In a previous lesson I explained the differences between classic and rapid spanning-tree and how rapid spanning-tree works.
Provided there is more than one link between two switches, the STP root bridge calculates the cost of each path based on bandwidth. STP will select the path with the lowest cost, that is the highest bandwidth, as the preferred link.
STP will enable this preferred link as the only path to be used for Ethernet frames between the two switches, and disable all other possible links by designating the switch ports that connect the preferred path as root port.
Its successor RSTP uses a similar formula with a larger numerator: These formulas lead to the sample values in the table on the right: This is either the port that connects the switch to the root bridge, or if there are several paths, the port with the preferred path as calculated by the root bridge.
Each switch adds the cost of its own path to the cost received from the neighboring switches to determine the total cost of a given path to the root bridge. Once the cost of all Spanning tree protocol paths to the root bridge have been added up, each switch assigns a port as root port which connects to the path with the lowest cost, or highest bandwidth, that will eventually lead to the root bridge.
No user data is sent or received over a blocking port, but it may go into forwarding mode if the other links in use fail and the spanning tree algorithm determines the port may transition to the forwarding state.
BPDU data is still received in blocking state. Prevents the use of looped paths. Listening - The switch processes BPDUs and awaits possible new information that would cause it to return to the blocking state.
It does not populate the MAC address table and it does not forward frames. Learning - While the port does not yet forward frames it does learn source addresses from frames received and adds them to the filtering database switching database. It populates the MAC address table, but does not forward frames.
Forwarding - A port receiving and sending data in Ethernet frames, normal operation. The Forwarding port monitors incoming BPDUs that would indicate it should return to the blocking state to prevent a loop.
Disabled - A network administrator has manually disabled a switch port When a device is first attached to a switch port, it will not immediately start to forward data.
It will instead go through a number of states while it processes BPDUs and determines the topology of the network. When a host is attached, such as a computer, printer or serverthe port always goes into the forwarding state, albeit after a delay of about 30 seconds while it goes through the listening and learning states see below.
The time spent in the listening and learning states is determined by a value known as the forward delay default 15 seconds and set by the root bridge.
However, if instead another switch is connected, the port may remain in blocking mode if it is determined that it would cause a loop in the network. TCNs are injected into the network by a non-root switch and propagated to the root. This flag is propagated to all other switches to instruct them to rapidly age out their forwarding table entries.
All switches must use the same version of STP. 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. 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. 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.
Lowest root bridge ID - Determines the root bridge Lowest cost to the root bridge - Favors the upstream switch with the least cost to root Lowest sender bridge ID - Serves as a tie breaker if multiple upstream switches have equal cost to root Lowest sender port ID - Serves as a tie breaker if a switch has multiple non-Etherchannel links to a single upstream switch, where: 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.
· The Spanning Tree Protocol that is the basis for the IEEE standard D was designed to provide "plug-and-play" operation for large Layer 2 networks based on half duplex shared Ethernet, which was the prevalent LAN metin2sell.com metin2sell.com · Spanning Tree Protocol RouterOS is capable of running bridge interfaces with (R/M)STP support in order to create loop-free and Layer2 redundant environment.
It is always recommended to manually set up each bridge priority, port priority and port path cost to metin2sell.com:Spanning_Tree_Protocol. · Ethernet networks are susceptible to broadcast storms if loops are introduced.
However, an Ethernet network needs to include loops because they provide redundant paths in case of a link failure. Spanning-tree protocols address both of these issues because they provide link redundancy while metin2sell.com /topic-map/metin2sell.com The Spanning Tree Protocol (STP) is a network protocol that builds a loop-free logical topology for Ethernet networks.
The basic function of STP is to prevent bridge loops and the broadcast radiation that results from them. · Spanning trees are important in parallel and distributed computing, as a way of maintaining communications between a set of processors; see for instance the Spanning Tree Protocol used by OSI link layer devices or the Shout (protocol) for distributed metin2sell.comations · Definitions · Counting spanning trees · Algorithmsmetin2sell.com The Spanning Tree Protocol (STP) is a link management protocol preventing media access control (MAC) bridge loops and broadcast delays on any local area network (LAN).