RAPID SPANNING TREE PROTOCOL PDF
This document provides information about the enhancements added by RSTP to the previous D standard. Rapid Spanning Tree Protocol (RSTP). ▫ Fast convergence Spanning tree: ▫ Less than 1 second. ▫ Standard IEEE w approved in fall 2. STP standards overview. 3. IEEE D STP protocol. 4. IEEE w RSTP Rapid STP. 5. IEEE Q CST Common Spanning Tree. 6. Cisco PVST+ and.
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implementation of the IEEE W Rapid Spanning Tree Protocol (RSTP). Switches send and receive spanning-tree frames, called bridge protocol data units. Spanning Tree Protocol (STP) and its much more recent update, the Rapid. Spanning most recent STP BPDUs (Bridge Protocol Data Units) with every port that receives them Ethernet Network: weinratgeber.info w Rapid Spanning Tree Protocol (RSTP) d Spanning Tree Protocol ( STP). Overview. Use spanning tree to ensure that only one active path at a time.
Then the administrator must determine which switch will be the root bridge, and the priority of the other switches in the spanning tree. If the root bridge goes down, the protocol will automatically assign a new root bridge based on the bridge ID.
Rapid Spanning Tree Protocol RSTP Tutorial
If all switches have the same bridge ID, such as the default ID, and the root bridge goes down, a tie situation arises and the protocol will assign one switch as root bridge based on the switch MAC addresses.
Once the switches have been assigned a bridge ID and the protocol has chosen the root bridge switch, the best path to the root bridge is calculated based on port cost, path cost and port priority. Administrators can influence the protocol's choice of the preferred path by configuring the port cost, the lower the port cost the more likely it is that the protocol will choose the connected link as root port for the preferred path.
The highest priority will mean the path will ultimately be less preferred. If all ports of a switch have the same priority, the port with the lowest number is chosen to forward frames. The numbered boxes represent bridges, that is switches in a LAN. The number is the bridge ID. The lettered clouds represent network segments. The smallest bridge ID is 3. Therefore, bridge 3 is the root bridge. The root bridge of the spanning tree is the bridge with the smallest lowest bridge ID.
For example, the ID of a bridge with priority and mac The bridge priority default is and can only be configured in multiples of The switch with the lowest priority of all the switches will be the root; if there is a tie, then the switch with the lowest priority and lowest MAC address will be the root.
Breaking ties in selecting the path to the root bridge[ edit ] Breaking ties for root ports: When multiple paths from a bridge are least-cost paths, the chosen path uses the neighbor bridge with the lower bridge ID.
The root port is thus the one connecting to the bridge with the lowest bridge ID. For example, in figure 3, if switch 4 were connected to network segment d instead of segment f, there would be two paths of length 2 to the root, one path going through bridge 24 and the other through bridge Because there are two least cost paths, the lower bridge ID 24 would be used as the tie-breaker in choosing which path to use. Breaking ties for designated ports: When the root bridge has more than one port on a single LAN segment, the bridge ID is effectively tied, as are all root path costs all equal zero.
It's put into Forwarding mode while all other ports on the root bridge on that same LAN segment become non-designated ports and are put into blocking mode. A final tie-breaker is required as noted in the section "The final tie-breaker.
Therefore, the designated port for network segment e is the port that connects bridge 92 to network segment e. When more than one bridge on a segment leads to a least-cost path to the root, the bridge with the lower bridge ID is used to forward messages to the root.
The port attaching that bridge to the network segment is the designated port for the segment. In the diagram on the right there are two least cost paths from network segment d to the root, one going through bridge 24 and the other through bridge The lower bridge ID is 24, so the tie breaker dictates that the designated port is the port through which network segment d is connected to bridge If bridge IDs were equal, then the bridge with the lowest MAC address would have the designated port.
In either case, the loser sets the port as being blocked. The final tie-breaker.
In some cases, there may still be a tie, as when the root bridge has multiple active ports on the same LAN segment see above, "Breaking ties for designated ports" with equally low root path costs and bridge IDs, or, in other cases, multiple bridges are connected by multiple cables and multiple ports.
In each case, a single bridge may have multiple candidates for its root port. In these cases, candidates for the root port have already received BPDUs offering equally-low i.
