What is a Loop Guard?
Quick Definition: A loop guard is a network feature that helps prevent network loops in Spanning Tree Protocol (STP) by blocking non-designated ports if BPDU (Bridge Protocol Data Unit) packets are not received.
Finding ways to optimize service while mitigating downtime is a never-ending goal in IT engineering. One of the tried-and-true ways of doing so is implementing loop guards. A loop guard does exactly what you expect—it guards against loops. Unfortunately, that's still a little cryptic.
In short, a loop guard is a protective mechanism that enhances network stability. It is critical in circular topologies like tree, mesh ring, and more. Loop guards, with Spanning Tree Protocol (STP), will prevent loops and stabilize your networks.
Let's get into the nitty-gritty of how loop guards work. This knowledge will not only help you in your day-to-day life as an engineer but is also critical for the CCNA.
What is a Loop?
To understand loop guards, let's first define a network loop in general. A loop is when data packets circulate indefinitely due to redundant or misconfigured paths on the network. Network loops can result in latency, broadcast storms, dropped packets, and more. If left unattended, loops can easily wreak havoc on a network. Loops occur on layer 2 in the OSI model.
To illustrate a loop, imagine a network with three switches (A, B, and C) connected in a triangle. If A sends a broadcast message without loop prevention, it could loop. It would travel from A to B, then from B to C, and finally back to A, consuming bandwidth and resources indefinitely. Let's discuss how STP resolves this issue.
Understanding Spanning Tree Protocol (STP) and Rapid PVST+
STP and Rapid PVST+ are pretty much the same thing. Rapid PVST+ optimizes for VLANs, which is the key difference. STP aims to prevent loops in Ethernet networks by creating a loop-free, logical topology. It does this by disabling redundant paths and keeping backup paths in case the primary link fails. This ensures network stability and redundancy.
Both PVST+ and STP use the same algorithm to mitigate looping. Let's review that algorithm step by step.
Elect a root bridge: The root bridge is the designated central switch that STP uses as a reference point. All switches calculate the quickest way to the root bridge, which is determined as the one with the lowest bridge ID. The MAC address and the switches' designated priority determine the bridge ID.
Put the root bridge in a forwarding state: When the root bridge is in a forwarding state, its only job is to simply forward packets down the tree.
The non-root switches select a root port: Each non-root switch selects one port as its root port. It is the port with the lowest cost to reach the root bridge, and the switch exclusively uses it to send traffic toward the root bridge.
Each non-root switch is designated a port for sending traffic: On each network segment, the switch with the lowest path cost to the root bridge becomes the designated switch. Its port on that segment becomes the designated port, which forwards traffic to other switches on the segment.
Block all other ports to prevent loop block: Block any remaining ports that are neither root nor designated ports. These ports do not forward traffic, preventing network loops and keeping the ports in reserve in case the topology changes.
And yeah—that's STP in a nutshell. The key difference with PVST+ is that each VLAN has its own root switch. Treating each VLAN as its own subnetwork increases the network's robustness.
What is a Loop Guard?
Loop guard is a mechanism that reduces the risk of network loops in an STP-enabled network. It accomplishes this by monitoring the reception of BPDUs (Bridge Protocol Data Units) into switches. BPDUs are packets exchanged between switches. They maintain the network's topology, elect the root bridge, and find the best traffic paths.
If a switch stops receiving BPDUs on a blocked or listening port, it may assume the path to the root bridge is down. It may then open that port to forwarding, which can cause a network loop. Loop guard prevents this by placing the port into a loop-inconsistent state if it loses BPDUs. This stops it from forwarding traffic and causing a loop.
How to Configure a Loop Guard
Luckily, configuring a loop guard on a Cisco switch isn't too difficult. There are two ways to do it.
You can configure loop guards on each switch or globally. Generally, you want to configure them globally, as the global configuration will apply to all point-to-point interfaces in the STP-enabled network.
So, let's go over that.
Switch# configure terminal
Switch(config)# interface <interface_id>
Switch(config-if)# spanning-tree loopguard default
Switch(config-if)# end
And that's about all there is to it. The key command here is spanning-tree loop guard default, which will enable the loop guard.
What are the Best Practices for Implementing Loop Guards?
Implementing loop guards isn’t complicated, but there are still a couple of points to keep in mind. Here's a quick list of best practices to help you along the way.
1. Enable Loop Guard on Point-to-Point Links
Loop guard is designed for use on point-to-point interfaces. Apply global settings to ensure it is enabled on all layer two devices.
2. Combine Loop Guard with Other STP Features
Optimize loop guard by combining it with other STP mechanisms. Here are a couple in particular:
Root Guard: Prevents switches from being elected as root in inappropriate locations.
BPDU Guard: Disables access ports if it receives unexpected BPDUs to protect them.
Use loop guard on trunk links, BPDU Guard on access ports, and Root Guard on ports where you want to prevent a switch from becoming the root bridge.
3. Document Your Configurations
To make troubleshooting easier, keep concise documentation of each interface with loop guard enabled. Also, annotate which STP technology is being used. Knowing quickly whether it is PVST+ or STP will save some time later.
Final Thoughts
Implementing a loop guard is an essential practice for maintaining network stability. It's critical for preventing disruptive loops, especially in complex, circular topologies. A loop guard adds a layer of protection to the STP and ensures your network can handle unexpected changes or failures.
To optimize network performance, follow best practices. Enable loop guard on point-to-point links. Combine it with other STP features, like BPDU Guard and Root Guard. Also, maintain thorough documentation. Using loop guard with Spanning Tree Protocol is a proactive measure. It improves your network's resilience and reliability.
Want to learn more about becoming a Network Engineer? Consider our CCNA online training.
delivered to your inbox.
By submitting this form you agree to receive marketing emails from CBT Nuggets and that you have read, understood and are able to consent to our privacy policy.