Layer 2 interfaces have the following configuration guidelines and limitations:

• Auto-negotiation is not supported.

• 1G autonegotiation not supported on N3K-C36180YC-R and N9K-X96136YC-R switches. To work around this issue, you must manually set speed to 1000. If autonegotiation is enabled on the neighbors, you must disable autonegotiation on those neighbors.

• On Cisco Nexus N3K-C3636C-R and N3K-C36180YC-R switches, auto-negotiation may not work on ports 49-64 when bringing up 100G links using QSFP-100G-CR4 cable. To work around this issue, you must hard-code the speed on ports 49-64 and disable auto-negotiation

Cisco Nexus 36180YC-R switches have 48 small form-factor pluggable (SFP) ports with a default speed of 10 G and 6 quad small form-factor pluggable (QSFP) ports with a default speed of 100 G. 48 SFP interface ports can support 25 G, 10 G, 1 G speeds. 6 QSFP interface ports can support 100 G and 40 G speeds.

In the first 48 ports, each 4 ports in the port group must have the same speed configured. You cannot configure one port at a time which might result in an error. For more information, see CSCve80686.

Note	The breakout ports are in administratively enabled state after the breakout of the 40G ports into 4x10G mode or the breaking of the 100G ports into 4x25G mode. On upgrade from the earlier releases, the configuration restored takes care of restoring the appropriate administrative state of the ports.

Note	When you break out from 40-Gigabit Ethernet to 10-Gigabit Ethernet, or break in from 10-Gigabit Ethernet to 40-Gigabit Ethernet, all interface configurations are reset, and the affected ports are administratively unavailable. To make these ports available, use the no shut command.

Note

A new QSFP+ 40-Gb transceiver is supported on the Cisco Nexus 3600 platform switches. The new QSFP+ (40-Gb) transceiver has a cable that splits into four 10Gb SFP-10G-LR transceivers. To use it, you need the port to be in 4x10G mode. If you are using the breakout cable, you need to run that 40G port in 4x10G mode.

The ability to break out a 40-Gigabit Ethernet port into four 10-Gigabit Ethernet ports and break in four 10-Gigabit Ethernet ports into a 40-Gigabit Ethernet port dynamically allows you to use any of the breakout-capable ports to work in the 40-Gigabit Ethernet or 10-Gigabit Ethernet modes without permanently defining them.

The Switch Virtual Interface (SVI) represents a logical interface between the bridging function and the routing function of a VLAN in the device. By default, when a VLAN interface has multiple ports in the VLAN, the SVI goes to the down state when all the ports in the VLAN go down.

Autostate behavior is the operational state of an interface that is governed by the state of the various ports in its corresponding VLAN. An SVI interface on a VLAN comes up when there is at least one port in that vlan that is in STP forwarding state. Similarly, this interface goes down when the last STP forwarding port goes down or goes to another STP state.

By default, Autostate calculation is enabled. You can disable Autostate calculation for an SVI interface and change the default value.

An interface is in the error-disabled (err-disabled) state when the inteface is enabled administratively (using the no shutdown command) but disabled at runtime by any process. For example, if UDLD detects a unidirectional link, the interface is shut down at runtime. However, because the interface is administratively enabled, the interface status displays as err-disabled. Once an interface goes into the err-disabled state, you must manually reenable it or you can configure an automatic timeout recovery value. The err-disabled detection is enabled by default for all causes. The automatic recovery is not configured by default.

When an interface is in the err-disabled state, use the errdisable detect cause command to find information about the error.

You can configure the automatic err-disabled recovery timeout for a particular err-disabled cause by changing the time variable.

The errdisable recovery cause command provides automatic recovery after 300 seconds. To change the recovery period, use the errdisable recovery interval command to specify the timeout period. You can specify 30 to 65535 seconds.

To disable recovery of an interface from the err-disabled state, use the no errdisable recovery cause command.

If you do not enable the err-disabled recovery for the cause, the interface stays in the err-disabled state until you enter the shutdown and no shutdown commands. If the recovery is enabled for a cause, the interface is brought out of the err-disabled state and allowed to retry operation once all the causes have timed out. Use the show interface status err-disabled command to display the reason behind the error.

You can use the default interface feature to clear the configured parameters for both physical and logical interfaces such as the Ethernet, loopback, management, VLAN, and the port-channel interface.

The debounce timer delays notification of a link change, which can decrease traffic loss due to network reconfiguration. You can configure the debounce timer separately for each Ethernet port and specify the delay time in milliseconds. The delay time can range from 0 milliseconds to 5000 milliseconds. By default, this parameter is set for 100 milliseconds, which results in the debounce timer not running. When this parameter is set to 0 milliseconds, the debounce timer is disabled.

Caution

Enabling the debounce timer causes the link-down detections to be delayed, which results in a loss of traffic during the debounce period. This situation might affect the convergence and reconvergence of some Layer 2 and Layer 3 protocols.