The bridges have to determine the root bridge and compute the port roles root, designated, or blocked with only the information that they have. Thanks again. If possible please include it in this tutorial. Omar June 14th. Vinay G 6. I am asking this for the completeness of RSTP. Vinay July 1st.
Thanks 9. Pawadan July 28th. What is the reconvergence time of rstp after link failure? Thanks in advance Moloy September 5th. Hab July 27th. Deepak S July 9th.
Amira July 26th. There is not a fixed time for RSTP convergence but typically it is less than 10 seconds.
Spanning Tree Protocol
Simba September 26th. But how it will be less than 10 seconds because it will take 6 seconds for max-age time and 15 seconds for learning to forwarding mode. Sajan September 25th. Moloy September 6th. I updated it! I am getting confused. You should write a cisco book!
Thanx Drei February 8th. Thanks a lot. Muhsin December 11th.. Nilesh December 17th. This is the best tutorial that i have come across. Ports may be configured as edge ports if they are attached to a LAN that has no other bridges attached.
These edge ports transition directly to the forwarding state. RSTP can also be configured to automatically detect edge ports.
As soon as the bridge detects a BPDU coming to an edge port, the port becomes a non-edge port. RSTP calls the connection between two or more switches as a "link-type" connection. A port that operates in full-duplex mode is assumed to be point-to-point link, whereas a half-duplex port through a hub is considered a shared port by default.
This automatic link type setting can be overridden by explicit configuration. RSTP improves convergence on point-to-point links by reducing the Max-Age time to 3 times Hello interval, removing the STP listening state, and exchanging a handshake between two switches to quickly transition the port to forwarding state.
An RSTP bridge will "propose" its spanning tree information to its designated ports. If another RSTP bridge receives this information and determines this is the superior root information, it sets all its other ports to discarding.
The bridge may send an "agreement" to the first bridge confirming its superior spanning tree information. This essentially creates a cascading effect away from the root bridge where each designated bridge proposes to its neighbors to determine if it can make a rapid transition.
As discussed in the port role details above, RSTP maintains backup details regarding the discarding status of ports. This avoids timeouts if the current forwarding ports were to fail or BPDUs were not received on the root port in a certain interval. Both standards implement a separate spanning tree for every VLAN. Very few switches from other vendors support Cisco's various proprietary protocols. If this protocol is in use, VLAN membership for trunk interfaces must be statically configured .
Cisco also published a proprietary version of Rapid Spanning Tree Protocol. Furthermore, the administrator can define alternate paths within a spanning tree. Each of these MSTI configuration messages conveys the spanning tree information for each instance.
This digest is then used by other MSTP bridges, along with other administratively configured values, to determine if the neighboring bridge is in the same MST region as itself.
The message age time is only incremented once when spanning tree information enters an MST region, and therefore RSTP bridges will see a region as only one "hop" in the spanning tree. As in RSTP, these ports can be configured as edge ports to facilitate rapid changes to the forwarding state when connected to endpoints. SPB allows redundant links between switches to be active through multiple equal cost paths, and provides much larger layer 2 topologies, faster convergence, and improves the use of the mesh topologies through increased bandwidth between all devices by allowing traffic to load share across all paths on a mesh network.
It is eight bytes in length. The first two bytes are the bridge priority, an unsigned integer of , The last six bytes are a MAC address supplied by the bridge. Prior to IEEE Since IEEE Disadvantages and current practice[ edit ] Spanning tree is an older protocol with a longer default hold-down time that governs convergence of the protocol state.
Improper use or implementation can contribute to network disruptions. The idea of blocking links is something that customers these days do not accept as a proper high availability solution.
Despite being designed primarily as a guard protocol to manage network topology loops, Spanning Tree Protocol was often misused as a high-availability method. Modern networks can make use of all connected links by use of protocols that inhibit, control or suppress the natural behavior of logical or physical topology loops.
A multi-chassis link aggregation group works like a normal LACP trunk , only distributed through multiple switches.RSTP can also be configured to automatically detect edge ports. Vicky Parasar. Administrators can influence the protocol's choice of the preferred path by configuring the port cost, the lower the port cost the more likely it is that the protocol will choose the connected link as root port for the preferred path.
When a RSTP bridge detects a topology change, these occur: It starts the TC While timer with a value equal to twice the hello-time for all its non-edge designated ports and its root port, if necessary. Root ports are normally in the forwarding state. Published in: