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CX11x, CX31x, CX710 (Earlier Than V6.03), and CX91x Series Switch Modules V100R001C10 Configuration Guide 12

The documents describe the configuration of various services supported by the CX11x&CX31x&CX91x series switch modules The description covers configuration examples and function configurations.
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This section describes the implementation of BFD.

BFD Implementation

Two network devices establish a BFD session to detect the forwarding path between them and serve upper-layer applications. BFD does not provide neighbor discovery. Instead, BFD obtains neighbor information from the upper-layer application BFD serves to establish a BFD session. After the BFD session is set up, the local device periodically sends BFD packets. If the local device does not receive a response from the peer system within the detection time, it considers the forwarding path faulty. BFD then notifies the upper-layer application for processing. The following uses association between OSPF and BFD as an example to describe the BFD session setup process.

Figure 11-5 BFD session setup

As shown in Figure 11-5, OSPF and BFD are configured on SwitchA and SwitchB. The BFD session setup process is as follows:

  1. OSPF uses the Hello mechanism to discover neighbors and establishes a neighbor relationship.

  2. OSPF notifies BFD of neighbor information including source and destination addresses.

  3. BFD sets up a BFD session based on received neighbor information.

  4. After the BFD session is set up, BFD starts to detect link faults and rapidly responds to link faults.

Figure 11-6 BFD detecting link faults

As shown in Figure 11-6:

  1. The detected link is faulty.

  2. BFD rapidly detects the link fault and the BFD session becomes Down.

  3. BFD notifies the local OSPF process that the neighbor is unreachable.

  4. The local OSPF process ends the OSPF neighbor relationship.

BFD Session Establishment Modes

BFD sessions can be set up statically and dynamically. Static and dynamic BFD sessions differ in that local and remote discriminators are configured in different modes. BFD uses local and remote discriminators in control packets to differentiate BFD sessions.

  • Statically establishing a BFD session

    BFD session parameters, including the local and remote discriminators, are specified using commands. Then a request for BFD session establishment is distributed manually.

  • Dynamically establishing a BFD session

    When a BFD session is set up dynamically, the system processes the local and remote discriminators as follows:

    • Dynamically allocated local discriminator

      When an application triggers dynamic setup of a BFD session, the system allocates a value as the local discriminator of the BFD session. Then the local system sends a BFD control packet with Remote Discriminator as 0 to the remote system to negotiate on the BFD session.

    • Self-learned remote discriminator

      When one end of a BFD session receives a BFD control packet with Remote Discriminator as 0, this end checks the BFD control packet. If the packet matches the local BFD session, this end learns the value of Local Discriminator in the received BFD control packet to obtain the remote discriminator.

BFD Detection Mechanism

Two systems set up 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 considers that a fault has occurred on the path.

BFD provides the Asynchronous mode. In asynchronous mode, two systems periodically send BFD control packets to each other. If one system receives no packets consecutively, the system considers the BFD session Down.

BFD Session Management

The BFD session has the following status: Down, Init, Up, and AdminDown. The State field of a BFD control packet shows the session status. The system changes the session status based on the local session status and the received session status of the peer. The BFD state machine implements a three-way handshake for BFD session setup or deletion to ensure that the two systems detect the status change. The following uses BFD session setup as an example to describe the state machine transition process.

Figure 11-7 BFD session setup
  1. SwitchA and SwitchB start BFD state machines respectively. The initial state of BFD state machine is Down. SwitchA and SwitchB send BFD control packets with the State field as Down. If BFD sessions are configured statically, the values of Remote Discriminator in BFD packets are specified. If BFD sessions are configured dynamically, the value of Remote Discriminator is set to 0.

  2. After receiving the BFD packet with the State field as Down, SwitchB switches the session status to Init and sends a BFD packet with State field as Init.

  3. After the local BFD session status of SwitchB changes to Init, SwitchB no longer processes the received BFD packets with the State field as Down.

  4. The BFD session status change on SwitchA is similar to that on SwitchB.

  5. After receiving the BFD packet with the State field as Init, SwitchB changes the local BFD session status to Up.

  6. The BFD session status change on SwitchA is similar to that on SwitchB.

Updated: 2019-08-09

Document ID: EDOC1000041694

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