Graceful Restart and Layer 2 and Layer 3 VPNs
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. If all of the conditions are satisfied, VPN graceful restart imposes the following rules on a restarting router:
The router must wait to receive all BGP NLRI information from other PE routers before advertising routes to the CE routers.
The router must wait for all protocols in all routing instances to converge (or complete the restart process) before it sends CE router information to other PE routers. In other words, the router must wait for all instance information (whether derived from local configuration or advertisements received from a remote peer) to be processed before it sends this information to other PE routers.
The router must preserve all forwarding state in the instance.mpls.0 tables until the new labels and transit routes are allocated and announced to other PE routers (and CE routers in a carrier-of-carriers scenario).
If any condition is not met, VPN graceful restart does not succeed in providing uninterrupted forwarding between CE routers across the VPN infrastructure.