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Why Are Network Loops a Problem?

By definition, broadcast traffic is flooded to every port on a switch within a layer 2 domain, except for the received port. With a switching loop, the broadcast traffic is transmitted around the loop indefinitely, and each looped packet is sent back to every other port in the domain. This leads to a lot of unnecessary broadcast traffic in the network, which can impact other network traffic.

Broadcast Storms

If left unchecked, you can end up with a broadcast storm, where the number of frames can eventually take your network down. A broadcast storm most commonly occurs because of a switching loop, either due to a cabling error or the failure of a protocol that protects a deliberate physical loop.

MAC Address Thrashing

Loops can also confuse the switch’s database of MAC address entries. If a loop means that you can reach a destination through two different ports (for example port 1 or port 6), the MAC address table will constantly swap (thrash) between the two.

How Do Network Loops Form?

Modern networks are often complex and distributed, so it can be difficult to know how devices are connected. This can lead to loops from human errors in cabling. But physical loops are most often added deliberately because they’re actually a good thing, for redundancy. Physical loops make your network resilient to switch or port failure, by providing alternative paths. The trick is to protect the network from the negative consequences of loops.

Protocols to manage topologies

Physical loops are useful in networks, because they provide backup, which makes a network resilient if a device or link fails. If there are multiple possible paths to each destination, if a link fails, traffic can simply change to another path.

To take advantage of this, you need to use a layer 2 protocol that manages the paths, blocking and unblocking ports as needed to create a single logical path at any time. AlliedWare Plus supports several of these protocols, for ring and other topologies.

Ring topologies are popular in networks that need a high level of resiliency. AlliedWare Plus offers two protocols to protect your ring from loops: the standards-based G.8032 Ethernet Ring Protection and our own Ethernet Protection Switching Ring (EPSR). Both can react to changes in topology as fast as 50 ms, so end users don’t even notice a link failure.

Other topologies can be protected with the popular Spanning Tree Protocol (STP). AlliedWare Plus supports the original STP protocol, the faster RSTP (Rapid STP), and MSTP (Multiple STP) for large networks. These protocols use special frames, called BPDUs, to constantly check link status and work out the best paths available. You can add BPDU Guard to provide extra protection for edge ports. RSTP is enabled by default on all switch ports.

Through all these mechanisms, Allied Telesis switches keep your network safely loop- and storm-free without compromising service or reliability.

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