Rapid Spanning Tree Protocol Topology/ Port Role Election Help

[reesenorman]

I'm just trying to review my course material for one of my university classes and one think that I don't quite understand is the difference between alternate and backup ports. I have a topology that I am looking at and trying to practice on and after an hour of trying to find some answers I'm still coming up short and don't understand. A lot of the diagrams I've seen are simple enough to understand and don't teach me anything.

I've already started labelling the topology but I was hoping someone could tell me what roles all of the unlabelled ports be given and why? Assuming all links between switches to be 100Mbps links. Any help is greatly appreciated. 

[kapslock]

Since path costs are the same the desg port will be on the lowest mac add switch.

On the segment between S2 and S3, because S2 has lower MAC than S3, it will have a better BPDU. Port Fa0/2 on S2 will be desg port and Fa0/3 on S3 will an alt port.

Between S2 and S4, S4 will be desg on port Fa0/2 and S2 will be alt on Fa0/3 because S4 has a lower mac add than S2.

Same rules applies on every other segment in your diagram because path cost are equal, if they weren't, the switch with lowest path cost would be desg.

Below is taken from following link, which is a really good spanning-tree resource.
https://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/24062-146.html
"Designated Port Role:
A port is designated if it can send the best BPDU on the segment to which it is connected. 802.1D bridges link together different segments, such as Ethernet segments, to create a bridged domain. On a given segment, there can only be one path toward the root bridge. If there are two, there is a bridging loop in the network. All bridges connected to a given segment listen to the BPDUs of each and agree on the bridge that sends the best BPDU as the designated bridge for the segment. The port on that bridge that corresponds is the designated port for that segment.

Alternate and Backup Port Roles:
These two port roles correspond to the blocking state of 802.1D. A blocked port is defined as not being the designated or root port. A blocked port receives a more useful BPDU than the one it sends out on its segment. Remember that a port absolutely needs to receive BPDUs in order to stay blocked. RSTP introduces these two roles for this purpose.

An alternate port receives more useful BPDUs from another bridge and is a port blocked.

A backup port receives more useful BPDUs from the same bridge it is on and is a port blocked.

This distinction is already made internally within 802.1D. This is essentially how Cisco UplinkFast functions. The rationale is that an alternate port provides an alternate path to the root bridge and therefore can replace the root port if it fails. Of course, a backup port provides redundant connectivity to the same segment and cannot guarantee an alternate connectivity to the root bridge. Therefore, it is excluded from the uplink group."

[Deepak Kumar]

Hi,

Let's check the definition of the Backup port and Alternate Port.

Alternate Port: In Simple Words, the alternate port is used as an alternate path up to the root bridge. In this Diagram Port between S2 to S3 and S2 to S4 point to point links are alternate links. 

Backup Port: Cisco has created this backup port for the function of supporting connectivity between switches that have some form of an unsupported spanning-tree device between them.  Note, that the backup port is hardly ever seen in real-life production networks because it’s only used where a switch has multiple links to the same layer 1 segment (i.e. multiple ports in the same collision domain), which can only really be apparent when using something like a hub. In the mentioned diagram there is no backup port.

I think this CCIE blog will help you more to understand the same in more details: https://ccieblog.co.uk/spanning-tree/rstp-alternate-and-backup-ports