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NE20E-S2 V800R010C10SPC500 Feature Description - Network Reliability 01

This is NE20E-S2 V800R010C10SPC500 Feature Description - Network Reliability
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Huawei uses machine translation combined with human proofreading to translate this document to different languages in order to help you better understand the content of this document. Note: Even the most advanced machine translation cannot match the quality of professional translators. Huawei shall not bear any responsibility for translation accuracy and it is recommended that you refer to the English document (a link for which has been provided).
BFD Basic Concepts

BFD Basic Concepts

Bidirectional Forwarding Detection (BFD) detects communication faults between forwarding engines. Specifically, BFD checks the continuity of a data protocol on the path between systems. The path can be a physical or logical link or a tunnel.

BFD interacts with upper-layer applications in the following manner:

  • An upper-layer application provides BFD with parameters, such as the detection address and time.

  • BFD creates, deletes, or modifies sessions based on these parameters and notifies the upper-layer application of the session status.

BFD has the following characteristics:

  • Provides a low-overhead, short-duration method to detect faults on the path between adjacent forwarding engines.

  • Provides a single, unified mechanism to monitor any media and protocol layers in real time.

The following sections describe the basic principles of BFD, including the BFD detection mechanism, detected link types, session establishment modes, and session management.

BFD Detection Mechanism

Two systems establish a BFD session and periodically send BFD control packets along the path between them. If one system does not receive BFD control packets within a specified period, the system regards it as a fault occurrence on the path.

BFD control packets are encapsulated using the User Datagram Protocol (UDP). In the initial phase of a BFD session, both systems negotiate BFD parameters with each other using BFD control packets. These parameters include discriminators, required minimum intervals at which BFD control packets are sent and received, and local BFD session status. After the negotiation is successful, both systems send BFD control packets along the path between them at the negotiated intervals.

BFD provides two types of detection modes:

  • Asynchronous mode: a major BFD detection mode. In this mode, both systems periodically send BFD control packets to each other. If one system fails to receive BFD control packets consecutively, the system considers the BFD session Down.

The echo function is used for two modes. When the echo function is activated, the local system sends a BFD control packet and the remote system loops back the packet through the forwarding channel. If several consecutive echo packets are not received, the session is declared to be Down.

Types of Links Detected by BFD

Table 2-1 Types of links detected by BFD

Link Type

Classification

Description

IP links

  • Layer 3 physical interfaces
  • Ethernet sub-interfaces (including Eth-Trunk sub-interfaces)

If a physical Ethernet interface has multiple sub-interfaces, BFD sessions can be separately established on the physical Ethernet interface and its sub-interfaces.

IP-Trunks

  • IP-Trunk links
  • IP-Trunk member links

Separate BFD sessions can be established to detect link faults on an IP-Trunk and its member interfaces at the same time.

Eth-Trunks

  • Layer 2 Eth-Trunk links
  • Layer 2 Eth-Trunk member links
  • Layer 3 Eth-Trunk links
  • Layer 3 Eth-Trunk member links

Separate BFD sessions can be established to detect link faults on an Eth-Trunk and its member interfaces at the same time.

VLANIF

  • VLAN Ethernet member links
  • VLANIF interfaces

Separate BFD sessions can be established to detect link faults on a VLANIF interface and its member interfaces at the same time.

MPLS LSPs

  • In static mode, BFD can detect the following types of LSPs:
    • LDP LSPs
    • TE tunnels, static CR-LSPs bound to tunnels, and RSVP CR-LSPs bound to tunnels
  • In dynamic mode, BFD can detect the following types of LSPs:
    • LDP LSPs
    • RSVP CR-LSPs bound to tunnels
A BFD session used to check the continuity of a Multiprotocol Label Switching label switched path (MPLS LSP) can be established in either of the following modes:
  • Static mode: Local and remote discriminators are manually configured on interconnected devices to allow them to negotiate a BFD session.
  • Dynamic mode: BFD discriminator type-length-value (TLV) carried in an LSP ping packet is used to allow the interconnected devices to negotiate a BFD session.

BFD can detect a TE tunnel that uses CR-Static or RSVP-TE as its signaling protocol and detect the primary LSP bound to the TE tunnel.

A dynamic BFD session cannot detect the entire TE tunnel.

PWs

  • SS PWs
  • MS PWs

BFD can monitor a PW in static (manually configured discriminator) or dynamic mode.

BFD Session Establishment Modes

BFD sessions can be established in either static or dynamic mode.

BFD identifies sessions based on the My Discriminator (local discriminator) and Your Discriminator (remote discriminator) fields carried in BFD control packets. The difference between the two modes lies in different configurations for the two fields.

Table 2-2 BFD session establishment modes

BFD Session Establishment Mode

Description

Static mode

BFD session parameters, such as the local and remote discriminators, are manually configured and delivered for BFD session establishment.
NOTE:
In static mode, configure unique local and remote discriminators for each BFD session. This mode prevents incorrect discriminators from affecting BFD sessions that have correct discriminators and prevents BFD sessions from alternating between Up and Down.

Dynamic mode

When a BFD session is dynamically established, the system processes the local and remote discriminators as follows:
  • Dynamically allocates the local discriminator. When a system triggers the dynamic establishment of a BFD session, the system allocates a dynamic discriminator as the local discriminator of the BFD session. Then, the system sends a BFD control packet with Your Discriminator set to 0 to the peer for session negotiation.
  • Automatically learns the remote discriminator. The local end of a BFD session sends a BFD control packet with Your Discriminator set to 0 to the remote end. After the remote end receives the packet, it checks whether the value of Your Discriminator in this packet is the same as the value of its My Discriminator. If the value of Your Discriminator matches that of My Discriminator, the remote end learns the value of My Discriminator of the local end and obtains its Your Discriminator.

BFD Session Management

A BFD session has the following states:

  • Down: A BFD session is in the Down state or a request has been sent.

  • Init: The local end can communicate with the remote end, and the local end expects the BFD session to go Up.

  • Up: A BFD session is successfully established.

  • AdminDown: A BFD session is in the AdminDown state.

Session status changes are transmitted using the State field carried in a BFD control packet. The system changes its session status based on the local session status and received remote session status from the peer system.

When a BFD session is to be established or deleted, the BFD state machine implements a three-way handshake to ensure that the two systems detect the status change.

Figure 2-1 shows the status change process of the state machine during the establishment of a BFD session.

Figure 2-1 Status change process of the state machine

  1. BFD configured on both Device A and Device B independently starts state machines. The initial status of BFD state machines is Down. Device A and Device B send BFD control packets with the State field set to Down. If BFD sessions are established in static mode, the value of Your Discriminator in BFD control packets is manually specified. If BFD sessions are established in dynamic mode, the value of Your Discriminator is set to 0.

  2. After receiving a BFD control packet with the State field set to Down, Device B switches the session status to Init and sends a BFD control packet with the State field set to Init.

    NOTE:

    After the local BFD session status of Device B changes to Init, Device B no longer processes the received BFD control packets with the State field set to Down.

  3. The BFD session status change of Device A is the same as that of Device B.

  4. After receiving a BFD control packet with the State field set to Init, Device B changes the local session status to Up.

  5. The BFD session status change of Device A is the same as that of Device B.

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Updated: 2019-01-02

Document ID: EDOC1100055473

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