Switching loop
Encyclopedia
A Switching loop or Bridge loop occurs in computer network
s when there is more than one Layer 2 (OSI model
) path between two endpoints (e.g. multiple connections between two network switch
es or two ports on the same switch connected to each other). The loop creates broadcast radiation
as broadcasts and multicast
s are forwarded by switches out every port
, the switch or switches will repeatedly rebroadcast the broadcast messages flooding the network. Since the Layer 2 header does not support a time to live
(TTL) value, if a frame is sent into a looped topology, it can loop forever.
A physical topology that contains switching or bridge loop is necessary for reliability, yet a switched network must not have loops. The solution is to allow physical loops, but create a loop-free logical topology using the spanning tree protocol
(STP) on the switches.
) over a switching loop, the situation may develop into a broadcast storm.
(MAC) database and can cause endless unicast frames
to be broadcast throughout the network. In a redundant switched network it is possible for switches to learn the wrong information. A switch can incorrectly learn that a MAC address is on one port, when it is actually on a different port.
(TTL) value expires as no TTL concept exists at Layer 2.
Computer network
A computer network, often simply referred to as a network, is a collection of hardware components and computers interconnected by communication channels that allow sharing of resources and information....
s when there is more than one Layer 2 (OSI model
OSI model
The Open Systems Interconnection model is a product of the Open Systems Interconnection effort at the International Organization for Standardization. It is a prescription of characterizing and standardizing the functions of a communications system in terms of abstraction layers. Similar...
) path between two endpoints (e.g. multiple connections between two network switch
Network switch
A network switch or switching hub is a computer networking device that connects network segments.The term commonly refers to a multi-port network bridge that processes and routes data at the data link layer of the OSI model...
es or two ports on the same switch connected to each other). The loop creates broadcast radiation
Broadcast radiation
Broadcast radiation is the accumulation of broadcast and multicast traffic on a computer network. Extreme amounts of broadcast traffic constitute a broadcast storm. A broadcast storm can consume sufficient network resources so as to render the network unable to transport normal traffic.-Causes:Most...
as broadcasts and multicast
Multicast
In computer networking, multicast is the delivery of a message or information to a group of destination computers simultaneously in a single transmission from the source creating copies automatically in other network elements, such as routers, only when the topology of the network requires...
s are forwarded by switches out every port
Computer port (hardware)
In computer hardware, a port serves as an interface between the computer and other computers or peripheral devices. Physically, a port is a specialized outlet on a piece of equipment to which a plug or cable connects...
, the switch or switches will repeatedly rebroadcast the broadcast messages flooding the network. Since the Layer 2 header does not support a time to live
Time to live
Time to live is a mechanism that limits the lifespan of data in a computer or network. TTL may be implemented as a counter or timestamp attached to or embedded in the data. Once the prescribed event count or timespan has elapsed, data is discarded. In computer networking, TTL prevents a data...
(TTL) value, if a frame is sent into a looped topology, it can loop forever.
A physical topology that contains switching or bridge loop is necessary for reliability, yet a switched network must not have loops. The solution is to allow physical loops, but create a loop-free logical topology using the spanning tree protocol
Spanning tree protocol
The Spanning Tree Protocol is a network protocol that ensures a loop-free topology for any bridged Ethernet local area network. The basic function of STP is to prevent bridge loops and ensuing broadcast radiation...
(STP) on the switches.
Broadcasts
In the case of broadcast packets (broadcast radiationBroadcast radiation
Broadcast radiation is the accumulation of broadcast and multicast traffic on a computer network. Extreme amounts of broadcast traffic constitute a broadcast storm. A broadcast storm can consume sufficient network resources so as to render the network unable to transport normal traffic.-Causes:Most...
) over a switching loop, the situation may develop into a broadcast storm.
MAC database instability
Switching loops can cause misleading entries in a switch's Media Access ControlMedia Access Control
The media access control data communication protocol sub-layer, also known as the medium access control, is a sublayer of the data link layer specified in the seven-layer OSI model , and in the four-layer TCP/IP model...
(MAC) database and can cause endless unicast frames
Call stack
In computer science, a call stack is a stack data structure that stores information about the active subroutines of a computer program. This kind of stack is also known as an execution stack, control stack, run-time stack, or machine stack, and is often shortened to just "the stack"...
to be broadcast throughout the network. In a redundant switched network it is possible for switches to learn the wrong information. A switch can incorrectly learn that a MAC address is on one port, when it is actually on a different port.
Multiple frame transmissions
In a redundant switched network it is possible for an end device to receive the same frame multiple times.Misinterpretations
It is not true that within a switching loop packets will circulate the network until their time to liveTime to live
Time to live is a mechanism that limits the lifespan of data in a computer or network. TTL may be implemented as a counter or timestamp attached to or embedded in the data. Once the prescribed event count or timespan has elapsed, data is discarded. In computer networking, TTL prevents a data...
(TTL) value expires as no TTL concept exists at Layer 2.