Port?? [7:39811]
> Hello,If every non-root bridge elects one root port to get to the
> root-bridge, then why do we still need a designated switch/port per
> segment? Do these two have different functions altogether?Thank you.
I did a few searches on cisco.com and google and they appear to be different
works for the same thing. I'll agree that the explanation I read in my
Examcram wasn't that explicit.
http://netcert.tripod.com/ccna/switches/2switch.html
"Ports that have the lowest cost to the root bridge are called designated
ports. The other ports on the bridge are considered non designated and will
not send or receive traffic, (blocking mode)."
Message Posted at:
http://www.groupstudy.com/form/read.php?f=7&i=39870&t=39811
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