3Com Switch 8800 Advanced Software V5 Configuration Guide

796 CHAPTER 55: GR CONFIGURATION
Figure 244 The GR Restarter obtains topology and routing information from the GR
Helper
As illustrated in Figure 244, the GR Restarter obtains the necessary topology and
routing information from all its neighbors through the GR sessions between them
and calculates its own routing table based on this information.
Graceful Restart
Mechanism for Several
Commonly Used
Protocols
Graceful restart is currently implemented in BGP, OSPF, IS-IS, and LDP.
BGP-based Graceful Restart
1 To establish a session with its peer, a BGP-based GR Restarter needs to send an
OPEN message first to its peer including its Graceful Restart Capability.
2 Upon receipt of this message, the peer is aware that the sending router is capable
of Graceful Restart. It will use the OPEN messages to exchange the Graceful
Restart Capability with the GR Restarter and to establish a GR session with it. If
neither party has the Graceful Restart Capability, the session established between
them will not be Graceful Restart Capable.
3 The GR session between the GR Restarter and the GR Helper goes down when a
BGP restarts. A Graceful Restart Capable GR Helper will mark all routes associated
with the GR Restarter as stale. However, during the configured GR Time, it still
uses these routes in packet forwarding, ensuring that no packet will be lost when
routing information from its peer is recollected.
4 After the restart, the GR Restarter will reestablish the GR session with its peer and
send new GR messages notifying the completion of restart. Routing information
can be exchanged between the peers and used by the GR Restarter in creating the
new routing table (forwarding table) in place of the stale routing information.
Thus the BGP routing convergence is complete.
OSPF-based Graceful Restart
After an OSPF-based GR Restarter restarts its OSPF, it needs to perform the
following two tasks in order to update its link-state database with its neighbor.
1 To obtain once again effective OSPF adjacency while retaining the original one.
2 To obtain once again link-state database information.
GR helper
Router D
Router B
Router C
Router A
GR helper GR helper
GR restarter
Signals to establish GR session