Configuring BGP GR
Graceful Restart (GR) ensures forwarding continuous when a routing protocol restarts or an active/standby switchover occurs. Two routers are required to complete a GR process. The following are router roles in a GR process:
GR restarter—Performs GR upon a BGP restart or active/standby switchover.
GR helper—Helps the GR restarter to complete the GR process.
A device can act as a GR restarter and GR helper at the same time.
BGP GR works as follows:
The BGP GR restarter and helper exchange Open messages for GR capability negotiation. If both parties have the GR capability, they establish a GR-capable session. The GR restarter sends the GR timer set by the graceful-restart timer restart command to the GR helper in an Open message.
When an active/standby switchover occurs or BGP restarts, the GR restarter does not remove existing BGP routes from Routing Information Base (RIB) and Forwarding Information Base (FIB). It still uses these routes for packet forwarding, and it starts the RIB purge timer (set by the graceful-restart timer purge-time command). The GR helper marks all routes learned from the GR restarter as stale instead of deleting them. It continues to use these routes for packet forwarding. During the GR process, packet forwarding is not interrupted.
After the active/standby switchover or BGP restart completes, the GR restarter re-establishes a BGP session to the GR helper. If the BGP session fails to be established within the GR timer advertised by the GR restarter, the GR helper removes the stale routes.
If the BGP session is established, routing information is exchanged for the GR restarter to retrieve route entries and for the GR helper to recover stale routes.
Both the GR restarter and the GR helper start the End-Of-RIB marker waiting timer.
The End-Of-RIB marker waiting time is set by 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. The GR restarter updates the RIB with the BGP routes already learned, and removes the aged routes from the RIB. The GR helper removes the stale routes.
The GR restarter quits the GR process if routing information exchange is not completed within the RIB purge timer. It updates the RIB with the BGP routes already learned, and removes the aged routes.
Follow these guidelines when you configure BGP GR:
The End-Of-RIB 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. It controls the time for the local end to receive updates from the peer.
As a best practice, 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. Enter BGP instance view. | bgp as-number [ instance instance-name ] | N/A |
3. Enable GR capability for BGP. | graceful-restart | By default, GR capability is disabled for BGP. |
4. Configure the GR timer. | graceful-restart timer restart timer | The default setting is 150 seconds. The time that a peer waits to re-establish a session must be less than the hold time. |
5. Configure the maximum time to wait for the End-of-RIB marker. | graceful-restart timer wait-for-rib timer | The default setting is 180 seconds. |
6. Configure the RIB purge timer. | graceful-restart timer purge-time timer | The default setting is 480 seconds. |