Root port vs Designated Port

I'm sorry if this is a totally beginners question. At one point I thought I had a full understanding of STP, but then I started thinking a bit deeper and realized I had no understanding of the differences between root ports and designated ports.

This is what I understand:
- The election process for the root bridge, root port, and designated ports
- I understand that BPDUs are sent on all ports, no matter what port state

My question is basically, are designated ports only ports that will forward traffic that is received from a switch below them? And if a switch has fa0/1 as the root port, and fa0/2 as the designated port, it will only forward traffic out of fa0/1, even if a host it's trying to reach is directly connected to a switch on it's designated port?

Thanks!

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networker050184

The root port on a switch is the port receiving the lowest cost path to the root. The designated port is a port that is sending that lowest cost path onto a segment. Hence all ports on the root are designated ports. Forwarding is done on root and designated ports.

kerx

networker050184 wrote: »

The root port on a switch is the port receiving the lowest cost path to the root. The designated port is a port that is sending that lowest cost path onto a segment. Hence all ports on the root are designated ports. Forwarding is done on root and designated ports.

So in an example where we have SWITCH_A that has three ports connected to switches. It will always select the one of it's port as the ROOT PORT, another for DESIGNATED, and the one with the lowest cost back to the root as the BLOCKED (or ALTERNATE)?

Thanks for your support.

networker050184

No, the ports are designated/root on a per segment basis. It can have two designated ports and a root port. Depends on the topology.

kerx

networker050184 wrote: »

No, the ports are designated/root on a per segment basis. It can have two designated ports and a root port. Depends on the topology.

I guess what I'm looking for, basically falls under graph theory.

So there will be cases where non-root switches w/ for example three links, could have one port as the root port, and the other two as designated ports.

I guess the best way for me to fully understand is to draw up some complicated STP layouts to see what kind of results I receive...

ALSO, if a switch has a host directly connected that has to send out a broadcast. Will it forward the broadcast out of the RP and the DP? Or will it only forward frames originating from itself out of the RP?

Zartanasaurus

Some simple rules to remember.

All ports on a root port are designated. There are no root ports on a root bridge.
Only one root port per switch.
The root port is the lowest cost path back to the root bridge.

A switch can have any number of designated ports. Only one of the ports per segment will be designated though. One of the switches on the link will assign it the designated role. The other switch connected by that link will not. It could be that switch's root port. It could be an alternate or backup port (find out the difference). It will not be a designated port.

Basically the thought process you go through if you're given a diagram is something like:
1) Identify the root bridge. Mark all its ports D.
2) Identify the root port for each switch. Mark those ports P.
3) On every other link you haven't marked, identify which is the designated port for that link. The other connecting port becomes A or B.
4) You will never see a DD link (insert boob joke).

networker050184

You basically build a tree from the root. All down stream forwarding interfaces will be designated and all up stream forwarding ports will be root. Any non forwarding are blocking/alternate ports.

kerx

Zartanasaurus wrote: »

A switch can have any number of designated ports. Only one of the ports per segment will be designated though. One of the switches on the link will assign it the designated role. The other switch connected by that link will not. It could be that switch's root port. It could be an alternate or backup port (find out the difference). It will not be a designated port.

What do you mean by segment when you mention "Only one of the ports per segment will be designated though." Is that per VLAN? Inter-connected switches only?

networker050184

kerx wrote: »

What do you mean by segment when you mention "Only one of the ports per segment will be designated though." Is that per VLAN? Inter-connected switches only?

A segment is a link between two switches, but that segment can have different states depending on what mode of STP you are running. No offense, but if I were you I'd start at the beginning and learn STP again. It seems as though you are missing the very basics.

kerx

networker050184 wrote: »

No offense, but if I were you I'd start at the beginning and learn STP again. It seems as though you are missing the very basics.

None taken. That's what I'm here for

Zartanasaurus

kerx wrote: »

None taken. That's what I'm here for

The Foundation Learning Guide was really good for this topic. Don't skim over MST either and think b/c it's not a Cisco thing that it won't be emphasized.

saavedra29

Zartanasaurus I was confused by this too and (after searching in many sites) only with your explanation i understood the difference..
Thanks:D