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bgpd: BGP Graceful Restart documentation.
This change list contains the documentation of BGP Greacful Restart feature and all the commnads to enable/disable the feature Signed-off-by: Biswajit Sadhu <sadhub@vmware.com>
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doc/user/bgp.rst
175
doc/user/bgp.rst
@ -694,6 +694,181 @@ from eBGP peers, :ref:`bgp-route-selection`.
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MED as an intra-AS metric to steer equal-length AS_PATH routes to, e.g.,
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desired exit points.
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.. _bgp-graceful-restart:
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Graceful Restart
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----------------
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BGP graceful restart functionality as defined in
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`RFC-4724 <https://tools.ietf.org/html/rfc4724/>`_ defines the mechanisms that
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allows BGP speaker to continue to forward data packets along known routes
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while the routing protocol information is being restored.
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Usually, when BGP on a router restarts, all the BGP peers detect that the
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session went down and then came up. This "down/up" transition results in a
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"routing flap" and causes BGP route re-computation, generation of BGP routing
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updates, and unnecessary churn to the forwarding tables.
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The following functionality is provided by graceful restart:
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1. The feature allows the restarting router to indicate to the helping peer the
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routes it can preserve in its forwarding plane during control plane restart
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by sending graceful restart capability in the OPEN message sent during
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session establishment.
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2. The feature allows helping router to advertise to all other peers the routes
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received from the restarting router which are preserved in the forwarding
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plane of the restarting router during control plane restart.
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::
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(R1)-----------------------------------------------------------------(R2)
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1. BGP Graceful Restart Capability exchanged between R1 & R2.
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<--------------------------------------------------------------------->
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2. Kill BGP Process at R1.
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---------------------------------------------------------------------->
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3. R2 Detects the above BGP Restart & verifies BGP Restarting
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Capability of R1.
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4. Start BGP Process at R1.
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5. Re-establish the BGP session between R1 & R2.
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<--------------------------------------------------------------------->
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6. R2 Send initial route updates, followed by End-Of-Rib.
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<----------------------------------------------------------------------
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7. R1 was waiting for End-Of-Rib from R2 & which has been received
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now.
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8. R1 now runs BGP Best-Path algorithm. Send Initial BGP Update,
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followed by End-Of Rib
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<--------------------------------------------------------------------->
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.. _bgp-end-of-rib-message:
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End-of-RIB (EOR) message
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^^^^^^^^^^^^^^^^^^^^^^^^
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An UPDATE message with no reachable Network Layer Reachability Information
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(NLRI) and empty withdrawn NLRI is specified as the End-of-RIB marker that can
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be used by a BGP speaker to indicate to its peer the completion of the initial
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routing update after the session is established.
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For the IPv4 unicast address family, the End-of-RIB marker is an UPDATE message
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with the minimum length. For any other address family, it is an UPDATE message
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that contains only the MP_UNREACH_NLRI attribute with no withdrawn routes for
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that <AFI, SAFI>.
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Although the End-of-RIB marker is specified for the purpose of BGP graceful
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restart, it is noted that the generation of such a marker upon completion of
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the initial update would be useful for routing convergence in general, and thus
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the practice is recommended.
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.. _bgp-route-selection-deferral-timer:
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Route Selection Deferral Timer
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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Specifies the time the restarting router defers the route selection process
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after restart.
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Restarting Router : The usage of route election deferral timer is specified
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in https://tools.ietf.org/html/rfc4724#section-4.1
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Once the session between the Restarting Speaker and the Receiving Speaker is
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re-established, the Restarting Speaker will receive and process BGP messages
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from its peers.
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However, it MUST defer route selection for an address family until it either.
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1. Receives the End-of-RIB marker from all its peers (excluding the ones with
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the "Restart State" bit set in the received capability and excluding the ones
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that do not advertise the graceful restart capability).
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2. The Selection_Deferral_Timer timeout.
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.. index:: bgp graceful-restart select-defer-time (0-3600)
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.. clicmd:: bgp graceful-restart select-defer-time (0-3600)
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This is command, will set deferral time to value specified.
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.. index:: bgp graceful-restart rib-stale-time (1-3600)
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.. clicmd:: bgp graceful-restart rib-stale-time (1-3600)
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This is command, will set the time for which stale routes are kept in RIB.
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.. _bgp-per-peer-graceful-restart:
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BGP Per Peer Graceful Restart
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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Ability to enable and disable graceful restart, helper and no GR at all mode
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functionality at peer level.
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So bgp graceful restart can be enabled at modes global BGP level or at per
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peer level. There are two FSM, one for BGP GR global mode and other for peer
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per GR.
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Default global mode is helper and default peer per mode is inherit from global.
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If per peer mode is configured, the GR mode of this particular peer will
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override the global mode.
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.. _bgp-GR-globla-mode-cmd:
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BGP GR Global Mode Commands
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^^^^^^^^^^^^^^^^^^^^^^^^^^^
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.. index:: bgp graceful-restart
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.. clicmd:: bgp graceful-restart
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This command will enable BGP graceful restart ifunctionality at the global
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level.
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.. index:: bgp graceful-restart disable
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.. clicmd:: bgp graceful-restart disable
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This command will disable both the functionality graceful restart and helper
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mode.
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.. _bgp-GR-peer-mode-cmd:
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BGP GR Peer Mode Commands
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^^^^^^^^^^^^^^^^^^^^^^^^^
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.. index:: neighbor A.B.C.D graceful-restart
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.. clicmd:: neighbor A.B.C.D graceful-restart
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This command will enable BGP graceful restart ifunctionality at the peer
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level.
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.. index:: neighbor A.B.C.D graceful-restart-helper
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.. clicmd:: neighbor A.B.C.D graceful-restart-helper
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This command will enable BGP graceful restart helper only functionality
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at the peer level.
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.. index:: neighbor A.B.C.D graceful-restart-disable
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.. clicmd:: neighbor A.B.C.D graceful-restart-disable
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This command will disable the entire BGP graceful restart functionality
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at the peer level.
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.. _bgp-network:
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Networks
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