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Cisco Nexus 3000 Series NX-OS Layer 2 Switching Configuration Guide, Release 5.0(3)U1(1)
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Preface
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Overview
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Configuring Ethernet Interfaces
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Configuring VLANs
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Configuring Private VLANs
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Configuring Access and Trunk Interfaces
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Configuring EtherChannels
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Configuring Rapid PVST+
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Configuring Multiple Spanning Tree
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Configuring STP Extensions
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Configuring LLDP
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Configuring the MAC Address Table
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Configuring IGMP Snooping
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Configuring Traffic Storm Control
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Index
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Feedback
Contents
- Configuring Ethernet Interfaces
- Information About Ethernet Interfaces
- About the Interface Command
- About the Unidirectional Link Detection Parameter
- Default UDLD Configuration
- UDLD Aggressive and Nonaggressive Modes
- About Interface Speed
- About the Cisco Discovery Protocol
- Default CDP Configuration
- About the Error-Disabled State
- About Port Profiles
- Guidelines and Limitations for Port Profiles
- About the Debounce Timer Parameters
- About MTU Configuration
- Configuring Ethernet Interfaces
- Configuring the UDLD Mode
- Configuring Interface Speed
- Configuring the CDP Characteristics
- Enabling or Disabling CDP
- Enabling the Error-Disabled Detection
- Enabling the Error-Disabled Recovery
- Configuring the Error-Disabled Recovery Interval
- Configuring the Debounce Timer
- Configuring the Description Parameter
- Disabling and Restarting Ethernet Interfaces
- Displaying Interface Information
- Default Physical Ethernet Settings
Configuring Ethernet Interfaces
This section describes the configuration of the Ethernet interfaces on a Cisco Nexus 3000 Series switch. It contains the following sections:
- Information About Ethernet Interfaces
- Configuring Ethernet Interfaces
- Displaying Interface Information
- Default Physical Ethernet Settings
Information About Ethernet Interfaces
- About the Interface Command
- About the Unidirectional Link Detection Parameter
- About Interface Speed
- About the Cisco Discovery Protocol
- About the Error-Disabled State
- About Port Profiles
- About the Debounce Timer Parameters
- About MTU Configuration
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:
The interface numbering convention is extended to support use with a Cisco Nexus 2000 Series Fabric Extender as follows:
switch(config)# interface ethernet [chassis/]slot/port
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 3000 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.
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.
Default UDLD Configuration
The following table shows the default UDLD configuration.
Table 1 UDLD Default Configuration Feature
Default Value
UDLD global enable state
Globally disabled
UDLD aggressive mode
Disabled
UDLD per-port enable state for fiber-optic media
Enabled on all Ethernet fiber-optic LAN ports
UDLD per-port enable state for twisted-pair (copper) media
Disabled on all Ethernet 10/100 and 1000BASE-TX LAN ports
Related Information
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 frames, 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.
About Interface Speed
A Cisco Nexus 3000 Series switch has a number of fixed 10-Gigabit ports, each equipped with SFP+ interface adapters. The Cisco Nexus 5010 switch has 20 fixed ports, the first 8 of which are switchable 1-Gigabit and 10-Gigabit ports. The Cisco Nexus 5020 switch has 40 fixed ports, the first 16 of which are switchable 1-Gigabit and 10-Gigabit ports.
About the Cisco Discovery Protocol
The Cisco Discovery Protocol (CDP) is a device discovery protocol that runs over Layer 2 (the data link layer) on all Cisco-manufactured devices (routers, bridges, access servers, and switches) and allows network management applications to discover Cisco devices that are neighbors of already known devices. With CDP, network management applications can learn the device type and the Simple Network Management Protocol (SNMP) agent address of neighboring devices running lower-layer, transparent protocols. This feature enables applications to send SNMP queries to neighboring devices.
CDP runs on all media that support Subnetwork Access Protocol (SNAP). Because CDP runs over the data-link layer only, two systems that support different network-layer protocols can learn about each other.
Each CDP-configured device sends periodic messages to a multicast address, advertising at least one address at which it can receive SNMP messages. The advertisements also contain time-to-live, or holdtime information, which is the length of time a receiving device holds CDP information before discarding it. Each device also listens to the messages sent by other devices to learn about neighboring devices.
The switch supports both CDP Version 1 and Version 2.
About the Error-Disabled State
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.
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.
About MTU Configuration
The Cisco Nexus 3000 Series switch is a Layer 2 device. This means it does not fragment frames. As a result, the switch cannot have two ports in the same Layer 2 domain with different maximum transmission units (MTUs). A per-physical Ethernet interface MTU is not supported. Instead, the MTU is set according to the QoS classes. You modify the MTU by setting Class and Policy maps.
Note
When you show the interface settings, a default MTU of 1500 is displayed for physical Ethernet interfaces.
