H3C Technologies H3C S12500 Series Switches - Manual (Page 255)

239
Following is a GR process:
1. A BGP GR restarter and helper exchange Open messages with GR capability. If both parties have
the GR capability, the session established between them is GR capable.
2. When an active/standby switchover occurs on the GR restarter, the GR restarter does not remove
existing route entries; it still uses these routes for packet forwarding. The GR helper marks all routes
learned from the GR restarter as stale, instead of deleting them; it still uses these routes for packet
forwarding. During the GR process, packet forwarding is not interrupted.
3. After the active/standby switchover is completed, the GR restarter reestablishes a GR session with
the GR helper. If no BGP session is established within the interval set with the graceful-restart timer
restart command, the GR helper removes the stale routes.
4. If a BGP session is established, routing information is exchanged between them for the GR restarter
to retrieve route entries, and for the GR helper to recover stale routes. You can control the routing
information exchange interval by configuring the graceful-restart timer wait-for-rib command. If
routing information exchange is not completed within the time, the GR restarter does not receive
new routes; instead, it updates its routing table and forwarding table with the BGP routes already
learned to complete BGP routing convergence. The GR helper then removes the state routes.
Configuration guidelines
• A device can act as a GR restarter and GR helper at the same time.
• GR restarter sends the maximum time allowed for the peer to reestablish a BGP session to the GR
helper in an Open message. When the GR helper detects that the session to the GR restarter is
down, it waits until the time advertised by the GR restarter expires for a new session to be
established.
• In general, the maximum time allowed for the peer to reestablish a BGP session must be less than
the Holdtime carried in the Open message.
• The End-Of-RIB (End of Routing-Information-Base) indicates the end of route updates.
• The maximum time to wait for the End-of-RIB marker configured on the local end is not advertised
to the peer; the setting is used to control the time on the local end to receive updates from the peer
end.
Configuration procedure
Perform the following configuration on the GR restarter and GR helper.
To configure BGP GR:
Step Command Remarks
1. Enter system view. system-view N/A
2. Enable BGP, and enter its view. bgp as-number N/A
3. Enable GR Capability for BGP. graceful-restart Disabled by default.
4. Configure the maximum time
allowed for the peer to
reestablish a BGP session.
graceful-restart timer restart
timer Optional.
150 seconds by default.
5. Configure the maximum time to
wait for the End-of-RIB marker.
graceful-restart timer
wait-for-rib timer Optional.
180 seconds by default.