308 CHAPTER 31: OSPF C ONFIGURATIONWorking Mechanism ofOSPF GR1. Implementation standard of OSPF GRRFC3623:Graceful OSPF RestartIETF drafts:draft-nguyen-ospf-lls-05;draft-nguyen-ospf-oob-resync-05;draft-nguyen-ospf-restart-05;Work mechanism of RFC3623RFC3623 defines two main principles for GR: the network topology must remainstable 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 GRprocess is started with the restarter’s sending a Grace LSA advertisement.The restarter device does not generate LSAs during the GR process. When itreceives a self-generated LSA, it will accept and mark this LSA. Route items are notdelivered to the forwarding table. The restarter device finds out its state beforerestart through hello packets on the snooping interface. When all the neighborrelationships are rebuilt or the GR period timeouts, the Restarter devices will exitGR.The Restarter device will perform standard OSPF reflooding and routing operationafter exiting GR, no matter whether it has finished GR.The Helper device judges its relationship with the Restarter device when it receivesGrace LSA from the Restarter device. When its neighbor state machine is full, andit is not in the GR state, it will enter Helper mode and keep the received GraceLSAs. The Helper routes help the Restarter routes to regain the LSDB informationbefore restart.When Grace LSAs in the Helper device are updated or when the Helper deviceexits the Helper mode when a GR period is finished, the Helper device willcalculate DR routers of this network, generate class 1 LSAs and calculate routes.3. Work mechanism of IETF draftsTwo primary concepts are defined in the drafts: Link-local Signaling (LLS) which isused to negotiate about OOB capabilities and trigger GR processes andOut-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 thepeer about its own LLS data. The LR_bit set in the EO_TLV of the LLS data is usedto negotiate about the OOB capabilities.When a protocol is restarted, the protocol will notify the peer that it will berestarted and let the peer keep the neighbor relationship through the RS_bit set inthe EO_TLV of a HELLO packet. The R_bit set in a DD packet indicates that this isan OOB process.
