Comparing Loop Guard and UDLD

Loop Guard

UDLD

Configuration

Per port

Per port

Action granularity

Per VLAN

Per Port

Autorecovery

Yes

Yes, with errdisable timeout feature

Protection against STP failures caused by unidirectional links

Yes, when enabled on all root and alternative ports in redundant topology

Yes, when enabled on all links in redundant topology

Protection against STP failures caused by problem in software, resulting in designated switch not sending BPDU

Yes

No

Protection against miswiring

No

Yes

The functions of UDLD and loop guard partially overlap in that both protect against STP failures caused by unidirectional links. These two features are different in their approach to the problem and also in the way they function. The figure identifies the key differences.

Depending on various design considerations, you can choose either UDLD or loop guard. UDLD provides no protection against STP failures that are caused by software and that result in the designated switch not sending BPDUs. This type of failure, however, is less common than problems caused by hardware failure.

On an EtherChannel bundle, UDLD will disable individual failed links. The channel itself remains functional if other links are available. Loop guard will put the entire channel in a loop-inconsistent state if any physical link in the bundle fails.

Loop guard does not work on shared links or a link that has been unidirectional since its initial setup. Enabling both UDLD and loop guard provides the highest level of protection.

Minimizing Service Loss and Data Theft in a Campus Network

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