RSTP was then incorporated into IEEE 802.1D-2004 making the original STP standard obsolete. RSTP was designed to be backward-compatible with standard STP. Stelios Antoniou holds a BSc in Electronic Engineering and an MSc in Communication Networks. How to Optimize Query Performance in MySQL Databases He has over three years of experience in teaching MS Office applications, networking courses and GCE courses in Information Technology. Stelios is currently working as a VoIP Engineer in a Telecom company, where he uses his knowledge in practice.

For loop avoidance under switchport backup mode, use the Loop Protection feature. These STP commands, when they do not include an optional MST or VLAN parameter, apply to RSTP. Commands that configure MSTP instance 0 also apply to the RSTP instance. Ports at the edge of an MST region connecting to a bridge or to an endpoint are boundary ports.

Non-Root Bridge (NRB):

I need to find some way in Bridge 6 to get that direction. And so what happens is it reconfigures itself on the fly and changes. The root port now on Bridge 5 swaps over so that now it passes through Network Y, and Bridge 11 recognizes that and gets rid of its blocked port there. And now Network C and Network Y are directly connected to each other, and we still maintain communication from Network A back up to the root or anywhere else on the network if we want to. Alternatively the network administrator can configure the switch as a spanning tree root primary or secondary. When configuring the root primary and root secondary the switch will automatically change the priority accordingly, and respectively with the default configuration.

In the PVST mode, when the VLAN membership of a port is changed by a significant margin, it is advisable to disable interface BPDU rate limit on both ends of a port. For example, if three VLANs are present on a port initially, the operator must first add 300 more VLANs on one side of the port and then add the same 300 VLANs on the other side of the port. In this case, if the VLANs are increased towards the root bridge first, then the other side can cross the rate-limit threshold. The spanning-tree portfast command programs access ports to immediately enter the forwarding state. PortFast connects devices attached to an access port, such as a single workstation, to the network immediately without waiting for STP convergence. PortFast allows devices to gain immediate network access before convergence of the spanning tree.

The port does not participate in frame forwarding and discards frames received from the attached network segment. However, the port continues to listen for and process BPDUs. The root bridge is elected based on bridge ID and a priority.

Not configuring spanning tree at all

It’s also worth remembering that the switch is designated as the spanning-tree protocol instance’s root bridge. The network administrator can change the switch priority that we will see when we begin the configurations. Disabling STP is not recommended, even in topologies free of physical loops; STP guards against configuration mistakes and cabling errors. When disabling STP, ensure that there are no physical loops in the VLAN.When disabling STP on a VLAN, ensure that all switches and bridges in the network disable STP for the same VLAN. Command configures MSTP PVST border feature to automatically detect border ports facing PVST+ regions. By default, spanning-tree mst pvst border is disabled.

Detection of root switch failure is done in 3 hello times, which is 6 seconds if the default hello times have not been changed. The Path Cost is added to the received Root Path Cost for each BPDU received. Root switch has Root Path Cost of zero for all its ports.

spanning-tree root

In the example below, on both Acc3 and Acc4, the available uplink is the one facing towards CD1. In this Cisco CCNA tutorial, you’ll learn why we have the Spanning Tree protocol. To understand why we need to have it, I need to first start off with a review of ethernet path selection.

spanning tree loops

Now the packet goes up to the root bridge, Core Switch A, gets routed between the VLANs, and immediately switched out to the downstream device. It doesn’t cross the backbone at all in either direction. But for some reason, a lot of switch vendors disable it by default. So out of the box, you might have to enable the protocol.

Each switch is assigned a unique bridge ID number for each instance. All network switches collectively elect the root bridge by comparing bridge IDs. The root bridge is the switch with the lowest bridge ID. The following sections describe the supported STP versions, compatibility issues in networks containing switches running different STP versions, and supported alternatives to spanning tree. RSTP differs internally from its much older parent, STP, in a number of ways.

show spanning-tree topology

When having multiple root switches in a network used to provide this redundancy, the problem that arises is infinite loops between the switches and the end workstations they’re connected to. If a loop exists in the network topology, the potential exists for duplication of messages. When loops occur, some switches see stations appear on both sides of the switch. This confuses the forwarding algorithm and allows duplicate frames to be forwarded. Both ends of the link should ideally have bridge assurance enabled.

spanning tree loops

The BPDU filter default setting for portfast ports is configured by the spanning-tree edge-port bpdufilter default command; BPDU filter is disabled by default on all non-portfast ports. Loop guard, when enabled globally, applies to all point-to-point ports. Loop guard is configurable on individual ports and applies to all STP instances of an enabled port. Loop-inconsistent ports transition to learning state when loop guard is disabled.

If we do have loops on the network with all that broadcast traffic going around, we’re going to get a broadcast storm. The network will crash because the amount of looping broadcast traffic will quickly overwhelm the switches, CPU, and bandwidth. You can see the ARP request comes in, Acc3 sends it to CD2, CD2 sends it to CD1, CD1 to Acc3, Acc3 back to CD2, and so on. We have loops running News and resources for web developers in both directions, both clockwise and counterclockwise between Acc3, CD1, and CD2. Still, it doesn’t stop there because the broadcast traffic was also forwarded out interface FastEthernet 0/21 by switch CD1. It floods the broadcast traffic out all ports apart from the one it was received on and that includes port FastEthernet 0/21 which is facing back towards switch Acc3, again.

Enabling PortFast on ports connected to another switch can create loops. STP uses the port priority interface parameter to select ports when resolving loops. The port with the lower port priority numerical value is placed in forwarding mode.

We’ve got our example network topology on the left-hand side here again. Our Layer 2 ethernet path selection is controlled by the switch’s MAC address tables. Switch#configure terminal – It is used to enter the global configuration mode. Switch#spanning-tree vlan vlan-id root secondary – We use this to Change the switch’s status to a secondary root switch.

In this example, assume that all paths have similar costs. Therefore, the root port for Switch 2 is the port that receives packets through the direct path from Switch 1 , because the other path is through Switch 3 (cost 4 + 4) as shown in Figure 3. Similarly, for Switch 3, the root port is the one that uses the direct path from Switch 1. By default, a spanning-tree protocol interface that stops receiving bridge protocol data unit data frames will transition to the designated port state, creating a potential loop.

Assuming the root bridge is at the end of a long daisy chain of switches, the most important question is how long the outage will be. And in that case I would actually recommend setting up an after hours test and measuring it on your network. I have a question relating to your example where you have 2 core switches 4096 and 8192. What bridge priority should the other distribution switches connecting to the core be?

In the event that a link in the network goes down, Spanning Tree will failover to the alternate link, if one exists. However, in Ethernet switched environments where multiple VLANs exist, it is often desirable to create multiple spanning trees so that traffic on different VLANs uses different links. 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.

The first two bytes are the bridge priority, an unsigned integer of 0–65,535. The last six bytes are a MAC address supplied by the bridge. Prior to IEEE 802.1D-2004, the first two bytes gave a 16-bit bridge priority. Since IEEE 802.1D-2004, the first four bits are a configurable priority, and the last twelve bits carry the bridge system ID extension. In the case of MST, the bridge system ID extension carries the MSTP instance number.

We have also set the switch priority and changed the switch’s status to a secondary root switch to lower the switch’s priority value. Ports not receiving a BPDU packet within a hello time period enter inconsistent state. In this case, the show spanning-tree transmit activecommand will show a bridge assurance status of inconsistent for the port. If the other side of the link has bridge assurance enabled, or if the other switch is the root bridge, it will send periodic BPDUs, preventing an inconsistent blocking state.

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