L3VPN: BGP: Graceful restart

VPN graceful restart uses three types of restart functionality:

•  BGP graceful restart functionality is used on all PE-to-PE BGP sessions. This affects sessions carrying any service signaling data for network layer reachability information (NLRI) - for example, an IPv4 VPN or Layer 2 VPN NLRI.
• OSPF, IS-IS, LDP, or RSVP graceful restart functionality is used in all core routers. Routes added by these protocols are used to resolve Layer 2 and Layer 3 VPN NLRI.
• Protocol restart functionality is used for any Layer 3 protocol (RIP, OSPF, LDP, and so on) used between the PE and customer edge (CE) routers. This does not apply to Layer 2 VPNs because Layer 2 protocols used between the CE and PE routers do not have graceful restart capabilities.

Before VPN graceful restart can work properly, all of the components must restart gracefully. In other words, the routers must preserve their forwarding states and request neighbors to continue forwarding to the router in case of a restart.

This feature introduces the first component of Layer 3 VPN graceful restart providing graceful restart support for BGP inet-vpn unicast and inet6-vpn unicast address families.