In addition, due to the constant changes in the MAC address table, the switch does not know which port to use to forward unicast frames. In this example just shown, the switches will have the incorrect ports listed for PC1.
Any unicast frame destined for PC1 loops around the network, just as the broadcast frames do. More and more frames looping around the network eventually creates a broadcast storm. A broadcast storm occurs when there are so many broadcast frames caught in a Layer 2 loop that all available bandwidth is consumed. Consequently, no bandwidth is available for legitimate traffic, and the network becomes unavailable for data communication. This is an effective denial of service DoS.
Broadcast storms are inevitable on a looped network. As more devices send broadcasts over the network, more traffic is caught in the loop, consuming resources. This eventually creates a broadcast storm that causes the network to fail. There are other consequences of broadcast storms. Because broadcast traffic is forwarded out every port on a switch, all connected devices have to process all the broadcast traffic that is being flooded endlessly around the looped network.
This can cause the end device to malfunction because of the processing requirements needed to sustain such a high traffic load on the NIC. The broadcast frame loops between all the interconnected switches on the network.
The PC4 broadcast frame gets caught in the loop between all the interconnected switches, just like the PC1 broadcast frame. When the network is fully saturated with broadcast traffic that is looping between the switches, the switch discards new traffic because it is unable to process it. Figure displays the resulting broadcast storm. Figure Broadcast Storm Example.
A broadcast storm can develop in seconds because devices connected to a network regularly send out broadcast frames, such as ARP requests.
As a result, when a loop is created, the switched network is quickly brought down. Broadcast frames are not the only type of frames that are affected by loops. Unknown unicast frames sent onto a looped network can result in duplicate frames arriving at the destination device.
An unknown unicast frame occurs when the switch does not have the destination MAC address in its MAC address table and must forward the frame out all ports except the ingress port. In an attempt to find PC4, it floods the unknown unicast frame out all switch ports except the port that received the traffic.
Figure shows a snapshot during sequences 5 and 6. Most upper-layer protocols are not designed to recognize duplicate transmissions. In general, protocols that make use of a sequence-numbering mechanism assume that the transmission has failed and that the sequence number has recycled for another communication session. Other protocols attempt to hand the duplicate transmission to the appropriate upper-layer protocol to be processed and possibly discarded.
Layer 2 LAN protocols, such as Ethernet, do not include a mechanism to recognize and eliminate endlessly looping frames. Some Layer 3 protocols implement a TTL mechanism that limits the number of times a Layer 3 networking device can retransmit a packet.
Layer 2 devices do not have this mechanism, so they continue to retransmit looping traffic indefinitely. STP, a Layer 2 loop-avoidance mechanism, was developed to address these problems. To prevent these issues from occurring in a redundant network, some type of spanning tree must be enabled on the switches.
Spanning tree is enabled by default on Cisco switches to prevent Layer 2 loops from occurring. In this activity, you will observe how STP operates by default, and how it reacts when faults occur. For the purpose of this activity, the bridge priority covered later in the chapter was modified. Redundancy increases the availability of the network topology by protecting the network from a single point of failure, such as a failed network cable or switch. When physical redundancy is introduced into a design, loops and duplicate frames occur.
Loops and duplicate frames have severe consequences for a switched network. STP ensures that there is only one logical path between all destinations on the network by intentionally blocking redundant paths that could cause a loop.
A port is considered blocked when user data is prevented from entering or leaving that port. Blocking the redundant paths is critical to preventing loops on the network. The physical paths still exist to provide redundancy, but these paths are disabled to prevent the loops from occurring. If the path is ever needed to compensate for a network cable or switch failure, STP recalculates the paths and unblocks the necessary ports to allow the redundant path to become active.
S2 is configured with STP and has set the port for Trunk2 to a blocking state. The blocking state prevents ports from being used to forward user data, which prevents a loop from occurring. S2 forwards a broadcast frame out all switch ports except the originating port from PC1 and the port for Trunk2. S1 receives the broadcast frame and forwards it out all of its switch ports, where it reaches PC4 and S3. S3 forwards the frame out the port for Trunk2, and S2 drops the frame.
