A switching loop or bridge loop occurs in

computer network A computer network is a set of computers sharing resources located on or provided by network nodes. The computers use common communication protocols over digital interconnections to communicate with each other. These interconnections are ...

s when there is more than one

layer 2 The data link layer, or layer 2, is the second layer of the seven-layer OSI model of computer networking. This layer is the protocol layer that transfers data between nodes on a network segment across the physical layer. The data link layer p ...

path between two endpoints (e.g. multiple connections between two

network switch A network switch (also called switching hub, bridging hub, and, by the IEEE, MAC bridge) is networking hardware that connects devices on a computer network by using packet switching to receive and forward data to the destination device. A netw ...

es or two ports on the same switch connected to each other). The loop creates broadcast storms as broadcasts and

multicast In computer networking, multicast is group communication where data transmission is addressed to a group of destination computers simultaneously. Multicast can be one-to-many or many-to-many distribution. Multicast should not be confused with ...

s are forwarded by switches out every

port A port is a maritime facility comprising one or more wharves or loading areas, where ships load and discharge cargo and passengers. Although usually situated on a sea coast or estuary, ports can also be found far inland, such as Ham ...

, the switch or switches will repeatedly rebroadcast the broadcast messages flooding the network. Since the layer-2 header does not include a

time to live Time to live (TTL) or hop limit is a mechanism which limits the lifespan or lifetime 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 times ...

(TTL) field, if a frame is sent into a looped topology, it can loop forever. A physical topology that contains switching or bridge loops is attractive for redundancy reasons, yet a switched network must not have loops. The solution is to allow physical loops, but create a loop-free logical topology using

link aggregation In computer networking, link aggregation is the combining ( aggregating) of multiple network connections in parallel by any of several methods, in order to increase throughput beyond what a single connection could sustain, to provide redundan ...

, shortest path bridging,

spanning tree protocol The Spanning Tree Protocol (STP) 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 al ...

TRILL TRILL (Transparent Interconnection of Lots of Links) is an Internet Standard implemented by devices called TRILL switches. TRILL combines techniques from bridging and routing, and is the application of link-state routing to the VLAN-aware cus ...

on the network switches.

Broadcasts

In the case of broadcast packets over a switching loop, the situation may develop into a

broadcast storm A broadcast storm or broadcast radiation is the accumulation of broadcast and multicast traffic on a computer network. Extreme amounts of broadcast traffic constitute a "broadcast storm". It can consume sufficient network resources so as to render ...

. In a very simple example, a switch with three ports A, B, and C has a normal

node In general, a node is a localized swelling (a "knot") or a point of intersection (a vertex). Node may refer to: In mathematics *Vertex (graph theory), a vertex in a mathematical graph *Vertex (geometry), a point where two or more curves, lines, ...

connected to port A while ports B and C are connected to each other in a loop. All ports have the same link speed and run in

full duplex A duplex communication system is a point-to-point system composed of two or more connected parties or devices that can communicate with one another in both directions. Duplex systems are employed in many communications networks, either to allow ...

mode. Now, when a broadcast frame enters the switch through port A, this frame is forwarded to all ports but the source port, i.e. ports B and C. Both frames exiting ports B and C traverse the loop in opposite directions and reenter the switch through their counterpart port. The frame received on port B is then forwarded to ports A and C, the frame received on port C to ports A and B. So, the node on port A receives two copies of its own broadcast frame while the other two copies produced by the loop continue to cycle. Likewise, each broadcast frame entering the system continues to cycle through the loop in both directions, rebroadcasting back to the network in each loop, and broadcasts accumulate. Eventually, the accumulated broadcasts exhaust the egress capacity of the links, the switch begins dropping frames, and communication across the switch becomes unreliable or even impossible.

MAC database instability

Switching loops can cause misleading entries in a switch's

media access control In IEEE 802 LAN/MAN standards, the medium access control (MAC, also called media access control) sublayer is the layer that controls the hardware responsible for interaction with the wired, optical or wireless transmission medium. The MAC sublay ...

(MAC) database and can cause endless unicast frames to be broadcast throughout the network. A loop can make a switch receive the same broadcast frames on two different ports, and alternatingly associate the sending MAC address with the one or the other port. It may then incorrectly direct traffic for that MAC address to the wrong port, effectively causing this traffic to be lost, and even causing other switches to incorrectly associate the sender's address with a wrong port as well.

Multiple frame transmissions

In a redundant switched network it is possible for an end device to receive the same frame multiple times.

TTL

Routing loop A routing loop is a common problem with various types of networks, particularly computer networks. They are formed when an error occurs in the operation of the routing algorithm, and as a result, in a group of nodes, the path to a particular destin ...

s are tempered by a

(TTL) field in

layer-3 In the seven-layer OSI model of computer networking, the network layer is layer 3. The network layer is responsible for packet forwarding including routing through intermediate routers. Functions The network layer provides the means of transf ...

packet header; Packets will circulate the routing loop until their TTL value expires. No TTL concept exists at

and packets in a switching loop will circulate until dropped, e.g. due to resource exhaustion.

References

{{DEFAULTSORT:Switching Loop Network performance Network topology