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alanchia2000
Beginner

Is there a difference between designated port and root port?

Hi,

I am reading up the Cisco press books and have some difficult understanding whether there is a difference in designated port and a root port?

11 REPLIES 11
sstudsdahl
Enthusiast

A root port is the port that data would pass through to get to the root bridge within the spanning tree. A designated port is the port facing away from the root bridge that has the lowest cost path to get to the root bridge.

Have a look at the link below for more info.

http://www.cisco.com/warp/public/473/146.html#roles

HTH

Steve

Hi,

I am confused, does that definition only apply to RSTP ? What about STP? Does it refer to the same thing?

The defination for the root port and designated ports remain same for STP and RSTP. A designated port is one which is other than root port and is in forwarding mode. There can be only one root port but can more than one designated port in any spanning tree scenario,

Hi alancia,

Well to get things clearer, a root port is the Port that recieves the lowes cost BPDU's (Bridge Protocol Data Unit) from its root in Spanning tree topology.

A designated Port (for a segment) is the port that SENDS the lowest Cost BPDU's onto a Segment.

E.G. now Imagine a ring topology. Three switches connected with one link to each other... On of them is the root. A root Bridge has only designated Ports, because it is the root and generates the BPDU's. the switches directly connected to the root have at least a root Port. Its the Port that recieves these BPDU's generated by the root. With the connectiopn between the switches one port one one switch blocks and one port sends these BPDU's on the "segment" between the switches. Remember that a few Years ago Ethernet was a shared LAN. (Not only one Node on each end of the cable) The port that sends the BPDU's on the segment is the designated Port.

With STP, RSTP, PVSTP and MST the ports are called the same.

Regards,

Sebastian

shaikh.rizwan
Beginner

the root port is the port that is connected to the root bridge , root bridge is the bridge which takes decisions about another ports about ports which can be disabled , listening , or ports who can be in forwarding state

the designated port is the one who has the right for forwarding frames

Hi,

To make it very simple the root port is the port pointing towards the root bridge and the designated port is the port which points against the root bridge i.e: which points towards the lower end switches.

On the root bridge the ports are termed as both the root port as well as the designated port.

The terminology remains the same for PVST,RSTP and MSTP.

HTH

Ankur

Hi Ankur,

u can explain it like this until u interconnect your root with two links each to every switch(eg. for backup).... Now two Ports are pointing at the root.... Same is when u connect your lower end switches with two or more links... (not a channel).

Regards,

Sebastian

hi shaikh,

Sorry but that is just wrong. The root cannot take decisions about designated and root ports on other switches.

Regards,

Sebastian

the root port is the port which is directly connected to the root bridge , if there are mutliple links connected then port cost is taken into consideration , which is determined by the bandwidth , if multiple links have the same bandwidth , then bridge with lower advertising bridge id is taken

A designated is the port which is in the forwarding state , meaning it can forward frames , the decision of decision port is done on the basis of the port cost , a port with lowest cost will be the designated port

cost is determined by the bandwidth of the link

Hi Shaik,

This explanation is too simple. What if u cascade some switches? Not all are directly connected to your root.

Next, bridge ID is only taken into consideration when determining which one will become the root. If u connect a switch to another with two links, the switch with the lower bridge-id becomes the root, but now u still don't know whitch port is the root port. U'r right when saying port cost is determined, but port cost is only the bandwith divided by a default constant (FE 100Mbit the constant is 10000000, i think), if its equal again u only have the port-ID as the last choice.

BUT only if the switch recieves on both Ports equal cost BPDU's from its root(refer to my first posting in this thread)!

U'r right.... a designated port is in forwarding state. It is forwarding the (lowest cost) BPDU's onto the connected segment. But if u have your lower end switch connected with two links to a non-root switch, u will encouter the same problem(s.o.).

I don't want to go against everyone here. But understanding STP is a nessassity in LAN Troubleshooting. So please be lenient against me.

Regards,

Sebastian