Contents
What is loop protection?
Loop protection increases the efficiency of STP, RSTP, and MSTP by preventing ports from moving into a forwarding state that would result in a loop opening up in the network. When loop protection is enabled, the spanning-tree topology detects root ports and blocked ports and makes sure both keep receiving BPDUs.
What causes a loop?
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 destination forms a loop. A Physical loop is caused by a loop link between devices. A common example is two switches with two active Ethernet links between them.
What causes a spanning tree loop?
A bridging loop or spanning tree loop caused a network outage. To break the loop you’ve pulled one of the redundant links or shut down one of the switches that are participating in the loop but now you’re unsure of what to do to both find the source of the loop and prevent it from occurring again.
How do you fix a spanning tree loop?
Action Plan:
- Implement Spanning Tree PortFast and BPDUGuard on all edge ports.
- Verify that currently the proper switch is STP root for all VLANs.
- Enable loop guard on all distribution/access layer switches*
- Enable BPDU guard on all distribution/access layer switches*
- Enable UDLD on all fiber uplinks*
Are there any other configuration to prevent possible loop?
I have enabled spanning-tree bpduguard on the Cisco 2960 but it seems like the unmanaged switch does not have spanning tree protocol. Any other configuration to prevent loop? Spanning-tree is the protocol designed to detect and prevent loops. bpduguard is intended for access ports — ports connected to end users/machines (a single node.)
How to prevent possible loop in Cisco switches?
In my experience, non-managed switches come in two flavors with regards to BPDUs. Switches that produce, relay, or forward BPDUs normally. Switches that drop or fail to relay BPDUs. In the first case, BPDU guard will disable the port the instant it sees a spanning-tree BPDU, even if that BPDU isn’t its own (thus a loop.)
Is there a way to prevent a L2 loop?
In the second case, if the switch is not participating in STP itself, this will result in a L2 loop. Depending on your environment, there are a number of ways to mitigate a loop in this situation. Feel free to mix and match as you like.
Is it bad to not write for loops?
The original title was “Never Write For-Loops Again” but I think it misled people to think that for-loops are bad. This wasn’t my intent. This article provides several alternatives for cases, IMHO, don’t need explicit for-loops, and I think it’s better not writing them, or at least, do a quick mental exercise to think of an alternative.