3Com 8807 Configuration Manual page 308

308 C 31: OSPF C
HAPTER ONFIGURATION
Working Mechanism of 1. Implementation standard of OSPF GR
OSPF GR
RFC3623:Graceful OSPF Restart
IETF drafts:
draft-nguyen-ospf-lls-05;
draft-nguyen-ospf-oob-resync-05;
draft-nguyen-ospf-restart-05;
Work mechanism of RFC3623
RFC3623 defines two main principles for GR: the network topology must remain
stable and the forwarding tables can be kept when a router is being restarted.
During the GR process, the behaviors of restarters and helpers are defined. A GR
process is started with the restarter's sending a Grace LSA advertisement.
The restarter device does not generate LSAs during the GR process. When it
receives a self-generated LSA, it will accept and mark this LSA. Route items are not
delivered to the forwarding table. The restarter device finds out its state before
restart through hello packets on the snooping interface. When all the neighbor
relationships are rebuilt or the GR period timeouts, the Restarter devices will exit
GR.
The Restarter device will perform standard OSPF reflooding and routing operation
after exiting GR, no matter whether it has finished GR.
The Helper device judges its relationship with the Restarter device when it receives
Grace LSA from the Restarter device. When its neighbor state machine is full, and
it is not in the GR state, it will enter Helper mode and keep the received Grace
LSAs. The Helper routes help the Restarter routes to regain the LSDB information
before restart.
When Grace LSAs in the Helper device are updated or when the Helper device
exits the Helper mode when a GR period is finished, the Helper device will
calculate DR routers of this network, generate class 1 LSAs and calculate routes.
3. Work mechanism of IETF drafts
Two primary concepts are defined in the drafts: Link-local Signaling (LLS) which is
used to negotiate about OOB capabilities and trigger GR processes and
Out-of-band LSDB resynchronization (OOB) which is used to synchronize LSDB.
The L_bit set in a HELLO packet can negotiate about LLS capabilities and notify the
peer about its own LLS data. The LR_bit set in the EO_TLV of the LLS data is used
to negotiate about the OOB capabilities.
When a protocol is restarted, the protocol will notify the peer that it will be
restarted and let the peer keep the neighbor relationship through the RS_bit set in
the EO_TLV of a HELLO packet. The R_bit set in a DD packet indicates that this is
an OOB process.