Configuring Ethernet Interfaces
The section includes the following topics:
- Configuring the UDLD Mode
- Configuring Interface Speed
- Configuring the CDP Characteristics
- Enabling or Disabling CDP
- Enabling the Error-Disabled Detection
- Enabling the Error-Disabled Recovery
- Configuring the Error-Disabled Recovery Interval
- Configuring the Debounce Timer
- Configuring the Description Parameter
- Disabling and Restarting Ethernet Interfaces
Configuring the UDLD Mode
SUMMARY STEPSYou 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.
To configure the UDLD mode, perform this task:
1. switch# configure terminal
2. switch(config)# feature udld
3. switch(config)# no feature udld
4. switch(config)# show udld global
5. switch(config)# interface type slot/port
6. switch(config-if)# udld {enable | disable | aggressive}
7. switch(config-if)# show udld interface
DETAILED STEPSThis example shows how to enable the UDLD for the switch:
switch# configure terminalswitch(config)# feature udldThis example shows how to enable the normal UDLD mode for an Ethernet port:
switch# configure terminalswitch(config)# interface ethernet 1/4switch(config-if)# udld enableThis example shows how to enable the aggressive UDLD mode for an Ethernet port:
switch# configure terminalswitch(config)# interface ethernet 1/4switch(config-if)# udld aggressiveThis example shows how to disable UDLD for an Ethernet port:
switch# configure terminalswitch(config)# interface ethernet 1/4switch(config-if)# udld disableThis example shows how to disable UDLD for the switch:
switch# configure terminalswitch(config)# no feature udldConfiguring Interface Speed
SUMMARY STEPS
1. switch# configure terminal
2. switch(config)# interface type slot/port
3. switch(config-if)# speed speed
DETAILED STEPSThe following example shows how to set the speed for a 1-Gigabit Ethernet port:
switch# configure terminal switch(config)# interface ethernet 1/4 switch(config-if)# speed 1000
Note
If the interface and transceiver speed is mismatched, the SFP validation failed message is displayed when you enter the show interface ethernet slot/port command. For example, if you insert a 1-Gigabit SFP transceiver into a port without configuring the speed 1000 command, you will get this error. By default, all ports are 10 Gigabits.
Configuring the CDP Characteristics
SUMMARY STEPSYou can configure the frequency of Cisco Discovery Protocol (CDP) updates, the amount of time to hold the information before discarding it, and whether or not to send Version-2 advertisements.
To configure CDP characteristics for an interface, perform this task:
1. switch# configure terminal
2. (Optional) switch(config)# [no] cdp advertise {v1 | v2 }
3. (Optional) switch(config)# [no] cdp format device-id {mac-address | serial-number | system-name}
4. (Optional) switch(config)# [no] cdp holdtime seconds
5. (Optional) switch(config)# [no] cdp timer seconds
DETAILED STEPSEnabling or Disabling CDP
SUMMARY STEPSYou can enable or disable 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:
1. switch# configure terminal
2. switch(config)# interface type slot/port
3. switch(config-if)# cdp enable
4. switch(config-if)# no cdp enable
DETAILED STEPSEnabling the Error-Disabled Detection
SUMMARY STEPSYou can enable error-disable (err-disabled) detection in an application. As a result, when a cause is detected on an interface, the interface is placed in an err-disabled state, which is an operational state that is similar to the link-down state.
1. config t
2. errdisable detect cause {all | link-flap | loopback}
3. shutdown
4. no shutdown
5. show interface status err-disabled
6. copy running-config startup-config
DETAILED STEPSEnabling the Error-Disabled Recovery
SUMMARY STEPSYou can specify the application to bring the interface out of the error-disabled (err-disabled) state and retry coming up. It retries after 300 seconds, unless you configure the recovery timer (see the errdisable recovery interval command).
1. config t
2. errdisable recovery cause {all | udld | bpduguard | link-flap | failed-port-state | pause-rate-limit}
3. show interface status err-disabled
4. copy running-config startup-config
DETAILED STEPSConfiguring the Error-Disabled Recovery Interval
SUMMARY STEPSYou can use this procedure to configure the err-disabled recovery timer value. The range is from 30 to 65535 seconds. The default is 300 seconds.
1. config t
2. errdisable recovery interval interval
3. show interface status err-disabled
4. copy running-config startup-config
DETAILED STEPSConfiguring the Description Parameter
Disabling and Restarting Ethernet Interfaces
SUMMARY STEPSYou 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.
