BGP Fast-External-Fallover and Graceful Restart

BGP Fast-External-Fallover

BGP Fast-external-fallover command terminates external BGP sessions of any directly adjacent peer if the link used to reach the peer goes down; without waiting for the hold-down timer to expire.

Although this feature improves the BGP conversion time, it may lead to great instability in your BGP table due to a flapping interface.

Ref: Cisco

BGP Graceful Restart

BGP Graceful Restart is a capability of BGP protocol that enables BGP sessions to be restarted without causing a disruption in the network. It works by allowing routers to maintain their established routes even after a session reset or restart.

The Graceful Restart Capability is a BGP capability that can be used by a BGP speaker to indicate its ability to preserve its forwarding state during BGP restart.

This capability is defined as follows:

         +--------------------------------------------------+
         | Restart Flags (4 bits)                           |
         +--------------------------------------------------+
         | Restart Time in seconds (12 bits)                |
         +--------------------------------------------------+
         | Address Family Identifier (16 bits)              |
         +--------------------------------------------------+
         | Subsequent Address Family Identifier (8 bits)    |
         +--------------------------------------------------+
         | Flags for Address Family (8 bits)                |
         +--------------------------------------------------+
         | ...                                              |
         +--------------------------------------------------+
         | Address Family Identifier (16 bits)              |
         +--------------------------------------------------+
         | Subsequent Address Family Identifier (8 bits)    |
         +--------------------------------------------------+
         | Flags for Address Family (8 bits)                |
         +--------------------------------------------------+

Restart Time: This is the estimated time (in seconds) it will take for theBGP session to be re-established after a restart.

How BGP Graceful Restart Works

• A BGP router advertises the Graceful Restart Capability for an address family to its peer if it has the ability to preserve its forwarding state for the address family when BGP restarts

• When the Restarting Speaker restarts, it MUST retain, the forwarding state for the BGP routes in the Loc-RIB and MUST mark them as stale. It MUST NOT differentiate between stale and other information during forwarding.

• When the Receiving Speaker detects termination of the TCP session for a BGP session with a peer that has advertised the Graceful Restart Capability, it MUST retain the routes received from the peer for all the address families that were previously received in the Graceful Restart Capability and MUST mark them as stale routing information.
  The router MUST NOT differentiate between stale and other routing information during forwarding.

• In re-establishing the session, the “Restart State” bit in the Graceful Restart Capability of the OPEN message sent by the Receiving Speaker MUST NOT be set unless the Receiving Speaker has restarted.
  
• If the session does not get re-established within the “Restart Time” that the peer advertised previously, the Receiving Speaker MUST delete all the stale routes from the peer that it is retaining.

Ref: RFC4724