IMPLMENTING SPANNING TREE DESCRIBING THE STP Because switches have replaces bridges as the network device for implmenting transparent bridging in modern networks, the basic functionality of a switch is identical to that od a transparent bridge on per VLAn basim. To understand STP, it is important first to look at the behavior of a transparent bridge has these characteristics:It must not be modified the frames that are foward. It learns addresses by listening on a port for the source addresses of a device. If a source MAC address is read in frames coming ina specific port, the bridge assumes that frames destined does that MAC address can be sent out of that port. Then bridge builds a table tha records what source addresses are seen on what port. A bridge is always listning and learning MAC addresses in this manner. It must foward all broadcast out all ports, except for that port the initialy received the broadcast. If a destination address is unknown to the bridge, it fowards the frame out all ports excepts for the port that initially recieved the frames. This is known as unicast flooding. Transparent bridging, by defenition must be trandparent to the devices on the network.End ststaions require no configuration. The existence of the bridging protocol operation is not directly visible to them-hence the term, transparent bridging> As with traditional share Ethernet, transaprent bridging inherently lack the capability to provide redundancy. The STP inserts a mechanism into the Ethernetb transparent bridge enviornment to dynamically discover the network topology and ensure only one pathe through the network. Without STP, there is no way to make a transpresnt bridge enviornment redundant. STP also protects a network against accidental miscabeling because it prevents unwanted bridging loops in the transparent bridging enviornment.
What is STP? A bridge loop is observed when a frame that is fowarded circulates cyclically and redundantly; this ocuurs where there is no mechnism to the redundant Layer2 paths. Example: Flooded Unicats Frames and bridhge loop Stataion A has 2 potentials to station B by way of the 2 intermediate bridges.. The following describes what happens. if stsaion A send frames to station B, if there were no provisions enabled to deal with redundant paths> Stepactiob station A transmits teh frames destined for satation B onto egment A. Both bridges on segment A pick up the frame on their bridge port1/1 and 2/1. Both bridges populate their respective MAC table, indicating that staionA resides on segment A on bridge ports 1/1 and 2/1. Both bridge foward the frame to segment B > Station B recives also see the same frame, with the MAC address of ststion A in the Source Address (SA) field, coming from the other bridge> The bridge will now incorrectly foward all fraes for satation A and segment B . When station B responds to station A the frame will be dropped by both bridges because it will be recievd on the same bridge ports that it considers the destination of station A. If satation A, or any ststion, sends a broadcast, the effects of the Layer 2 lopp[ would be much worse. The destination MAC address would be ff-ff-ff-ff-ff-ff. This would cuause each bridge to foward the frame out all bridge ports execept the bridge ports upon which the frame was recieved. The broadcats frame would also be fowarded to the originating bridge, which would again
foward the same broadcats out all bridge ports. This broad cast woulsd continue until the lopop was shut down or until the bridge could no longer handle the load.
Preventing bridge loops A lopps fre network is one in which no Layer 2 loops exist; therfor, the network cannot creat Layer 2 broadcast storms or floeded unicast staorms. a loop free network can be achieved manually by shutting down or disconnecting all redundant links betwenn bridge> hoiwever this leave sn redundancy in the network and requiers manual intervention in the event of a link afilure > STP resolves the problem. Where there are alternative links to a destination on a switch, only link will be used to foward data unless there is a failure on that link. The switch ports asscoiated wityh alternative paths remain aware of the topology of the network and can be enabled is a failure occurs ona primary link. In the case of primary link failure, the switch will begin fowarding over an alternative link. The spanning treee algorithems (STA) runs on each swicthe to activate or block redundant links. To find the redundant links, the STA chooses a reference point in the netwoork and determines if there are redundant paths to that reference point. If the STA finds a redundant path it chooses which path will be fowarded frames and which
redundant path or paths wiill be blocked. this effectively severs the redundant links within the network until they are needed when the primary link toward the reference points fails. Spanning tree standards often refere to a (bridge) < but it is likely that all the devices exchanging spnaning tree information will be Layer 2 switches.
802.1D Spanning Tree Protocol the 802.21D STP mechanism for switches to reconfigure the paths over which they foward frames, making possible a loop free path when ther are redundant switch path through the network. This is accomplished by fowarding traffic over specific ports and by disabling other ports to prevent fraames from being sent repeateadly or in a loop. STP prevents loops by using the following mechanisms: STP is implemented through the exchange of bridge protocol data unit (BPDU) messages between adjacent switches> A single root bridge is elected to serve as the refernce point from whihc aloop free topology is built for all switches exchanging BPDUs> each switch, except for the bridge, determins a "root bridge " is elected to serve as the refernce point from which a loop free topology is built for all swithces exchanging BPDUs. Each switch, except for the root bridge, determins a "root port" that provides the best path to the root bridge. In a triangular design similar to the one in the figure, on the link between the two nonroot switch ports a
port on one switch will become a designated port , and the port on the other swithc will be ina blocking state, not fowarding frames. This effectiveley breaks any loops. Typically, the designate port will be on the switch with the best path to the root bridge. Any port state change on any switch is considered a network topology change ( for example, if a port goes up or down and the STA must be run on all switches to adapt to adapt to the new topology).
Bridge Protocol Data units STP send configuration messges out every port of the bridge> These messages are called BPDUs. The information provided ina BPDU imcludes the following: Root ID: The lowest bridge ID (BID) in the toplogy Cost of path: Coats all links from the transmitting swithc BID: BID of the transmission switch port ID STP timer valued: Max age , hello time, foward delay BPDUs contain teh apprpoiate information for STP configuration. The type field for the BPDU message is 0x00 and it usesb the multicast MAC address 01-80-c2-00-00-00 . The switch compares the BPDU recieved on all the ports t its own values to dtermine whta role the recieving switch and it ports will play in the STP topology.