Graceful restart

BGP4 has the capability to gracefully restart.

In some situations, route flap is caused by routers that appear to be offline but the hardware portion of the router (control plane) can continue to function normally. One example of this is when some software is restarting or being upgraded, but the hardware can still function normally.

Graceful restart is best used for these situations where routing will not be interrupted, but the router is unresponsive to routing update advertisements. Graceful restart does not have to be supported by all routers in a network, but the network will benefit when more routers support it.

FortiGate HA clusters can benefit from graceful restart. When a failover takes place, the HA cluster will advertise it is going offline, and will not appear as a route flap. It will also enable the new HA main unit to come online with an updated and usable routing table — if there is a flap the HA cluster routing table will be out of date.

For example, your FortiGate unit is one of four BGP routers that send updates to each other. Any of those routers may support graceful starting—when a router plans to go offline, it will send out a message to its neighbors how long it expects to be before being back online. That way its neighbor routers don’t remove it from their routing tables. However if that router isn’t back online when expected, the routers will mark it offline. This prevents routing flap and its associated problems.

 

Scheduled time offline

Graceful restart is a means for a router to advertise it is going to have a scheduled shutdown for a very short period of time. When neighboring routers receive this notice, they will not remove that router from their routing table until after a set time elapses. During that time if the router comes back online, everything continues to function as normal. If that router remains offline longer than expected, then the neighboring routers will update their routing tables as they assume that router will be offline for a long time.

FortiGate units support both graceful restart of their own BGP routing software, and also neighboring BGP routers.

For example, if a neighbor of your FortiGate unit, with an IP address of 172.20.120.120, supports graceful restart, enter the command:

config router bgp config neighbor

edit 172.20.120.120

set capability-graceful-restart enable end

end

If you want to configure graceful restart on your FortiGate unit where you expect the Fortigate unit to be offline for no more than 2 minutes, and after 3 minutes the BGP network should consider the FortiGate unit offline, enter the command:

config router bgp

set graceful-restart enable set graceful-restart-time 120 set graceful-stalepath-time 180

end

The BGP commands related to BGP graceful restart are:

 

config router bgp

set graceful-restart { disable| enable}

set graceful-restart-time <seconds_integer> set graceful-stalepath-time <seconds_integer> set graceful-update-delay <seconds_integer> config neighbor

set capability-graceful-restart {enable | disable}

end end

 

execute router restart

Before the restart, the router sends its peers a message to say it is restarting. The peers mark all the restarting router’s routes as stale, but they continue to use the routes. The peers assume the router will restart and check its routes and take care of them if needed after the restart is complete. The peers also know what services the restarting router can maintain during its restart. After the router completes the restart, the router sends its peers a message to say it is done restarting.

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