To disable an interface, perform this task:
1. switch# configure terminal
2. switch(config)# interface type slot/port
3. switch(config-if)# shutdown
4. switch(config-if)# no shutdown
DETAILED STEPS
Command or Action Purpose The following example shows how to disable an Ethernet port:
switch# configure terminalswitch(config)# interface ethernet 1/4switch(config-if)# shutdownThe following example shows how to restart an Ethernet interface:
switch# configure terminalswitch(config)# interface ethernet 1/4switch(config-if)# no shutdownDisplaying Interface Information
To view configuration information about the defined interfaces, perform one of these tasks:
Command
Purpose
switch# show interface type slot/port Displays the detailed configuration of the specified interface.
switch# show interface type slot/port capabilities Displays detailed information about the capabilities of the specified interface. This option is only available for physical interfaces
switch# show interface type slot/port transceiver Displays detailed information about the transceiver connected to the specified interface. This option is only available for physical interfaces.
switch# show interface brief Displays the status of all interfaces.
switch# show interface flowcontrol Displays the detailed listing of the flow control settings on all interfaces.
The show interface command is invoked from EXEC mode and displays the interface configurations. Without any arguments, this command displays the information for all the configured interfaces in the switch.
The following example shows how to display the physical Ethernet interface:
switch# show interface ethernet 1/1 Ethernet1/1 is up Hardware is 1000/10000 Ethernet, address is 000d.eca3.5f08 (bia 000d.eca3.5f08) MTU 1500 bytes, BW 10000000 Kbit, DLY 10 usec, reliability 255/255, txload 190/255, rxload 192/255 Encapsulation ARPA Port mode is trunk full-duplex, 10 Gb/s, media type is 1/10g Input flow-control is off, output flow-control is off Auto-mdix is turned on Rate mode is dedicated Switchport monitor is off Last clearing of "show interface" counters never 5 minute input rate 942201806 bytes/sec, 14721892 packets/sec 5 minute output rate 935840313 bytes/sec, 14622492 packets/sec Rx 129141483840 input packets 0 unicast packets 129141483847 multicast packets 0 broadcast packets 0 jumbo packets 0 storm suppression packets 8265054965824 bytes 0 No buffer 0 runt 0 Overrun 0 crc 0 Ignored 0 Bad etype drop 0 Bad proto drop Tx 119038487241 output packets 119038487245 multicast packets 0 broadcast packets 0 jumbo packets 7618463256471 bytes 0 output CRC 0 ecc 0 underrun 0 if down drop 0 output error 0 collision 0 deferred 0 late collision 0 lost carrier 0 no carrier 0 babble 0 Rx pause 8031547972 Tx pause 0 resetThe following example shows how to display the physical Ethernet capabilities:
switch# show interface ethernet 1/1 capabilities Ethernet1/1 Model: 734510033 Type: 10Gbase-(unknown) Speed: 1000,10000 Duplex: full Trunk encap. type: 802.1Q Channel: yes Broadcast suppression: percentage(0-100) Flowcontrol: rx-(off/on),tx-(off/on) Rate mode: none QOS scheduling: rx-(6q1t),tx-(1p6q0t) CoS rewrite: no ToS rewrite: no SPAN: yes UDLD: yes MDIX: no FEX Fabric: yesThe following example shows how to display the physical Ethernet transceiver:
switch# show interface ethernet 1/1 transceiver Ethernet1/1 sfp is present name is CISCO-EXCELIGHT part number is SPP5101SR-C1 revision is A serial number is ECL120901AV nominal bitrate is 10300 MBits/sec Link length supported for 50/125mm fiber is 82 m(s) Link length supported for 62.5/125mm fiber is 26 m(s) cisco id is -- cisco extended id number is 4The following example shows how to display a brief interface status (some of the output has been removed for brevity):
switch# show interface brief -------------------------------------------------------------------------------- Ethernet VLAN Type Mode Status Reason Speed Port Interface Ch # -------------------------------------------------------------------------------- Eth1/1 200 eth trunk up none 10G(D) -- Eth1/2 1 eth trunk up none 10G(D) -- Eth1/3 300 eth access down SFP not inserted 10G(D) -- Eth1/4 300 eth access down SFP not inserted 10G(D) -- Eth1/5 300 eth access down Link not connected 1000(D) -- Eth1/6 20 eth access down Link not connected 10G(D) -- Eth1/7 300 eth access down SFP not inserted 10G(D) -- ...The following example shows how to display the CDP neighbors:
switch# show cdp neighbors Capability Codes: R - Router, T - Trans-Bridge, B - Source-Route-Bridge S - Switch, H - Host, I - IGMP, r - Repeater, V - VoIP-Phone, D - Remotely-Managed-Device, s - Supports-STP-Dispute Device ID Local Intrfce Hldtme Capability Platform Port ID d13-dist-1 mgmt0 148 S I WS-C2960-24TC Fas0/9 n5k(FLC12080012) Eth1/5 8 S I s N5K-C5020P-BA Eth1/5Default Physical Ethernet Settings
The following table lists the default settings for all physical Ethernet interfaces:
Parameter
Default Setting
Duplex
Auto (full-duplex)
Encapsulation
ARPA
MTU1
1500 bytes
Port Mode
Access
Speed
Auto (10000)
1 MTU cannot be changed per-physical Ethernet interface. You modify MTU by selecting maps of QoS classes.
