Network Loop Prevention with the Rapid Spanning Tree Protocol RSTP Overview

Spanning-tree protocol loop protection enhances the normal checks that spanning-tree protocols perform on interfaces. Loop protection performs a specified action when BPDUs are not received on a nondesignated port interface. You can choose to block the interface or issue an alarm when bridge protocol data units are not received on the port. The above rules describe one way of determining what spanning tree will be computed by the algorithm, but the rules as written require knowledge of the entire network. The bridges have to determine the root bridge and compute the port roles with only the information that they have.

If the link between the switches goes down, devices on Switch1 won’t be able to communicate with devices on Switch2 and vice versa. Part of selecting the best path is calculating the best path to the root switch is calculating the link costs, based on the speed of the links. If your switches are connected in a loop without STP, each switch would infinitely duplicate the first broadcast packet heard because there’s nothing at Layer 2 to prevent a loop. The root bridge in a spanning tree is the logical center and sees all traffic on a network.

Chapter 12. Protecting against bridging loops by using the Spanning Tree Protocol

Because the “best ports” are put into forwarding state and the other ports are put into blocking state, there are no loops in the network. When a new switch is introduced to the network, the algorithm and port states are recalculated to prevent a new loop. The root bridge is elected based on bridge ID and a priority.

It tweaks the way that STP works, in an effort to minimize converge times on switches, both on startup and on link failure. We’ll examine how RSTP improves on STP and how to configure it on a Cisco switched device. Loop guard causes the non-designated port to go into the MSTP loop inconsistent state instead of the forwarding state.

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Above, you see that SW1 has been elected as the root bridge and the “D” on the interfaces stands for designated. By default, VSTP uses the RSTP protocol as its core spanning-tree protocol, but usage of STP can be forced if the network includes old bridges. More information about configuring VSTP on Juniper Networks switches was published in the official documentation. If this protocol is in use, VLAN membership for trunk interfaces must be statically configured. In 2001, the IEEE introduced Rapid Spanning Tree Protocol as 802.1w.

I have seen very RARE cases where a blocked port is actually forwarding traffic. Once you identify which port should be blocking and confirm it is blocking via the “show span vlan detail” command you can check the interface stats with “show interface ” and look at input / output rates. Note if it is a trunk and not all vlans are blocked then this command will not be very useful. If you find a switch generating TCNs, you will want to look for two uplink ports or trunks in a spanning tree forwarding state for the same VLAN. If you find two ports in a forwarding state, shut one port down and see if this  breaks the loop.

External links

The original Spanning Tree Protocol is defined in the IEEE 802.1D 1998 specification. A newer version called Rapid Spanning Tree Protocol was originally defined in the IEEE 802.1w draft specification and later incorporated into the IEEE 802.1D-2004 specification. A recent version called Multiple Spanning Tree Protocol was originally defined in the IEEE 802.1s draft specification and later incorporated into the IEEE 802.1Q-2003 specification.

A big difference between the original STP and RSTP is in how they converge. With RSTP, a proposal and agreement (P/A) process is put in place, which is automatically used as soon as an interface comes up. This P/A process drastically shortens the time needed for an interface to move into a forwarding state. However, this process only happens on what RSTP calls point-to-point links. Occasionally a hardware or software failure can cause MSTP to fail, creating forwarding loops that can cause network failures where unidirectional links are used. The non-designated port transitions in a faulty manner because the port is no longer receiving MSTP BPDUs.

The costs shown are revised costs from the original STP default port costs. The IEEE did not anticipate the massive increase in speed now offered by Ethernet. Because of this 1Gb and 10Gb links could not be accommodated by the old default costs and the costs had to be revised. Ethernet frames don’t have a TTL value, so they will loop around forever. For example, whenever the switch doesn’t know about a destination MAC address, it will be flooded. SW1 will forward this broadcast frame on all it interfaces, except the interface where it received the frame on.

2. Rapid Spanning Tree

Sniff the CPU and see if  the packets share a common source (this is only an option on certain platforms.  You’ll need to contact TAC to assist with setting it up and analyzing the data). If they are  STP or CDP packets (or packets destined IT consulting rates per hour 2022 Latest statistics to the 0100.0CCC.CCCX reserved  multicast address) trace where the source mac is learned. Table 5 shows the components that will be configured for loop protection. Table 4 shows the components that will be configured for loop protection.

The first part is 2 bytes of information known as bridge priority. In this example, the default value is used for all the switches. The remaining 6 bytes consist of the MAC address of the switch. In this example, Switch1 is elected as the root switch because it has the lowest MAC address. Radia Perlman developed TRILL as a way to improve on the spanning tree algorithm.

Rapid Spanning Tree Protocol Operation

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• All traffic is stopped for seconds while a spanning tree calculation takes place.
• The designated port forwards data to the downstream network segment or device.
• But the STP implementation must be carefully planned and deployed to ensure that it delivers the necessary level of service.
• Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
• The first spanning tree protocol was invented in 1985 at the Digital Equipment Corporation by Radia Perlman.
• RSTP will revert to legacy STP on an interface if a legacy version of an STP BPDU is detected on that port.

As a result, the network is much less likely to come to a halt. It can complete a convergence in seconds, so it greatly diminishes the possible impact the process can have on your network compared to STP. STP and RSTP prevent loops from forming by ensuring that only one path exists between the end nodes in your network. RSTP is designed as a general replacement for the slower, legacy STP. With STP, convergence can take up to a minute to complete in a larger network.

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The spanning-tree protocol family is responsible for breaking loops in a network of bridges with redundant links. However, hardware failures can create forwarding loops and cause major network outages. However, errors occur when a blocked port transitions erroneously to a forwarding state. A blocking interface can transition to the forwarding state in error if the interface stops receiving BPDUs from its designated port on the segment. Such a transition error can occur when there is a hardware error on the switch or software configuration error between the switch and its neighbor.

Some bridges implement both the IEEE and the DEC versions of the Spanning Tree Protocol, but their interworking can create issues for the network administrator. STP is based on an algorithm that was invented by Radia Perlman while she was working for Digital Equipment Corporation. The Spanning Tree Protocol 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 tree also allows a network design to include backup links providing fault tolerance if an active link fails. 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.

Unlike some proprietary per-VLAN spanning tree implementations, MSTP includes all of its spanning tree information in a single BPDU format. Not only does this reduce the number of BPDUs required to communicate spanning tree information for each VLAN, but it also ensures backward compatibility with RSTP and, in effect, classic STP too. MSTP does this by encoding an additional region of information after the standard RSTP BPDU as well as a number of MSTI messages . Each of these MSTI configuration messages conveys the spanning tree information for each instance. Each instance can be assigned a number of configured VLANs and frames assigned to these VLANs operate in this spanning tree instance whenever they are inside the MST region. 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.

The protocol was then standardized by the Internet Engineering Task Force . TRILL uses Layer 3 network routing techniques to create a cloud of links that appear to Internet Protocol nodes as a single IP subnet. When STP is enabled, each bridge learns which computers are on which segment by sending a first-time message to network segments. Through this process, the bridge discovers the computers’ locations and records the details in a table. When subsequent messages are sent, the bridge uses the table to determine which segment to forward them to. Enabling the bridge to learn about the network on its own is known as transparent bridging, a process that eliminates the need for an administrator to set up bridging manually.

You do not want root guard on the port-channel between core switches running HSRP. It should be enabled ONLY on the uplinks to other switches that you do NOT want to become spanning tree root. Typically, root ports use the least-cost paths from one switch to the other. 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. The bridge ID is 8 bytes in length and consists of two parts.