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Like others, I'm trying to wrap my head around the difference between root ports (I under stand this one) and designated ports (I do not yet understand this one). Wendell Odom's CCNA book mentions the concept of a LAN segment. What exactly is that in the context below? So far I've only read about segments as a synonym for L4PDUs. Is the definition of a LAN segment even relevant to understanding the difference? I know that a designated port cannot be the root port on a non-root switch. But isn't the root port supposed to provide the shortest path to the root switch?

STP/RSTP’s final step to choose the STP/RSTP topology is to choose the designated port on each LAN segment. The designated port (DP) on each LAN segment is the switch port that advertises the lowest-cost Hello onto a LAN segment. When a nonroot switch forwards a Hello, the nonroot switch sets the root cost field in the Hello to that switch’s cost to reach the root. In effect, the switch with the lower cost to reach the root, among all switches con- nected to a segment, becomes the DP on that segment.

Also, what is the point of having the port of one of two switches on both ends of a link be in a blocking state, while the other is in a forwarding state? Doesn't the switch at the blocking end drop all traffic forwarded by the switch with the designated port?

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In this context, segment means the link between two STP bridges. It may be a physical link or a logical one.

The designated port is the non-root port forwarding traffic on the segment. Only root ports and designated ports can forward traffic.

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  • The sentence "In effect, the switch with the lower cost to reach the root, among all switches connected to a segment, becomes the DP on that segment." implies that one switch may be connected to multiple other switches. Sounds like the author means something along the lines of a junction or something.
    – L.S. Roth
    Aug 29 at 19:05
  • @L.S.Roth In theory, a switch port could be connected to a half-duplex LAN segment with multiple switches - this is reflected in the standard. In practice, all ports are used with point-to-point links, and a port linked to a root port is a designated port.
    – Zac67
    Aug 29 at 19:15
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LAN segment means whole layer 2 topology.You should also be aware that while different switch ports are representing same LAN segment for each VLAN, router ports aren't. Router ports two different LAN segment, even they are connected to same switch with same untagged native VLAN's. A router port can not traverse broadcast for this reason, they are two independent interfaces.

LAN segment also determined by instance ID's in MSTP.

As i said LAN segment means L2 topology for each VLAN but STP and RSTP can't create different tree for each VLAN. So in data plane, a access port that mapped to VLAN 2 on different LAN segment than a access port that mapped to VLAN 3. On the control plane, STP and RSTP can't distinguish this difference and sees all VLANs as they are same LAN segment.

MSTP provides independent tree structure with VLAN to instance maps. So in MSTP network a LAN segment means different VLAN's to also control plane, not just to users.

Root ports are ports that has least cost to root bridge, Designated ports are ports that not close as rott port as to root bridge. Because of the there can be only one root bridge, and one path to that, there are no chance to become multiple root ports on same switch.

Actually root port name comes from path selection, It means, this port is best for to reach root bridge, Because of that, in each hop, ports that serves an uplink to far switches gives name designated, far switches selected us as best to root, we are designated.

Same concept applies to root bridge. All root switch ports are designated. We are root, we've no root ports and everyone else designated us we are given name designated to all forward state ports.

I hope it helps. Sorry for my bad English.

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