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We know that a Layer 2 loop can be devastating to the network. The STP Toolkit feature of LoopGuard is there to help ensure this condition never occurs. How does it do this? It prevents a port from going to a forwarding state in the event of a loss of BPDUs received. The idea behind this feature is that we will sacrifice the ability of the network to “self heal” and converge around a failure to ensure that the network never incorrectly converges and creates a loop.
For example, in the event of a duplex mismatch, BPDUs might not show up on a port, even though there is intermittent connectivity that could still produce a loop. LoopGuard ensures the loop condition does not happen by placing a port that would have converged into forwarding into a loop inconsistent state. This is a blocking port state, and the admin is alerted via a system message. Should an admin correct the duplex mismatch and BPDUs be received properly – the loop inconsistent port state can be dynamically resolved.
Now a big point of issue here for the written exam is the LoopGuard feature and its “overlap” with another STP-related feature – Unidirectional Link Detection or UDLD. You see LoopGuard can indeed protect against a loop caused by a unidirectional link just as UDLD can. So how do you know when to recommend one over the other? Here is what you need to keep in mind…UDLD is better at catching unidirectional conditions on links as a result of miswiring, while LoopGuard is better at preventing links caused by an overall much wider variety of circumstances. For example, LoopGuard could prevent the loop as a result of the CPU being too busy to catch and process the BPDUs that are being received.
In the official Cisco documentation, Cisco points out some key differences between UDLD and LoopGuard that I want you to be aware of:
- LoopGaurd works per VLAN; UDLD does not
- UDLD cannot auto recover without taking advantage of the Error Disable Recovery feature
- UDLD cannot catch loops from non-wiring issues
- LoopGaurd is not as good at catching miswiring issues
In an upcoming blog post here, we will go into more detail for you on UDLD.
Anthony Sequeira CCIE, CCSI
Twitter: @compsolv
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Tags: CCIE, CCIE R&S, ccie r&s written, exam, lab, layer 2, prep, Written
Very nicely explained Anthony !