This feature extends the graceful restart helper function supported under other protocols to OSPFv3.
The primary difference between graceful restart helper for OSPFv2 and OSPFv3 is in OSPFv3 a different grace-LSA format is used.
As SR OS platforms can support a fully non-stop routing model for control plane high availability, SR OSs have no need for graceful restart as defined by the IETF in various RFCs for each routing protocol. However, since the router does need to coexist in multi-vendor networks and other routers do not always support a true non-stop routing model with stateful failover between routing control planes, there is a need to support a graceful restart helper function.
Graceful restart helper mode allows SR OS-based systems to provide a grace period to other routers which have requested it, during which the SR OS systems continue to use routes authored by or transiting the router requesting the grace period. This is typically used when another router is rebooting the control plane but the forwarding plane is expected to continue to forward traffic based on the previously available FIB.
The format of the graceful OSPF restart (GRACE) LSA format is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS age |0|0|0| 11 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link State ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS checksum | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- TLVs -+
| ... |
For more information, see section 2.2 of RFC 5187, OSPFv3 Graceful Restart.
The Link State ID of a grace-LSA in OSPFv3 is the Interface ID of the interface originating the LSA.
The format of each TLV is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
TLV Format
Grace-LSA TLVs are formatted according to section 2.3.2 of RFC 3630, Traffic Engineering (TE) Extensions to OSPF Version 2. The Grace-LSA TLVs are used to carry the Grace period (type 1) and the reason the router initiated the graceful restart process (type 2).
Other information in RFC 5187 is directed to routers that require the full graceful restart mechanism as they do not support a stateful transition from primary or backup control plane module (CPM).