Interfaces and Hardware Component Configuration Guide for Cisco 8000 Series Routers, Cisco IOS XR Releases

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Interfaces and Hardware Component Configuration Guide for Cisco 8000 Series Routers, Cisco IOS XR Releases

Network interface speed and autonegotiation

Updated: September 8, 2026

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Describes autonegotiation as an IEEE 802.3 Ethernet standard that enables connected routers to determine the highest common transmission parameters, such as mode, speed, and flow control, to optimize link bandwidth utilization.

Autonegotiation is an IEEE 802.3 Ethernet standard that

  • enables a router to connect with its peer by automatically determining and negotiating to the highest common supported transmission parameters between them,

  • exchanges parameters, such as mode, speed, and flow control, between the two connected routers, and

  • supports autonegotiation on 1 G copper ports and 1 G SFP copper transceivers that allow an SFP port to connect to a copper Ethernet network at 1 Gbps using a standard RJ45 connector.

Table 1. Feature history table

Feature Name

Release Information

Feature Description

Network interface speed support with autonegotiation

 Release 25.4.1

Introduced in this release on: Fixed Systems (8010 [ASIC: A100]) (select variants only*)

You can ensure compatibility between new and legacy routers by enabling autonegotiation on the Gigabit interfaces of the connected routers. This process allows the routers to exchange key parameters such as mode, speed, and flow control, enabling them to agree on common transmission settings. As a result, the link achieves optimal bandwidth utilization.

*This feature is supported on:

  • 8011-4G24Y4H-I

  • 8011-32Y8L2H2FH

  • 8011-12G12X4Y-A/D

Network interface speed configuration methods

Configure with speed option

When you configure a 1 Gbps network interface using the speed command, the interface is forced to operate at the specified value.

Procedure

  1. Run the show interfaces GigabitEthernet interface-name command to check the existing speed.

    Example:

    #show interfaces GigabitEthernet 0/0/0/31
GigabitEthernet0/0/0/31 is up, line protocol is up
  Interface state transitions: 7
  Hardware is GigabitEthernet, address is 0035.1a00.e62c (bia 0035.1a00.e62c)
  Internet address is Unknown
  MTU 1514 bytes, BW 100000 Kbit (Max: 100000 Kbit)
     reliability 255/255, txload 0/255, rxload 0/255
  Encapsulation ARPA,
  Full-duplex, 1000 Mb/s, TFD, link type is force-up
  output flow control is off, input flow control is off
  Carrier delay (up) is 10 msec
  loopback not set,
  Last link flapped 00:00:30
  Last input 00:00:00, output 00:00:00
  Last clearing of "show interface" counters never
  30 second input rate 1000 bits/sec, 1 packets/sec
  30 second output rate 0 bits/sec, 1 packets/sec
     90943 packets input, 11680016 bytes, 0 total input drops
     0 drops for unrecognized upper-level protocol
     Received 0 broadcast packets, 90943 multicast packets
              0 runts, 0 giants, 0 throttles, 0 parity
     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort
     61279 packets output, 4347618 bytes, 0 total output drops
     Output 0 broadcast packets, 8656 multicast packets
     0 output errors, 0 underruns, 0 applique, 0 resets
     0 output buffer failures, 0 output buffers swapped out
8	carrier transitions
    Checks the existing speed. Observe that the speed is 1000 Mbps.

  2. Use interface GigabitEthernet interface-path-id , speed speed-value command to set the configuration that forces 1 Gbps interface speed to 100 Mbps.

    Example:

    
 (config)#interface GigabitEthernet 0/0/0/31
 (config-if)#speed 100
 (config-if)#commit
 (config-if)#end
    Sets the configuration that forces the 1 Gbps interface speed to 100 Mbps. If the remote end is not also configured for 100 Mbps, the link goes down.

  3. Run the show controllers GigabitEthernet interface-path-id and show interfaces GigabitEthernet interface-path-id commands to verify if the speed is configured to 100 Mbps and autonegotiation is disabled.

    Example:

    #show controllers GigabitEthernet 0/0/0/31
Operational data for interface GigabitEthernet0/0/0/31:
State:
    Administrative state: enabled
    Operational state: Up
    LED state: Green On
Phy:
    Media type: Four-pair Category 5 UTP PHY, full duplex
    Optics:
        Vendor: CISCO
        Part number: SBCU-5740ARZ-CS1
        Serial number: AVC194525HW
        Wavelength: 0 nm
MAC address information:
    Operational address: 0035.1a00.e67c
    Burnt-in address: 0035.1a00.e62c
Autonegotiation disabled.

Operational values:
    Speed: 100 Mbps       /*Gig interface speed is set to 100 Mbps */
    Duplex: Full Duplex
    Flowcontrol: None
    Loopback: None (or external)
    MTU: 1514
    MRU: 1514
    Forward error correction: Disabled
    #show interfaces GigabitEthernet 0/0/0/31
GigabitEthernet0/0/0/31 is up, line protocol is up
  Interface state transitions: 7
  Hardware is GigabitEthernet, address is 0035.1a00.e62c (bia 0035.1a00.e62c)
  Internet address is Unknown
  MTU 1514 bytes, BW 100000 Kbit (Max: 100000 Kbit)
     reliability 255/255, txload 0/255, rxload 0/255
  Encapsulation ARPA,
  Full-duplex, 100 Mb/s, TFD, link type is force-up
  output flow control is off, input flow control is off
  Carrier delay (up) is 10 msec
  loopback not set,
  Last link flapped 00:00:30
     0 output errors, 0 underruns, 0 applique, 0 resets
     0 output buffer failures, 0 output buffers swapped out
      carrier transitions
    The show output confirms that GigabitEthernet0/0/0/31 is up and its line protocol is up. This occurs when both ends of the link are operating at 100 Mbps. If the remote end is not also configured for 100 Mbps, the link goes down.

    Verifies that the speed is configured to 100 Mbps and autonegotiation is disabled.

Configure with negotiation auto option

Procedure

  1. Set the configuration for interface speed to auto-negotiate. The router sets its interface speed to match the highest common speed supported by the peer router.

    Example:

    Router#configuration
 Router(config)#interface GigabitEthernet 0/0/0/31
 Router(config-if)#negotiation auto
 Router(config-if)#commit
 Router(config-if)#end
    Sets the configuration for the interface speed to autonegotiate. The router sets its interface speed to match the highest common speed supported by the peer router. Autonegotiation is not enabled by default. Use the negotiation auto command to enable autonegotiation.

  2. Run the show controllers GigabitEthernet interface-path-id and show interfaces GigabitEthernet interface-path-id commands to verify if the speed is autonegotiated.

    Example:

    Router#show interfaces GigabitEthernet 0/0/0/31
GigabitEthernet0/0/0/31 is up, line protocol is up
  Interface state transitions: 10
  Hardware is GigabitEthernet, address is 0035.1a00.e62c (bia 0035.1a00.e62c)
  Internet address is Unknown
  MTU 1514 bytes, BW 100000 Kbit (Max: 100000 Kbit)
     reliability 255/255, txload 0/255, rxload 0/255
  Encapsulation ARPA,
  Full-duplex, 100Mb/s, TFD, link type is autonegotiation
  output flow control is off, input flow control is off
  Carrier delay (up) is 10 msec
  loopback not set,
  Last link flapped 00:00:01
  Last input 00:00:00, output 00:00:00
  Last clearing of "show interface" counters never
  30 second input rate 1000 bits/sec, 1 packets/sec
  30 second output rate 0 bits/sec, 0 packets/sec
     91005 packets input, 11687850 bytes, 0 total input drops
     0 drops for unrecognized upper-level protocol
     Received 0 broadcast packets, 91005 multicast packets
              0 runts, 0 giants, 0 throttles, 0 parity
     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort
     61307 packets output, 4350024 bytes, 0 total output drops
     Output 0 broadcast packets, 8668 multicast packets
     0 output errors, 0 underruns, 0 applique, 0 resets
     0 output buffer failures, 0 output buffers swapped out
     15 carrier transitions
     #show controllers GigabitEthernet 0/0/0/31
Operational data for interface GigabitEthernet0/0/0/31:

State:
    Administrative state: enabled
    Operational state: Up
    LED state: Green On

Phy:
    Media type: Four-pair Category 5 UTP PHY, full duplex
    Optics:
        Vendor: CISCO
        Part number: SBCU-5740ARZ-CS1
        Serial number: AVC194525HW
        Wavelength: 0 nm

MAC address information:
    Operational address: 0035.1a00.e67c
    Burnt-in address: 0035.1a00.e62c

Autonegotiation enabled:
    No restricted parameters

Operational values:
    Speed: 100 Mbps
    Duplex: Full Duplex
    Flowcontrol: None
    Loopback: None (or external)
    MTU: 1514
    MRU: 1514
    Forward error correction: Disabled
    Verifies that the speed is autonegotiated.

Configure with speed and negotiation auto options

Procedure

  1. Set the configuration for Gigabit interface speed to 100 Mbps and autonegotiate other parameters. The interface speed at remote end is set to 100 Mbps.

    Example:

    
Router(config)#interface GigabitEthernet 0/0/0/31
Router(config-if)#negotiation auto
Router(config-if)#speed 100
Router(config-if)#end

    Sets the configuration for the Gigabit interface speed to 100 Mbps and autonegotiates the other parameters. The interface speed at the remote end is set to 100 Mbps. The interface does not attempt to negotiate the speed with its peer; it simply tries to establish a link at 100 Mbps. The autonegotiation process might still occur for other parameters like duplex and flow control, but the speed itself is fixed by the speed command.

  2. Use the show controllers GigabitEthernet interface-path-id and show interfaces GigabitEthernet interface-path-id command to verify if the link is up, speed is forced to 100 Mbps, and auto-negotiation is enabled.

    Example:

    Router#show interfaces GigabitEthernet 0/0/0/31
GigabitEthernet0/0/0/31 is up, line protocol is up
  Interface state transitions: 9
  Hardware is GigabitEthernet, address is 0035.1a00.e62c (bia 0035.1a00.e62c)
  Internet address is Unknown
  MTU 1514 bytes, BW 100000 Kbit (Max: 100000 Kbit)
     reliability 255/255, txload 0/255, rxload 0/255
  Encapsulation ARPA,
  Full-duplex, 100 Mb/s, TFD, link type is autonegotiation
  output flow control is off, input flow control is off
  Carrier delay (up) is 10 msec
  loopback not set,
  Last link flapped 00:00:03
  Last input 00:00:00, output 00:00:00
  Last clearing of "show interface" counters never
  30 second input rate 0 bits/sec, 1 packets/sec
  30 second output rate 0 bits/sec, 0 packets/sec
     90968 packets input, 11683189 bytes, 0 total input drops
     0 drops for unrecognized upper-level protocol
     Received 0 broadcast packets, 90968 multicast packets
              0 runts, 0 giants, 0 throttles, 0 parity
     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort
     61287 packets output, 4348541 bytes, 0 total output drops
     Output 0 broadcast packets, 8664 multicast packets
     0 output errors, 0 underruns, 0 applique, 0 resets
     0 output buffer failures, 0 output buffers swapped out
     12 carrier transitions

    The show output confirms that GigabitEthernet0/0/0/31 is up and line protocol is up. This occurs because the speed at both ends is 100 Mbps.

    #show controllers GigabitEthernet 0/0/0/31
Operational data for interface GigabitEthernet0/0/0/31:

State:
    Administrative state: enabled
    Operational state: Up
    LED state: Green On

Phy:
    Media type: Four-pair Category 5 UTP PHY, full duplex
    Optics:
        Vendor: CISCO
        Part number: SBCU-5740ARZ-CS1
        Serial number: AVC194525HW
        Wavelength: 0 nm

MAC address information:
    Operational address: 0035.1a00.e67c
    Burnt-in address: 0035.1a00.e62c

Autonegotiation enabled:
    Speed restricted to: 100 Mbps  /* autonegotiation is enabled and speed is forced to 100 Mbps*/

Operational values:
    Speed: 100 Mbps
    Duplex: Full Duplex
    Flowcontrol: None
    Loopback: None (or external)
    MTU: 1514
    MRU: 1514
    Forward error correction: Disabled
    Verifies that the link is up, the speed is forced to 100 Mbps, and autonegotiation is enabled.

QoS with autonegotiated speeds

QoS with autonegotiated speeds is a congestion management feature that

  • manages congestion on interfaces that automatically adjust their speed—such as 10 Mbps or 100 Mbps—to connect with legacy devices,

  • addresses reduced bandwidth and increased congestion potential when operating at lower speeds, and

  • prioritizes critical applications, controls bandwidth allocation, and minimizes packet loss or delay, even when connecting to legacy devices or working in mixed-speed environments.

Table 2. Feature history table

Feature Name

Release Information

Feature Description

QoS with auto-negotiated speeds

Release 25.3.1

Introduced in this release on: Fixed Systems (8010 [ASIC: A100]) (select variants only*)

You can now maintain optimal network efficiency by applying QoS on Gigabit Ethernet interfaces that operate at lower speeds through autonegotiation. Autonegotiation detects and matches the highest common speed between connected routers, allowing seamless interoperability. With this enhancement, you can use QoS on interfaces connecting to legacy devices that support only 10 Mbps or 100 Mbps speeds. This capability improves traffic management, leading to enhanced network performance and reliability across diverse deployment scenarios.

*This feature is supported on 8011-4G24Y4H-I.

Benefits of QoS with autonegotiated speeds

Applying QoS policies on interfaces that operate at autonegotiated speeds provides the following benefits.

  • Maximized bandwidth utilization: When an interface autonegotiates to a higher speed, the QoS policy dynamically scales up its allocations and limits. This ensures that the newly available bandwidth is fully leveraged by all traffic classes according to their defined priorities, preventing under-utilization of network resources.

  • Optimized resource allocation at lower speeds: If an interface autonegotiates down to a lower speed, QoS mechanisms, such as shapers and bandwidth allocations, automatically adjust downward, preventing enforcement of rates that exceed physical link capacity. This minimizes packet drops, control plane overhead, and inconsistent state conditions.

Supported interfaces for QoS with autonegotiation

QoS with autonegotiation is supported on the following interface types.

  • Gigabit Ethernet interfaces that need to connect to legacy devices operating at 10 Mbps or 100 Mbps speeds.

  • Bundle interfaces that include Gigabit Ethernet interfaces operating at autonegotiated speeds of 10 Mbps, 100 Mbps, or 1 Gbps.

Restrictions for QoS with autonegotiated speeds

  • Absolute rate values, such as fixed, specific numerical bandwidth or rate limits, cannot be configured on bundle interfaces.

  • Hierarchical QoS (HQoS) policies are not supported on 10 or 100 M ports on main and sub-interfaces of 8011-4G24Y4H-I, 8011-32Y8L2H2FH, and 8011-32Y8L2H2FH (ASIC: A100) systems.

Enable QoS with autonegotiated speeds

The purpose of this task is to enable QoS with autonegotiated speeds using relative values on Cisco IOS XR routers.

Before you begin

Confirm that the interface you intend to configure supports autonegotiation and QoS.

Procedure

  1. Configure the interface to allow auto-negotiation.

    Example:

    
Router(config)#interface GigabitEthernet0/0/0/28
Router(config)#negotiation auto
    Configures the interface to allow autonegotiation.

  2. Define the class-maps for traffic classification.

    Example:

    
Router(config)#class-map match-any voice_traffic
Router(config-cmap)#match dscp ef
Router(config-cmap)#end-class-map
Router(config)#class-map match-any data_traffic
Router(config-cmap)#match dscp af21
Router(config-cmap)#end-class-map

  3. Define the policy map using bandwidth percentages.

    Example:

    Router(config)#policy-map adaptive_qos_policy
Router(config-pmap)#class voice_traffic
Router(config-pmap)#priority level 1
Router(config-pmap)#bandwidth percent 20  ! Allocates 20% of the *negotiated* link speed
Router(config-pmap)#class data_traffic
Router(config-pmap)#bandwidth percent 50  ! Allocates 50% of the *negotiated* link speed
Router(config-pmap)#class class-default
Router(config-pmap)#end-policy-map

    We recommend using the percentile of port speed for interfaces with auto-negotiated speeds, as this allows the policy map to automatically adjust to any changes in speed.

  4. Apply the policy to the auto-negotiating interface.

    Example:

    
Router(config)#interface GigabitEthernet0/0/0/28
Router(config)#service-policy output adaptive_qos_policy
    Applies the policy to the autonegotiating interface.

  5. Execute the show qos inconsistency detail location location command to identify any conflict between a configured QoS policy and the interface’s current operational state.

    Example:

    
Router# show qos inconsistency detail location 0/rp0/cpu0
Fri Mar 28 14:53:36.542 UTC
Inconsistency Type: Policer conform rate greater than parent reference rate
============================================================================
Interface                                    Direction      Policy Name
----------------------------------------------------------------------------
GigabitEthernet0/0/0/28                      input          p1-ing-negative

Inconsistency Type: Shape bandwidth rate greater than parent reference rate
============================================================================
Interface                                   Direction      Policy Name
----------------------------------------------------------------------------
GigabitEthernet0/0/0/28                     output         p1-negative

Autonegotiation parameters

These parameters are crucial for Ethernet transmission because they directly define how data is sent and received over the link, impacting performance, efficiency, and reliability.

  • Speed – 1G copper ports and 1G SFP copper transceivers support autonegotiation, allowing them to establish the highest common speed and duplex mode with connected routers.

  • Duplex – Full.

  • Pause – Receive part (RX) and Transmit part (TX).

Auto-Negotiation software mode

A Auto-Negotiation Clause 37 software mode is a feature that

  • enables standards-based optical link negotiation on supported 1G SFP fiber ports,

  • provides user-controlled negotiation for flexibility, and

  • improves link reliability when optics are inserted or hot-swapped.

This feature addresses previous limitations on fiber links that lacked negotiation capabilities, which could lead to link failures when connecting to devices that require auto-negotiation. Clause 37 software mode increases flexibility by enabling user-controlled negotiation, improving link establishment reliability when optics are inserted or hot-swapped.

Table 3. Feature History Table

Feature Name

Release Information

Feature Description

Auto-Negotiation software mode for 1G SFP optical ports

Release 26.3.1

Introduced in this release on: Fixed Systems (8010 [ASIC: A100]) (select variants only*)

You can enable auto-negotiation in software mode on 1G SFP optical ports, allowing your fiber interfaces to dynamically negotiate speed and duplex settings with peer devices for improved link interoperability and reduced link failures.

*This feature is supported on:

  • 8011-4G24Y4H-I

  • 8011-32Y8L2H2FH

  • 8011-12G12X4Y-A/D

Attributes and supported environments for Clause 37 software mode

Describes key attributes and supported contexts for Auto-Negotiation Clause 37 software modes:

  • Clause 37 negotiation is essential for interoperability on some fiber links.

  • Software mode is enabled or disabled via CLI using the negotiation auto command.

  • Hardware mode applies to copper optics and is always enabled (not user-configurable).

Details key differences between hardware and software modes:

  1. Hardware mode: Always active for copper optics and not configurable by users.

  2. Software mode: User configurable for SFP fiber ports, not available on copper optics.

Supported CLI commands and optic types include:

  • negotiation auto, no negotiation auto, show interfaces, and show controllers optics.

Configure Auto-Negotiation Clause 37 software mode

Configure Auto-Negotiation Clause 37 (AN37) software mode on 1G SFP optical ports to ensure standards-based negotiation of link parameters, such as speed and duplex.

AN37 software mode enables the dynamic negotiation of link parameters on 1G SFP optical ports. This feature ensures standards-based interoperation with peer devices that require auto-negotiation to maintain a correct link state.

While copper interfaces operate in a hardware-controlled mode that handles auto-negotiation natively, SFP fiber optics require user-controlled activation via the CLI. Enabling software mode increases flexibility and improves link establishment reliability when optics are inserted or hot-swapped.

Before you begin

Before you begin, ensure that you meet the following prerequisites:

  • The interface is a supported 1G SFP optical port (not a copper or PHY module), and the system platform supports AN37 software mode.

  • The installed SFP optic is a compatible fiber transceiver (for example, GLC-SX-MMD, GLC-LH-SMD, GLC-EX-SMD, or GLC-ZX-SMD).

  • The peer device requires or supports IEEE 802.3 Clause 37 auto-negotiation.

Procedure

  1. Set the configuration for interface autonegotiation. For Gigabit fiber (1000BASE-X), autonegotiation primarily exchanges duplex settings, flow control, and remote fault information, as the speed is typically fixed at 1000 Mbps.

    Example:

    
Router#configure
Router(config)#interface GigabitEthernet 0/0/0/31
Router(config-if)#negotiation auto
Router(config-if)#commit
Router(config-if)#end

    Sets the configuration for Clause 37 software mode autonegotiation. Enabling this feature increases user-controlled flexibility, ensures interoperability, and improves link establishment reliability when fiber optics are inserted or hot-swapped. Software mode is not enabled by default on fiber ports. Use the negotiation auto command to enable autonegotiation.

  2. Troubleshoot common issues related to Clause 37 on SFP optical ports.

    Example:

    Run the show controllers GigabitEthernet interface-path-id and show interfaces GigabitEthernet interface-path-id commands to verify if autonegotiation is functioning correctly for the fiber link.

    
Router#show interfaces GigabitEthernet 0/0/0/31
GigabitEthernet0/0/0/31 is up, line protocol is up
  Interface state transitions: 2
  Hardware is GigabitEthernet, address is 0035.1a00.e62c (bia 0035.1a00.e62c)
  Internet address is Unknown
  MTU 1514 bytes, BW 1000000 Kbit (Max: 1000000 Kbit)
     reliability 255/255, txload 0/255, rxload 0/255
  Encapsulation ARPA,
  Full-duplex, 1000Mb/s, link type is autonegotiation
  output flow control is off, input flow control is off
  Carrier delay (up) is 10 msec
  loopback not set,
  Last link flapped 00:05:12
  Last input 00:00:00, output 00:00:00
  Last clearing of "show interface" counters never
  30 second input rate 15000 bits/sec, 12 packets/sec
  30 second output rate 12000 bits/sec, 10 packets/sec
     151005 packets input, 18687850 bytes, 0 total input drops
     0 drops for unrecognized upper-level protocol
     Received 0 broadcast packets, 121005 multicast packets
              0 runts, 0 giants, 0 throttles, 0 parity
     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort
     91307 packets output, 14350024 bytes, 0 total output drops
     Output 0 broadcast packets, 11668 multicast packets
     0 output errors, 0 underruns, 0 applique, 0 resets
     0 output buffer failures, 0 output buffers swapped out
     2 carrier transitions

Router#show controllers GigabitEthernet 0/0/0/31
Operational data for interface GigabitEthernet0/0/0/31:

State:
    Administrative state: enabled
    Operational state: Up
    LED state: Green On

Phy:
    Media type: 1000BASE-SX, full duplex
    Optics:
        Vendor: CISCO-FINISAR
        Part number: GLC-SX-MMD
        Serial number: FNS194525HW
        Wavelength: 850 nm

MAC address information:
    Operational address: 0035.1a00.e67c
    Burnt-in address: 0035.1a00.e62c

Autonegotiation enabled:
    No restricted parameters

Operational values:
    Speed: 1000 Mbps
    Duplex: Full Duplex
    Flowcontrol: None
    Loopback: None (or external)
    MTU: 1514
    MRU: 1514
    Forward error correction: Disabled

    Verifies that Clause 37 software mode autonegotiation is enabled and the fiber link is established.

After completing these steps, AN37 software mode is confirmed operational with the link in "Up" state and auto-negotiation active. Remaining problems are identified for resolution or escalation.

Configuration guidelines and restrictions for autonegotiation and speed

Speed and autonegotiation guidelines

  • Speed can be manually set or configured to autonegotiate with the remote interface.

  • In autonegotiation mode, the interface automatically determines whether to operate at 10 Mbps, 100 Mbps, or 1 Gbps based on the capabilities of the connected device.

  • Other settings such as full duplex and pause are also negotiated during this process.

  • Autonegotiation is an optional function of the Fast Ethernet standard that enables devices to automatically exchange information about speed and duplex abilities.

  • It is especially useful for ports where devices with different capabilities are connected and disconnected regularly.

  • Autonegotiation is enabled, by default, on integrated copper ports and copper transceivers on most devices.

  • Do not enable autonegotiation when you configure a link on K100-based systems. Ensure autonegotiation is disabled on both ends of the link.

  • For Cisco 8711-32FH-M routers, an autonegotiation mismatch between source and destination interfaces does not cause the link to go down; the link remains active. To avoid potential loss of traffic due to an autonegotiation mismatch, enable autonegotiation on both source and destination interfaces.