Must Knows about Spanning-Tree Roles, Ports, and States

There many things to know about Spanning-Tree, however knowing the difference between a few key terms will help you earn some points on the exam. The three things we'll be covering in this post is the difference between STP Roles, Modes and States.

Spanning-tree Port Roles

Spanning-tree roles describe what the switch's responsibility is for the LAN segment running STP. For example if a switch was elected the Root Switch, it's role would be to promote it's self as Root and start sending out BPDUs to other switches indicating the timers of STP such as the forward delay timer. There are only four types of Roles; Root, designated, alternate, and backup.

Root Port Role - The port that receives the best BPDU on a bridge is the root port. This is the port that is the closest to the root bridge in terms of path cost. The STA elects a single root bridge in the whole bridged network (per-VLAN). The root bridge sends BPDUs that are more useful than the ones any other bridge sends. The root bridge is the only bridge in the network that does not have a root port. All other bridges receive BPDUs on at least one port. 

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 & Backup - 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.

Information from http://www.cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a0080094cfa.shtml#topic2

Spanning-Tree Modes

Spanning-Tree modes are simply the version of spanning-tree running. Over the years several different versions have been developed some Cisco Proprietary some created by the IEEE. Below lists these modes.

802.1D Spanning-tree 
PVST+ or PVSTP - Per-vlan spanning tree "Cisco Proprietary"
802.1W Rapid Spanning-tree
Rapid PVST+ or PVSTP Cisco Proprietary version of RSTP

Spanning-tree Port States

For this post we're listing only the states for interfaces that are running 802.1D normal spanning tree protocol RSTP 802.1W has different names for these ports.

Transition States
Listening
Learning  

Stable States
Blocking
Forwarding
Disabled