Ana Sayfa Software Development Loop Protection for Spanning-Tree Protocols Junos OS

Loop Protection for Spanning-Tree Protocols Junos OS


This is a lot of time of inactivity especially in large networks where topology changes may happen relatively often. Every switch port has its own path cost based on the port’s bandwidth (equal to 1000Mbps divided by the port bandwidth in Mbps as specified in the original IEEE 802.1D standard). Above, you see that SW1 has been elected as the root bridge and the “D” on the interfaces stands for designated. In a Layer 2 environment, you can configure various spanning-tree protocol versions to create a loop-free topology in Layer 2 networks. To understand bridge loops, consider a scenario in which four switches are connected to four different subsections where each subsection is a collection of network nodes . For simplicity, Subsection i and Subsection ii are combined to form Section 1.

  • The switch with the smallest Bridge Priority is automatically elected as the Root Bridge.
  • All network switches collectively elect the root bridge by comparing bridge IDs.
  • The main difference is that you can designate groups of VLANs that are all part of the same tree structure with a single common root bridge.
  • Some vendors set the bridge system ID extension to carry a VLAN ID allowing a different spanning tree per VLAN, such as Cisco’s PVST.

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. For other bridges that are not the root bridge, the port that is closest to the root bridge is put in forwarding state. All interfaces on the root bridge are put in forwarding state. This is a critical piece to how this spanning tree technology works.

STP balances the states across ports to avoid bridge looping, while still making redundancy possible. A segmented LAN is often designed with redundant bridges and paths to ensure that communications can continue in the event that a network link becomes unavailable. However, this makes the network more susceptible to looping, so a system must be put into place to prevent this possibility, which is where STP comes in. STP can help prevent bridge looping on LANs that include redundant links. Without STP, it would be difficult to implement that redundancy and still avoid network looping.

Understanding Loop Protection for Spanning-Tree Instance Interfaces

Physical paths must be added to redundant networks, but logical redundancy must also be considered. Users can access network services despite path interruption. By providing other physical paths for data to travel through the network.

Spanning tree bridges continuously transmit topology information to notify all other bridges on the network when topology changes are required, such as when a link fails. Bridge Protocol Data Units are STP information packets that bridges exchange. An alternative implementation balances traffic between the primary and backup interfaces.

This will remove all BPDU going through the interface . It should be noted that you are breaking spanning-tree at this one interface and if you have a loop, it could take down your network. But bpdu-filter is a great tool to use, if you know your topology.

Spanning-tree is a protocol that runs on our switches that helps us to solve loops. Spanning-tree is one of the protocols that you must understand as a network engineer and you will encounter it for sure if you decide to face the Cisco CCNA R&S exam. This lesson is an introduction to spanning-tree, you will learn why we need it, how it works and how you can How To Build Forms in React check the spanning-tree topology on your Cisco switches. 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. Check for a unidirectional link or excessive link flaps.

spanning tree loops

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. However, a blocking interface can transition to the forwarding state in error if the interface stops receiving BPDUs from its designated port on the segment. Grab the mac address of your gateway on the data vlan and search for it on your voice switch.

So it’s possible for devices with both versions of the protocol to operate together on the same network. The result is that network communications flow seamlessly regardless of how complex or vast a network becomes. In a way, Spanning Tree creates single paths through a network for data to travel using software in much the same way that network engineers did using hardware on the old loop networks.

Non-Designated Ports

Below are the global settings, but on individual access ports I have bpdu guard and port fast enabled. The switch inserts the forward delay timer value in BPDU packets it sends as the root bridge. The forward delay value ranges from 4 to30 seconds with a default of15 seconds.

spanning tree loops

If you need to force a switch connection, what you need to do is turn on spanning-tree bpdufilter. I would like to go over a spanning-tree option that I use many times to “fix” some switch networks, BPDU-Filter. To start talking about BPDU-Filter, we will need to first talk about what is and why have spanning-tree. A loop on a switch not participating in STP isn’t detected if that switch filters BPDUs . Values range from0 to 240 and must be a multiple of 16. Max_bpdu configures the maximum number of BPDUs per timer interval.

