BFD and BGP Graceful Restart

So that BFD can maintain its BFD protocol sessions across a BGP graceful restart, BGP requests that BFD set the C bit to 1 in transmitted BFD packets. When the C bit is set to 1, BFD can maintain its session in the forwarding plane in spite of disruptions in the control plane. Setting the bit to 1 gives BGP neighbors acting as a graceful restart helper the most accurate information about whether the forwarding plane is up.

When BGP is acting as a graceful restart helper and the BFD session to the BGP peer is lost, one of the following actions takes place:

• If the C bit received in the BFD packets was 1, BGP immediately flushes all routes, determining that the forwarding plane on the BGP peer has gone down.
• If the C bit received in the BFD packets was 0, BGP marks all routes as stale but does not flush them because the forwarding plane on the BGP peer might be working and only the control plane has gone down.

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