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muthu.arul
Beginner

Protecting Multihop BFD during RLFA/TI-LFA Local Repair

Hi All,

 

I've a question on Multihop BFD protection on XR platforms.

 

Multihop BFD (RFC 5883) packets are sent over UDP/IP. The encapsulation used is identical to single hop BFD (RFC 5881) except that the UDP destination port is set to 4784.

 

Now, suppose on the ingress node there is no IP/LFA backup path for the destination address tracked by multihop BFD, but there exists an RLFA or TI-LFA (1-segment or 2-segment) backup path to that destination. In this case, does IPOS XR platforms support protecting multihop BFD packets over the RLFA or TI-LFA (1-segment or 2-segment) backup paths i.e do they keep the multihop BFD session up if the outgoing link or nexthop neighbor for the primary path goes down?

 

Regards,

Muthu

3 REPLIES 3
Aleksandar Vidakovic
Cisco Employee

On asr9k multi-hop BFD sessions are hosted on any line card. The async packets are generated and received by the LC CPU (we don't support echo on MH BFD sessions). It doesn't matter on which NP/interface are packets received. As long as valid path exists to the MH BFD peer, the BFD session will be up. The MH BFD session will flap only if the line card that hosts the MH BD session is reloaded. In that case another LC has to take ownership of that session and initialise it.

 

See the "Implementing BFD" chapter of latest ASR9000 Routing Configuration Guides at http://www.cisco.com/en/US/products/ps9853/products_installation_and_configuration_guides_list.html and the https://wiki.cisco.com/display/CSGCHENNAI/BFD+IOS-XR+Information document for more info on MH BFD sessions.

 

On NCS5500 the implementation is slightly different because the MH BFD session is offloaded to the NP0 of the host line card. Everything else I wrote applies to NCS5500 as well.

 

Hope this helps. 

Thanks for the inputs. A follow up question for me to understand better:

Suppose there is a primary IP path and an LSP path to the remote PE loopback (i.e BGP nexthop tracked using MH-BFD). In this case, are the MH-BFD packets generated by the LC CPU on ASR9K sent over the IP path or LSP path?

 

Regards,

Muthu

BFD will rely on FIB to provide the forwarding information. Based on that the appropriate next hop and encapsulation will be applied.