Configuring Dynamic BFD for IS-IS

Context

On an IS-IS network, a device periodically sends Hello packets to detect the neighbor status change. By default, the device considers a neighbor Down when it does not receive a Hello packet from the neighbor after sending three Hello packets (30 seconds). This IS-IS fault detection mechanism, however, cannot provide high reliability for the network that requires fast network convergence and no packet loss. BFD for IS-IS can solve this problem. BFD is a millisecond-level fault detection mechanism. It can fast detect faults on the link between IS-IS neighbors. Therefore, BFD can speed up IS-IS route convergence, ensure fast link switchover, and reduce traffic loss.

Dynamic BFD for IS-IS implements dynamic setup of BFD sessions. When a new IS-IS neighbor relationship is set up, BFD is notified of the neighbor parameters and the detection parameters (including source and destination IP addresses). Then a BFD session will be established based on the received neighbor parameters.

Dynamic BFD is more flexible than static BFD. In dynamic BFD, routing protocols trigger the setup of BFD sessions, preventing the configuration errors caused by manual configuration. Dynamic BFD is easy to configure and applies to the scenarios where BFD needs to be configured on the entire network. Dynamic BFD for IS-IS can fast detect neighbor status changes and implement fast network convergence.

A BFD session currently does not detect route switching. If the change of bound peer IP address causes a route to switch to another link, the BFD session is negotiated again only when the original link fails.

The priority of BFD configured on an interface is higher than that of BFD configured for a process. If BFD session parameters are configured for both a process and an interface, the parameters on the interface will be used to establish a dynamic BFD session.

Only the S5720-EI, S5720-HI, S5730-HI, S5731-H, S5731-S, S5731S-H, S5731S-S, S5732-H, S6720-EI, S6720-HI, S6720S-EI, S6730-H, S6730S-H, S6730-S, and S6730S-S support BFD for IS-IS.

Procedure

Configure dynamic BFD for IS-IS in a specified IS-IS process.
1. Run system-view
  The system view is displayed.
2. Run bfd
  BFD is enabled globally.
3. Run quit
  The system view is displayed.
4. Run isis process-id
  The IS-IS view is displayed.
5. Run bfd all-interfaces enable
  BFD for IS-IS is enabled to establish a BFD session.
  
  This command enables an IS-IS process to use default BFD parameters to create BFD sessions on all the interfaces in the IS-IS process.
6. (Optional) Run bfd all-interfaces { min-rx-interval receive-interval | min-tx-interval transmit-interval | detect-multiplier multiplier-value } ^*
  The parameters for establishing BFD sessions are set for all interfaces.
  
  The command execution result is applicable to BFD session parameters on all IS-IS interfaces.
7. (Optional) Run the isis bfd block command in the interface view:
  The interface is prohibited from dynamically establishing a BFD session.
  
  By default, an interface can dynamically establish BFD sessions.
Configure dynamic BFD for IS-IS on a specified interface.
1. Run system-view
  The system view is displayed.
2. Run bfd
  BFD is enabled globally.
3. Run quit
  The system view is displayed.
4. Run interface interface-type interface-number
  The interface view is displayed.
5. (Optional) On an Ethernet interface, run undo portswitch
  The interface is switched to Layer 3 mode.
  
  By default, an Ethernet interface works in Layer 2 mode.
  
  Only the S5720-EI, S5720-HI, S5730-HI, S5731-H, S5731-S, S5731S-H, S5731S-S, S5732-H, S6720-EI, S6720-HI, S6720S-EI, S6730-H, S6730S-H, S6730-S, and S6730S-S support switching between Layer 2 and Layer 3 modes.
6. Run isis bfd enable
  BFD is enabled on the interface to establish a BFD session.
  
  After BFD is configured globally and the neighbor status is Up (on a broadcast network, DIS is in the Up state), default BFD parameters will be used to establish BFD sessions on the specified interface.
7. (Optional) Run isis bfd { min-rx-interval receive-interval | min-tx-interval transmit-interval | detect-multiplier multiplier-value } ^*
  Run this command when BFD session parameters need to be configured for a specified interface.
8. (Optional) Run isis bfd block
  The interface is prohibited from dynamically establishing a BFD session.

Verifying the Configuration

After BFD is enabled on both ends of a link, run the display isis [ process-id | vpn-instance vpn-instance-name ] bfd session { all | peer ip-address | interface interface-type interface-number } command. The command output shows that BFD status is up.