This flag is propagated to all other switches and instructs them to rapidly age out their forwarding table entries. A spanning-tree protocol is a Layer 2 control protocol that calculates the best path through a switched network containing redundant paths. A spanning-tree protocol uses bridge protocol data unit data frames to exchange information with other switches.

spanning-tree port-priority

It should be enabled ONLY on the uplinks to other switches that you do NOT want to become spanning tree root. Designated ports are the only ports that can receive and forward frames on switches other than the root switch. They are generally the ports that use the least-cost paths. The reason people do this is that if you don’t you have to wait until spanning tree works its magic to see if a network loop is being created. This can take up to 45 seconds which delays traffic forwarding on the port. Bridge ID numbers range from 0 to65535 ; bridges with smaller bridge IDs are elected over other bridges.

  • It is a Layer 2 protocol that runs on bridges and switches, which should be enabled on the switch interfaces.
  • It sits at Layer 2 , so it is simply concerned with moving packets to their appropriate destination, not what kind of packets are being sent, or the data that they contain.
  • However, I recommend using Per-VLAN RSTP in most cases because it’s easier to configure.
  • STP attempts to avoid this endless loop problem by removing the potential for a loop in the first place.
  • For example, 802.1D was updated in the 802.1D-2004 revision, which also incorporated 802.1w.

Fast state transitions are allowed on point-to-point links that connect bridges. Fast state transitions are not allowed on shared ports regardless of the duplex setting. The default link type for half-duplex ports isshared. Static MAC addresses should be configured after primary-backup pairs are established. When Spanning Tree detects that a primary path is no longer active, it can quickly open another path that had been previously closed. It can then send data around the trouble spot, eventually designating the detour as the new primary path, or sending packets back to the original bridge should it again become available.

What Happens if I Do Not Use Loop Protection?

Most modern Cisco switches default to a protocol called Per-VLAN RSTP. This stands for Rapid Spanning Tree Protocol. It automatically operates a separate spanning tree domain with a separate root bridge on every VLAN. In practice, it’s common to make the same switch the root bridge on all or most of the VLANs, though. If there are multiple paths, then on each switch, spanning tree selects the best path and puts all the other ports into a blocking state. In this way, there’s a single path between any two devices on the network, although it might be rather circuitous.

The administrator must also identify root ports and designated ports. A root port is a bridge port that forwards frames to the root bridge, and a designated port is a bridge port that forwards frames away from the root bridge. Unlike some proprietary per-VLAN spanning tree implementations, MSTP includes all of its spanning tree information in a single BPDU format.

These changes greatly affect the time needed for a network to converge with initially brought up and during link changes and failures; this time is very valuable in modern organizations. Look over your existing switches to determine if they support RSTP; if they do, take the next step and enable its use. The initial duty of STP is to form a relationship to the switched network on which it exists. If this is the only switch in the network, by default it would become a root bridge . This root bridge is elected initially within the STP network, and the rest of the network is built on that switch being the center of the switching universe. This also means that you don’t want the switch that gains this title to be on an edge switch, or on a switch that exists on a remote site.

It uses the BPDUs to identify redundant links and select the best data path for forwarding messages. The algorithm also controls packet ASP Tutorial forwarding by setting the port state. Unlike in STP, RSTP will respond to BPDUs sent from the direction of the root bridge.

Root Bridge (RB):

Blocking means that the interface cannot forward or receive data frames. Forwarding means that the interface can send and receive data frames. By having a correct subset of the interfaces blocked, a single currently active logical path will exist between each pair of LANs. Command specifies the configuration mode interfaces port-priority number. The switch uses this number to determine which interface it places into forwarding mode when resolving a loop.

Those have their shortcomings as well but are pretty much irreplaceable in a complex network. Connect and share knowledge within a single location that is structured and easy to search. This hello time ranges from 0.2 seconds to10 seconds with a default of2 seconds. Ports not covered by a spanning-tree bpdufilter command use the global BPDU filter setting. A switch can continue support of MLAG operation when its peer is offline and the STP agent is unavailable. The following sections describe instructions that configure BPDU packet contents and transmissions.


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