VPN Graceful Restart

VPN graceful restart allows a router whose VPN control plane is undergoing a restart to continue to forward traffic while recovering its state from neighboring routers. Without graceful restart, a control plane restart disrupts any Layer 2 or Layer 3 VPN services provided by the router.

For VPN graceful restart to function properly, the following needs 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, ISIS, LDP, and RSVP graceful restart must be active, because routes added by these protocols are used to resolve Layer 2 and Layer 3 VPN NLRIs.
• For other protocols (static, RIP, OSPF, LDP 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 does the following:

• 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).