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VPN Graceful Restart

With routing protocols, any service interruption requires that an affected router recalculate adjacencies with neighboring routers, restore routing table entries, and update other protocol-specific information. An unprotected restart of the router results in forwarding delays, route flapping, wait times stemming from protocol reconvergence, and even dropped packets. Graceful restart allows a routing device undergoing a restart to inform its adjacent neighbors and peers of its condition. During a graceful restart, the restarting device and its neighbors continue forwarding packets without disrupting network performance.

For VPN graceful restart to function properly, the following items need to be configured on the PE router:

  • BGP graceful restart must be active on the PE-to-PE sessions carrying any service-signaling data in the session’s network layer reachability information (NLRI).

  • OSPF, IS-IS, LDP, and RSVP graceful restart must be active, because routes added by these protocols are used to resolve VPN NLRIs.

  • For other protocols (static, Routing Information Protocol [RIP], and so on), graceful restart functionality must also be active when these protocols are run between the PE and CE routers. Layer 2 VPNs do not rely on this, because protocols are not configured between the PE and CE routers.

In VPN graceful restart, a restarting router completes the following procedures:

  • Waits for all the BGP NLRI information from other PE routers before it starts advertising routes to its CE routers.

  • Waits for all protocols in all routing instances to converge (or finish graceful restart) before sending CE router information to the other PE routers.

  • Waits for all routing instance information (whether it is local configuration or advertisements from a remote peer router) to be processed before sending it to the other PE routers.

  • Preserves all forwarding state information in the MPLS routing tables until new labels and transit routes are allocated and then advertises them to other PE routers (and CE routers in carrier-of-carriers VPNs).

Graceful restart is supported on Layer 2 VPNs, Layer 3 VPNs, and virtual-router routing instances.

Benefit of a VPN graceful restart

The main benefit of a VPN graceful restart is that it allows a router whose VPN control plane is undergoing a restart to continue to forward traffic while recovering its state from neighboring routers. It temporarily suppresses all routing protocol updates and enables a router to pass through intermediate convergence states that are hidden from the rest of the network. Without graceful restart, a control plane restart disrupts the VPN services provided by the router.