Configuring Layer 2 Interfaces

About the Unidirectional Link Detection Parameter

About the Unidirectional Link Detection Parameter

The Cisco-proprietary Unidirectional Link Detection (UDLD) protocol allows ports that are connected through fiber optics or copper (for example, Category 5 cabling) Ethernet cables to monitor the physical configuration of the cables and detect when a unidirectional link exists. When the switch detects a unidirectional link, UDLD shuts down the affected LAN port and alerts the user. Unidirectional links can cause a variety of problems, including spanning tree topology loops.

UDLD is a Layer 2 protocol that works with the Layer 1 protocols to determine the physical status of a link. At Layer 1, autonegotiation takes care of physical signaling and fault detection. UDLD performs tasks that autonegotiation cannot perform, such as detecting the identities of neighbors and shutting down misconnected LAN ports. When you enable both autonegotiation and UDLD, Layer 1 and Layer 2 detections work together to prevent physical and logical unidirectional connections and the malfunctioning of other protocols.

A unidirectional link occurs whenever traffic transmitted by the local device over a link is received by the neighbor but traffic transmitted from the neighbor is not received by the local device. If one of the fiber strands in a pair is disconnected, as long as autonegotiation is active, the link does not stay up. In this case, the logical link is undetermined, and UDLD does not take any action. If both fibers are working normally at Layer 1, then UDLD at Layer 2 determines whether those fibers are connected correctly and whether traffic is flowing bidirectionally between the correct neighbors. This check cannot be performed by autonegotiation, because autonegotiation operates at Layer 1.

A Cisco Nexus device periodically transmits UDLD frames to neighbor devices on LAN ports with UDLD enabled. If the frames are echoed back within a specific time frame and they lack a specific acknowledgment (echo), the link is flagged as unidirectional and the LAN port is shut down. Devices on both ends of the link must support UDLD in order for the protocol to successfully identify and disable unidirectional links.

Note By default, UDLD is locally disabled on copper LAN ports to avoid sending unnecessary control traffic on this type of media.

The following figure shows an example of a unidirectional link condition. Device B successfully receives traffic from Device A on the port. However, Device A does not receive traffic from Device B on the same port. UDLD detects the problem and disables the port.

Figure 1: Unidirectional Link

Default UDLD Configuration

The following table shows the default UDLD configuration.

Cisco Nexus 5000 Series NX-OS Interfaces Configuration Guide, Release 5.2(1)N1(1)

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Cisco Systems N5KC5596TFA manual About the Unidirectional Link Detection Parameter, Default Udld Configuration
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N5KC5596TFA specifications

The Cisco Systems N5KC5596TFA is a high-performance data center switch designed specifically for modern architectures requiring low-latency, high-bandwidth connectivity. As part of the Nexus 5000 series, the N5KC5596TFA addresses the needs of virtualized environments, cloud computing, and highly dynamic workloads, making it an essential choice for enterprises and service providers.

One of the standout features of the N5KC5596TFA is its impressive port density. The switch offers 48 10 Gigabit Ethernet (10GbE) ports and 4 40 Gigabit Ethernet (40GbE) uplinks, providing flexibility to connect a variety of devices and ensuring high throughput across the network. This port configuration is ideal for connecting multiple servers and storage devices, making it perfect for environments with significant data traffic.

The switch supports a range of advanced technologies such as Virtual Extensible LAN (VXLAN) for improving scalability and multi-tenancy in data center networks. VXLAN encapsulates Layer 2 Ethernet frames within Layer 4 UDP packets, allowing for a larger address space and effective overlay networking, which is crucial for cloud deployments.

Another notable aspect of the N5KC5596TFA is its support for Data Center Bridging (DCB), which enhances the reliability and performance of Ethernet networks. DCB features like Priority Flow Control (PFC) and Enhanced Transmission Selection (ETS) help to minimize data loss and ensure Quality of Service (QoS) for various types of traffic, including storage and regular data, which is critical in a converged infrastructure.

Security is also a priority with the N5KC5596TFA, as it comes equipped with features such as Access Control Lists (ACLs) and port security, helping to safeguard the network from unauthorized access and potential threats.

In terms of management, the switch offers integration with Cisco's Data Center Network Manager (DCNM), providing tools for monitoring, management, and automation, which simplifies operational tasks. Additionally, it supports Cisco's Application Centric Infrastructure (ACI), enabling a more comprehensive and application-focused approach to networking.

Overall, the Cisco Systems N5KC5596TFA is engineered for organizations looking to build robust, highly efficient, and secure data center environments. Its combination of high density, versatile connectivity options, and advanced networking technologies makes it a powerful solution for meeting the demands of today’s enterprise workloads.
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