When BGP restarts, the peer relationship is re-established and the forwarding is interrupted. After Graceful Restart (GR) is enabled, traffic interruption can be avoided.
The following roles are involved in the GR process:
GR restarter: indicates a device that performs GR caused by a fault or triggered by the administrator. The GR restarter must be a GR-capable device. That is, the router must be enabled with GR and negotiates the GR capability with its peer.
GR helper: indicates a neighbor of the GR restarter. The GR helper must also have the GR capability.
The following concepts are involved in the GR process:
GR session: indicates the session with the GR capability. The GR session is a negotiation mechanism between the GR restarter and the GR helper. By controlling the session negotiation mechanism of the protocols, the GR restarter and the GR helper can know each other’s GR capability and set up a GR session.
GR time: indicates the time during which a GR helper retains the forwarding information after detecting that the GR restarter is Down. When detecting that the GR restarter is in the Down state, the GR helper retains the topology or routing information obtained from the GR restarter and does not delete the information during the GR time.
Principles of BGP GR are listed as follows:
Using the capability negotiation mechanism of BGP, BGP speakers negotiate the GR capability before setting up a BGP session with the GR capability.
When detecting the restart of the GR restarter, a GR helper does not delete the routing information and forwarding entries related to the GR restarter, but waits to re-establish a BGP connection with the GR restarter.
After setting up a new BGP connection, the GR restarter and the GR helper update BGP routes.
In this manner, the forwarding is not interrupted. In addition, the flapping of BGP occurs only on the neighbors of the GR restarter, and does not occur in the entire routing domain. This is important for BGP that needs to process a large number of routes.
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