Switching Techniques: STP Vs RSTP
Today we are going to talk about the difference between STP
and RSTP. In fact, RSTP is the originate from STP as a base technology where we
were trying to reduce the convergence time between the switches going up. Now
the question is how we were able to achieve this.
Before that we will shortly describe STP and RSTP in order
to understand the basics of these techniques.
Spanning Tree Protocol
The main drive of STP is to confirm that you do not create loops when you
have redundant paths in your network.Use STP in circumstances where you want redundant links, but
not loops. Redundant links are as significant as backups in the case of a
failover in a network. A disaster of your primary triggers the backup links so
that users can endure to use the network.
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Fig 1.1- STP Topology
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Without STP on the bridges and
switches, such a failure can result in a loop. If two connected switches run
different flavours of STP, they require different timings to converge. When
different flavors are used in the switches, it creates timing issues between
Blocking and Forwarding states. Therefore, it is recommended to use the same
flavors of STP.
With STP, the important is for all the switches
in the network to elect a root bridge that becomes the principal point in the
network. All other judgements in the network, such as which port to block and
which port to put in forwarding mode, are made from the standpoint of this root
bridge.
A switched ecosystem, which is different from a bridge ecosystem, most
likely deals with multiple VLANs. When you apply a root bridge in a switching
network, you typically refer to the root bridge as the root switch. Each VLAN
must have its own root bridge because each VLAN is an independent broadcast
domain. The roots for the different VLANs can all reside in a single switch or
in various switches.
Rapid Spanning Tree Protocol
RSTP achieves better than branded extensions of
Cisco without any added configuration. 802.1w can also revert back to 802.1D in
order to interoperate with old bridges on a per-port basis. This drops the
benefits it introduces.
RSTP provides faster convergence than 802.1D STP
when topology changes occur. RSTP defines three port states: discarding,
learning, and forwarding and five port roles: root, designated, alternate,
backup, and disabled.
A RSTP accomplished switch controls what spanning
tree will be computed by the algorithm, but the rules as written need knowledge
of the entire network. This info is provided bridges use special data
frames called Bridge Protocol Data Units (BPDUs) to interchange
information about bridge IDs and root path costs. The switch device needs to
manage BPDU in the proper way to ensure that the algorithm running on the CPU
it is able to compute the algorithm. You can check the below link to understand RSTP protocol in details
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Fig 1.2- STP Vs RSTP Comparison
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