[Configuration and Deployment Instructions] Configure Consistent Incoming and Outgoing Paths for a BFD for LSP Session

In Figure 1-1, two paths exist between Router A and Router B.

When deploying a BFD for LSP session on a network:

1. For a dynamic BFD for LSP session, configure a route constraint mode to ensure that the incoming and outgoing paths are the same (for example, use high-priority static routes).
2. For a static BFD for non-TE-LSP session, configure a route constraint mode to ensure that the incoming and outgoing paths are the same (for example, use high-priority static routes).
3. For a static BFD for TE-LSP session, configure strict explicit paths to constrain the incoming and outgoing LSPs and configure a BFD for TE-LSP session on both the incoming and outgoing LSPs to ensure that the incoming and outgoing paths are the same.

Misconfiguration Risks

Risk description:

The problem occurs if any of the following conditions is met:

1. A dynamic BFD for LSP session is configured, and paths 1 and 2 are the LSP and IP path, respectively.
2. A static BFD for LSP (excluding BFD for TE-LSP) session is configured. The session on path 2 is configured to travel along an LSP, and the LSP is configured not to share the LSP from Router A to Router B.
3. A static BFD for TE-LSP session is configured on both paths 1 and 2, and strict explicit paths are not configured for the TE-LSPs on devices A and B.

The BFD session aims to monitor link 1 (path 1). However, when link 2 (path 2) fails, the session goes Down because the return path fails, triggering the LSP on path 1 to be incorrectly switched. When traffic is switched to the faulty path 2, service traffic is lost.

Identification method:

1. In the following example, a dynamic BFD session monitors a forward LSP and a reverse IP routing path. The identification method depends on the network, and the forward and reverse paths of the BFD session must be consistent.

2. Check the BFD session's neighbor information on Router A.

   # Run the display bfd session all verbose command in the user view to check the BFD session's neighbor information from Router A to Router B. The content in bold indicates the BFD session's neighbor and next hop from Router A to Router B.

   <HUAWEI> display bfd session all verbose

   --------------------------------------------------------------------------------

     State : Up                    Name : dyn_16396

   --------------------------------------------------------------------------------

     Local Discriminator    : 16396            Remote Discriminator   : 16392 

     Session Detect Mode    : Asynchronous Mode Without Echo Function

     BFD Bind Type          : TE_LSP 

     Bind Session Type      : Dynamic  

     Bind Peer IP Address   : 2.2.2.2  

     NextHop Ip Address     : 10.1.1.2        

     ......

3. Check the BFD session's neighbor information on Router B.

   # Run the display bfd session all verbose command in the user view to check the BFD session's neighbor information from Router B to Router A. The content in bold indicates the BFD session's neighbor from Router B to Router A.

   <HUAWEI> display bfd session all verbose 

   --------------------------------------------------------------------------------

     (Multi Hop) State : Up                    Name : dyn_16392

   --------------------------------------------------------------------------------

     Local Discriminator    : 16392            Remote Discriminator   : 16396 

     Session Detect Mode    : Asynchronous Mode Without Echo Function

     BFD Bind Type          : Peer IP Address  

     Bind Session Type      : Entire_Dynamic  

     Bind Peer IP Address   : 1.1.1.1             

     ......

4. Check the routing information from Router B to Router A.

   # Run the display ip routing-table ip-address mask verbose command in the user view to check the routing information from Router B to Router A. The content in bold indicates the BFD session's next hop from Router B to Router A. The BFD session's next hop from Router A to Router B is 10.1.1.2, but the BFD session's next hop from Router B to Router A is 10.2.1.2. The BFD session's incoming and outgoing paths are different, indicating that the problem described exists.

   <HUAWEI> display ip routing-table 1.1.1.1 32 verbose

   Route Flags: R - relay, D - download to fib, T - to vpn-instance, B - black hole route

   ------------------------------------------------------------------------------

   Routing Table : _public_

   Summary Count : 1

   Destination: 1.1.1.1/32          

        Protocol: ISIS-L2         Process ID: 1              

      Preference: 15                    Cost: 10             

         NextHop: 10.2.1.2         Neighbour: 0.0.0.0        

           State: Inactive Adv           Age: 1d04h15m09s         

             Tag: 0                 Priority: high           

           Label: NULL               QoSInfo: 0x0           

      IndirectID: 0xE600087A    

    RelayNextHop: 0.0.0.0          Interface: GigabitEthernet0/5/5

        TunnelID: 0x0                  Flags:   

Recovery measures:

For a dynamic BFD for LSP session and a static BFD for non-TE-LSP session, configure a route constraint mode.

For a static BFD for TE-LSP session, configure strict explicit paths.