Rapid spanning tree protocol is extension of original spanning tree protocol. The reason behind the expansion is to speed up the process and configurations remains same. STP (802.1D) which is having five different port states called disabled, listening, learning, blocking, forwarding where disabled, blocking and listening merged into one and formed as discarding state in RSTP (802.1w). RSTP is having three different port states which are learning, forwarding, discarding. Each state has unique way of paths that learning port and forwarding port will be included in Active topology and grabs mac addresses where as discarding port will not be included in active topology and it doesn’t grabs mac addresses as well.
Port roles: Root port, Designated port, Alternate and backup port roles.These roles will be assigned by spanning tree algorithm also called STA.
Root port: The switch with the best bridge ID (priority + MAC address) will be the root bridge. Spanning tree algorithm picks one root bridge in the whole network where as that root bridge doesn’t have root ports while other non root brigdes configures with root ports.
Designated port: which port allows forwarding BPDUs for every segment will be the designated port
Alternate port: alternate port is the alternate path for the root bridge, if any designated port gets down then the alternate port will be in active that makes forwarding.
Backup port: The backup port is the redundant path of the topology, backup port will be applied on collision domains
Bridge port data unit: we have a little bit comparision from STP BPDU format to RSTP BPDU format which is called version 2 BPDU, we will have look on it :
In Spanning tree protocol BPDU flag field has only two bits in use:
- Topology change.
- Topology change acknowledgment.
In Rapid spanning tree protocol BPDU flag field all bits will be in usage, the roles involved in fields get added by the port role which has various options alternate port, backup port, root port, designated port and unknown.
RSTP makes it port in forwarding state to grab faster convergence on edge ports or point-to-point interfaces.
It has 2 link types:
- Point to point which is full duplex.
- Shared which is half duplex.
RSTP naturally includes the enrichment features of original spanning tree protocol such as uplink fast, Backbone fast, port fast. If link types has been configured well in the network by the engineer will converts into speed accessing. These all options makes RSTP faster convergence in the configured network.
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