Cisco Nexus 5000 Series NX-OS Software Configuration Guide

About the Interface Command

You can enable the various capabilities of the Ethernet interfaces on a per-interface basis using the interface command. When you enter the interface command, you specify the following information:

• Interface type—All physical Ethernet interfaces use the ethernet keyword.
• Slot number

– Slot 1 includes all the fixed ports.

– Slot 2 includes the ports on the upper expansion module (if populated).

– Slot 3 includes the ports on the lower expansion module (if populated),

• Port number

– Port number within the group.

Unidirectional Link Detection Parameter

This section includes the following topics:

• UDLD Overview
• Default UDLD Configuration
• UDLD Aggressive and Nonaggressive Modes

UDLD Overview

The Cisco-proprietary Unidirectional Link Detection (UDLD) protocol allows ports that are connected through fiber-optic 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 5000 Series switch 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.

Figure 1-1 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 1-1 Unidirectional Link

Default UDLD Configuration

Table 1-1 shows the default UDLD configuration.

For information on configuring the UDLD for the device and its port, see the “Configuring the UDLD Mode” section.

UDLD Aggressive and Nonaggressive Modes

UDLD aggressive mode is disabled by default. You can configure UDLD aggressive mode only on point-to-point links between network devices that support UDLD aggressive mode. If UDLD aggressive mode is enabled, when a port on a bidirectional link that has a UDLD neighbor relationship established stops receiving UDLD frame, UDLD tries to reestablish the connection with the neighbor. After eight failed retries, the port is disabled.

To prevent spanning tree loops, nonaggressive UDLD with the default interval of 15 seconds is fast enough to shut down a unidirectional link before a blocking port transitions to the forwarding state (with default spanning tree parameters).

When you enable the UDLD aggressive mode, the following occurs:

• One side of a link has a port stuck (both transmission and receive)
• One side of a link remains up while the other side of the link is down

In these cases, the UDLD aggressive mode disables one of the ports on the link, which prevents traffic from being discarded.

Interface Speed

A Cisco Nexus 5000 Series switch has a number of fixed 10-Gigabit ports, each equipped with SFP+ interface adapters. The Nexus 5010 switch has 20 fixed ports, the first eight of which are switchable 1-Gigabit/10-Gigabit ports. The Nexus 5020 switch has 40 fixed ports, the first 16 of which are switchable 1-Gigabit/10-Gigabit ports.

About the Debounce Timer Parameters

The port debounce time is the amount of time that an interface waits to notify the supervisor of a link going down. During this time, the interface waits to see if the link comes back up. The wait period is a time when traffic is stopped.

You can enable the debounce timer for each interface and specify the delay time in milliseconds.

Caution When you enable the port debounce timer the link up and link down detections are delayed, resulting in a loss of traffic during the debounce period. This situation might affect the convergence and reconvergence of some protocols.

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About MTU Configuration

A per-physical Ethernet interface maximum transmission unit (MTU) is not supported. Instead, MTU is set according to the QoS classes. You modify MTU by setting Policy and Class maps. See Chapter 1, “Configuring QoS” for more details.

When you show the interface settings, an MTU of 1500 is displayed for physical Ethernet interfaces and a receive data field size of 2112 is displayed for Fibre Channel interfaces.

Configuring the UDLD Mode

You can configure normal or aggressive unidirectional link detection (UDLD) modes for Ethernet interfaces on devices configured to run UDLD. Before you can enable a UDLD mode for an interface, you must make sure that UDLD is already enabled on the device that includes the interface. UDLD must also be enabled on the other linked interface and its device.

To use the normal UDLD mode, you must configure one of the ports for normal mode and configure the other port for the normal or aggressive mode. To use the aggressive UDLD mode, you must configure both ports for the aggressive mode.

Note Before you begin UDLD must be enabled for the other linked port and its device.

This example shows how to enable the UDLD for the switch:

This example shows how to enable the normal UDLD mode for an Ethernet port:

This example shows how to enable the aggressive UDLD mode for an Ethernet port:

This example shows how to disable UDLD for an Ethernet port:

This example shows how to disable UDLD for the switch:

Configuring Interface Speed

The first eight ports of a Nexus 5010 switch and the first 16 ports of a Nexus 5020 switch are switchable 1-Gigabit/10-Gigabit ports. The default interface speed is 10-Gigabit. To configure these ports for 1-Gigabit Ethernet, insert a 1-Gigabit Ethernet SFP transceiver into the applicable port then set its speed with the speed command.

To configure a 1-Gigabit Ethernet port, perform this task:

The following example shows how to set the speed for a 1-Gigabit Ethernet port:

This command can only be applied to a physical Ethernet interface.

Note If the interface and transceiver speed is mismatched, this shows as "SFP validation failed" when you run the show interface ethernet slot/port command. For example, if you insert 1-Gigabit SFP transceiver into a port without configuring speed 1000 you will get this error. By default, all ports are 10-Gigabit.

Configuring the CDP Parameter

You can enable or disable the Cisco Discovery Protocol (CDP) for Ethernet interfaces. This protocol works only when you have it enabled on both interfaces on the same link.

To enable or disable CDP for an interface, perform this task:

The following example shows how to enable CDP for an Ethernet port:

This command can only be applied to a physical Ethernet interface.

Configuring the Debounce Timer

You can enable the debounce timer for Ethernet ports by specifying a debounce time (in milliseconds) or disable the timer by specifying a debounce time of 0.

You can show the debounce times for all of the Ethernet ports by using the show interface debounce command.

To enable or disable the debounce timer, perform this task:

This example shows how to enable the debounce timer and set the debounce time to 1000 milliseconds for an Ethernet interface:

This example shows how to disable the debounce timer for an Ethernet interface:

This command can only be applied to a physical Ethernet interface.

Configuring the Description Parameter

To provide textual interface descriptions for the Ethernet ports, perform this task:

This example shows how to set the interface description to “Server 3 Interface”.

Disabling and Restarting Ethernet Interfaces

You can shut down and restart an Ethernet interface. This action disables all of the interface functions and marks the interface as being down on all monitoring displays. This information is communicated to other network servers through all dynamic routing protocols. When shut down, the interface is not included in any routing updates.

To disable an interface, perform this task:

The following example shows how to disable an Ethernet port:

To restart an interface, perform this task:

The following example shows how to restart an Ethernet interface:

Default Physical Ethernet Settings

The following table lists the default settings for all physical Ethernet interfaces: