Basic Concepts

Interface Types

Devices exchange data and interact with other devices on a network through interfaces. Interfaces are classified into physical and logical interfaces.

• Physical Interfaces

  Physical interfaces physically exist on boards. They are divided into the following types:

  • LAN interfaces: interfaces through which the router can exchange data with other devices on a LAN.
  • WAN interfaces: interfaces through which the router can exchange data with remote devices on external networks.

• Logical Interfaces

  Logical interfaces are manually configured interfaces that do not exist physically. Logical interfaces can be used to exchange data.

Interface Views and Prompts

Table 1 lists the commands, views, and prompts of physical interfaces supported by the NetEngine 8000 F. Table 2 lists the commands, views, and prompts of logical interfaces supported by the NetEngine 8000 F.

Commonly-used Link Protocols and Access Technologies

The link layer is responsible for accurately sending data from a node to a neighboring node. It receives packets from the network layer, encapsulates the packets in frames, and then sends the frames to the physical layer.

Major link layer protocols supported by the NetEngine 8000 F are listed as follows:

• Ethernet

  Currently, the LAN mostly refers to the Ethernet. The Ethernet is a broadcast network, which is flexible and simple in configuration as well as easy to expand. For these reasons, the Ethernet is widely used.

• Trunk

  The trunk technology has the following advantages:

  • Bandwidth increase: The bandwidth of a trunk is the total bandwidth of all member interfaces.
  • Reliability enhancement: When a link fails, other links in the same trunk automatically take over the services on the faulty link to prevent traffic interruption.

MTU

The maximum transmission unit (MTU) is the size (in bytes) of the longest packet that can be transmitted on a physical network. The MTU is very important for interworking between two devices on a network. If the size of a packet exceeds the MTU supported by a transit node or a receiver, the transit node or receiver may fragment the packet before forwarding it or may even discard it, increasing the network transmission loads. MTU values must be correctly negotiated between devices to ensure that packets reach the receiver.

• If fragmentation is disallowed, packet loss may occur during data transmission at the IP layer. To ensure that long packets are not discarded during transmission, configure forcible fragmentation for long packets.

• When an interface with a small MTU receives long packets, the packets have to be fragmented. Consequently, when the quality of service (QoS) queue becomes full, some packets may be discarded.

• If an interface has a large MTU, packets may be transmitted at a low speed.

Loopback

The physical interface of the router supports loopback local and loopback remote. The following figure shows the two loopback paths.

Figure 1 Local loopback

Control-Flap

The status of an interface on a device may alternate between up and down for various reasons, including physical signal interference and incorrect link layer configurations. The changing status causes Multiprotocol Label Switching (MPLS) and routing protocols to flap. As a result, the device may break down, causing network interruption. Control-flap controls the frequency of interface status alternations between up and down to minimize the impact on device and network stability.

The following two control modes are available.

• control-flap

  Interface flapping control controls the frequency of interface status alternations between Up and Down to minimize the impact on device and network stability.

  Interface flapping suppression involves the following concepts:

  • Penalty value and threshold

    An interface is suppressed or freed from suppression based on the penalty value.

    • Penalty value: This value is calculated based on the status of the interface using the suppression algorithm. The penalty value increases with the changing times of the interface status and decreases with the half life.
    • Suppression threshold (suppress): The interface is suppressed when the penalty value is greater than the suppression threshold.
    • Reuse threshold (reuse): The interface is no longer suppressed when the penalty value is smaller than the reuse threshold.
    • Ceiling threshold (ceiling): The penalty value no longer increases when the penalty value reaches the ceiling threshold.

    The parameter configuration complies with the following rule: reuse threshold (reuse) < suppression threshold (suppress) < maximum penalty value (ceiling).

  • Half life

    When an interface goes down for the first time, the half life starts. A device matches against the half life based on the actual interface status. If a specific half life is reached, the penalty value decreases by half. Once a half life ends, another half life starts.

    • Half life when an interface is up (decay-ok): When the interface is up, if the period since the end of the previous half life reaches the current half life, the penalty value decreases by half.
    • Half life when an interface is down (decay-ng): When the interface is down, if the period since the end of the previous half life reaches the current half life, the penalty value decreases by half.
  • Maximum suppression time: The maximum suppression time of an interface is 30 minutes. When the period during which an interface is suppressed reaches the maximum suppression time, the interface is automatically freed from suppression.
  • You can set the preceding parameters to restrict the frequency at which an interface alternates between up and down.

  Comply with the following rules when configuring parameters.

  Table 4 Flapping control parameter configuration recommendations

  Configuration Objective	Recommendations
  	suppress	reuse	decay-ok	decay-ng
  To delay interface suppression	Increase	N/A	Decrease	Decrease
  To accelerate interface suppression	Decrease	N/A	Increase	Increase
  To accelerate disabling interface suppression	N/A	Increase	Decrease	Decrease
  To delay disabling interface suppression	N/A	Decrease	Increase	Increase

  decay-ok and decay-ng can be configured separately:

  • If an interface remains up for a long period and the interface needs to be used as soon as it goes up, decreasing decay-ok is recommended.

  • If an interface remains down for a long period and the interface needs to be suppressed as soon as it goes down, increasing decay-ng is recommended.

  Example:

  Table 5 Example for setting the control-flap parameter

  Parameter	Examples for the Impact of Flapping Control Parameters on Interface Suppression
  suppress
  reuse
  decay-ok and decay-ng

  Principles of interface flapping control:

  In Figure 3, the default penalty value of an interface is 0. The penalty value increases by 400 each time the interface goes down. When an interface goes down for the first time, the half life starts. The system checks whether the specific half life expires at an interval of 1s. If the specific half life expires, the penalty value decreases by half. Once a half life ends, another half life starts.

  • If the penalty value exceeds the interface suppressing threshold, the interface is suppressed. When the interface is suppressed, the outputs of the display interface, display interface brief, and display ip interface brief commands show that the protocol status of the interface remains DOWN(dampening suppressed) and does not change with the physical status.

  • If the penalty value falls below the interface reuse threshold, the interface is freed from suppression. When the interface is freed from suppression, the protocol status of the interface is in compliance with the actual status and does not remain Down (dampening suppressed).

  • If the penalty value reaches ceiling, the penalty value no longer increases.

  Figure 3 Flapping control
  
  

• damp-interface

  Related concepts:

  • penalty value: a value calculated by a suppression algorithm based on an interface's flappings. The suppression algorithm increases the penalty value by a specific value each time an interface goes down and decreases the penalty value exponentially each time the interface goes up.
  • suppress: An interface is suppressed if the interface's penalty value is greater than the suppress value.
  • reuse: An interface is no longer suppressed if the interface's penalty value is less than the reuse value.
  • ceiling: calculated using the formula of reuse x 2 ^{(MaxSuppressTime/HalfLifeTime)}. ceiling is the maximum penalty value. An interface's penalty value no longer increases when it reaches ceiling.
  • half-life-period: period that the penalty value takes to decrease to half. A half-life-period begins to elapse when an interface goes Down for the first time. If a half-life-period elapses, the penalty value decreases to half, and another half-life-period begins.
  • max-suppress-time: maximum period during which an interface's status is suppressed. After max-suppress-time elapses, the interface's actual status is reported to upper layer services.

  Figure 4 shows the relationship between the preceding parameters. To facilitate understanding, figures in Figure 4 are all multiplied by 1000.

  Figure 4 Suppression on physical interface flappings
  

  At t1, an interface goes down, and its penalty value increases by 1000 (The actual penalty value on the device is 1. The penalty values in this example are 1000 times the actual penalty values). Then, the interface goes up, and its penalty value decreases exponentially based on the half-life rule. At t2, the interface goes down again, and its penalty value increases by 1000, reaching 1600, which has exceeded the suppress value 1500. At this time if the interface goes up again, its status is suppressed. As the interface keeps flapping, its penalty value keeps increasing until it reaches the ceiling value 10000 at tA. As time goes by, the penalty value decreases and reaches the reuse value 750 at tB. The interface status is then no longer suppressed.

Loopback interfaces, Layer 2 interfaces that are converted from Layer 3 interfaces using the portswitch command, and Null interfaces do not support MTU or control-flap configuration.