The Layer 2 loop is prevented. Figure shows how STP recalculates the path when a failure occurs:. As shown in the figure, the trunk link between S2 and S1 fails, resulting in the previous path being disrupted. S2 unblocks the previously blocked port for Trunk2 and allows the broadcast traffic to traverse the alternate path around the network, permitting communication to continue.
If this link comes back up, STP reconverges, and the port on S2 is again blocked. The switches running STP are able to compensate for failures by dynamically unblocking the previously blocked ports and permitting traffic to traverse the alternate paths.
However, these terms can be misleading. To communicate spanning tree concepts correctly, it is important to refer to the particular implementation or standard in context. Because the two protocols share much of the same terminology and methods for the loop-free path, the primary focus is on the current standard and the Cisco proprietary implementations of STP and RSTP. IEEE STA designates a single switch as the root bridge and uses it as the reference point for all path calculations.
In Figure , the root bridge switch S1 is chosen through an election process. For simplicity, assume until otherwise indicated that all ports on all switches are assigned to VLAN 1. The lowest BID value is determined by the combination of these three fields. After the root bridge has been determined, the STA calculates the shortest path to the root bridge. Each switch uses the STA to determine which ports to block. While the STA determines the best paths to the root bridge for all switch ports in the broadcast domain, traffic is prevented from being forwarded through the network.
The STA considers both path and port costs when determining which ports to block. The path costs are calculated using port cost values associated with port speeds for each switch port along a given path. The sum of the port cost values determines the overall path cost to the root bridge. If there is more than one path to choose from, STA chooses the path with the lowest path cost.
When the STA has determined which paths are most desirable relative to each switch, it assigns port roles to the participating switch ports. The port roles describe their relationship in the network to the root bridge and whether they are allowed to forward traffic:. Root port —A root port is selected on all non-root bridge switches on a per-switch basis.
Root ports are the switch ports closest to the root bridge, based on the overall cost to the root bridge. There can be only one root port per non-root switch. Root ports could be single-link interfaces or an EtherChannel port channel interface. Designated port —A designated port is a non-root port that is permitted to forward traffic. Designated ports are selected on a per-segment basis, based on the cost of each port on either side of the segment and the total cost calculated by STP for that port to get back to the root bridge.
If one end of a segment is a root port, then the other end is a designated port. All ports on the root bridge are designated ports. Alternate port and b ackup port —An alternate port and a backup port are in a blocking state or discarding state to prevent loops. Alternate ports are selected only on links where neither end is a root port. Only one end of the segment is blocked, while the other end remains in forwarding state, allowing for a faster transition to the forwarding state when necessary.
The port roles displayed are those defined by RSTP. The role originally defined by the Next, the interconnecting link between S2 and S3 must negotiate to see which port will become the designated port and which port will transition to alternate. As shown in Figure , every spanning-tree instance STP instance has a switch designated as the root bridge.
The root bridge serves as a reference point for all spanning-tree calculations to determine which redundant paths to block. Figure shows the BID fields. The bridge priority value is automatically assigned but can be modified. All switches in the broadcast domain participate in the election process. After a switch boots, it begins to send out BPDU frames every two seconds. The switch with the lowest BID becomes the root bridge. At first, all switches declare themselves as the root bridge.
But through the exchange of several BPDUs, the switches eventually agree on the root bridge. The receiving switch compares its current root ID with the received root ID identified in the received frames. Eventually, the switch with the lowest BID is identified as the root bridge for the spanning-tree instance.
Figure The Root Bridge. A root bridge is elected for each spanning-tree instance. It is possible to have multiple distinct root bridges for different sets of VLANs. If all ports on all switches are members of VLAN 1, then there is only one spanning-tree instance. Bridge priority is a value between 0 and 65, The default is 32, If two or more switches have the same priority, the switch with the lowest MAC address becomes the root bridge.
When the root bridge has been elected for the spanning-tree instance, STA starts determining the best paths to the root bridge. Switches send BPDUs, which include the root path cost. This is the cost of the path from the sending switch to the root bridge. It is calculated by adding the individual port costs along the path from the switch to the root bridge. When a switch receives the BPDU, it adds the ingress port cost of the segment to determine its internal root path cost. It then advertises the new root path cost to its adjacent peers.
The default port cost is defined by the speed at which the port operates. As shown in Table , 10 Gbps Ethernet ports have a port cost of 2, 1 Gbps Ethernet ports have a port cost of 4, Mbps Fast Ethernet ports have a port cost of 19, and 10 Mbps Ethernet ports have a port cost of Specifically, 1 Gbps links were assigned a port cost of 1, Mbps link a cost of 10, and 10 Mbps links a cost of Any link faster than 1 Gbps i.
As Ethernet technologies evolve, the port cost values may change to accommodate the different speeds available. The nonlinear numbers in the table accommodate some improvements to the older Ethernet standard. Although switch ports have a default port cost associated with them, the port cost is configurable. The ability to configure individual port costs gives the administrator the flexibility to manually control the spanning-tree paths to the root bridge.
To configure the port cost of an interface, enter the spanning-tree cost value command in interface configuration mode.
The value can be between 1 and ,, Example shows how to restore the port cost to the default value, 19, by entering the no spanning-tree cost interface configuration mode command. The internal root path cost is equal to the sum of all the port costs along the path to the root bridge. Paths with the lowest cost become preferred, and all other redundant paths are blocked. In Figure , the internal root path cost from S2 to the root bridge S1 using Path 1 is 19 based on Table , while the internal root path cost using Path 2 is Path 1 has a lower overall path cost to the root bridge and therefore becomes the preferred path.
STP configures the redundant path to be blocked, which prevents a loop from occurring. Use the show spanning-tree command as shown in Example to verify the root ID and internal root path cost to the root bridge. Figure Root Path Cost Example. The output generated identifies the root BID as An Alternate port provides a backup of your own Root port.
If your Root port fails, the Alternate port is allowed to immediately transition into the Forwarding state and become the new Root port in essence, the Alternate port is the one that receives the second best BPDU. The basic function of STP is to prevent bridge loops and the broadcast radiation that results from them.
These ports are allowed to immediately enter the forwarding state rather than passively wait for the network to converge. The alternative port moves to the forwarding state if there is a failure on the designated port for the segment. The port that receives the best BPDU on a bridge is the root port. And also: - "A designated switch for each LAN segment is selected. The designated switch is the one closest to the root switch through which frames are forwarded to the root.
Above topology, omnisecu. SW4 has two paths to reach the Root Switch omnisecu. Designated bridge switch is the bridge closest to the root switch through which frames will be forwarded to the root.
What is the difference between root port and designated port in STP? Category: technology and computing computer networking. What is a root port? What is Portfast? How is STP path cost calculated? How does STP protocol work? What is PCI Express root port? Only nonroot switches have root ports. The root switch does not have any root ports for the VLAN it is the root of. The PCI Express Root Port is a port on the root complex — the portion of the motherboard that contains the host bridge.
The host bridge allows the PCI ports to talk to the rest of the computer; this allows components plugged into the PCI Express ports to work with the computer.
Because of its cost and difficulty with termination, STP is rarely used in Ethernet networks. The basic function of STP is to prevent bridge loops and the broadcast radiation that results from them. A Blocked port is neither the Root port nor the Designated port, but is part of the redundant links between switches.
A Blocked port is the one that actually stops the loop, so it is just as important as the Root or Designated. PortFast causes a switch or trunk port to enter the spanning tree forwarding state immediately, bypassing the listening and learning states. When you enable PortFast on a switch or trunk port, the port is immediately transitioned to the spanning tree forwarding state. Your email address will not be published. Save my name, email, and website in this browser for the next time I comment.
Skip to content What is the difference between root port and designated port in STP? What are the root ports? What is STP and its types? Types of Spanning Tree Protocols 3. What port is the root port? What are STP port roles? Port Roles Determine Participation in the Spanning Tree Root port—The port closest to the root bridge has the lowest path cost from a bridge.
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