dampening (BGP)
Function
The dampening command enables BGP route flap damping or modifies various BGP route flap damping parameters.
The undo dampening command disables the route flap damping.
By default, route flap damping is not configured.
Format
dampening [ ibgp ] [ half-life-reach reuse suppress ceiling | route-policy route-policy-name ] *
undo dampening [ ibgp ]
Parameters
| Parameter | Description | Value |
|---|---|---|
| ibgp | Specifies the half life reachable of the ibgp route. Only the BGP-VPN instance IPv4 address family view support this parameter. |
- |
| half-life-reach | Specifies the half life reachable of a route. | The value is an integer that ranges from 1 to 45, in minutes. By default, it is 15. |
| reuse | Specifies the threshold value for the route to be unsuppressed. If the penalty of the route is lower than the value, the route is reused. | The value is an integer that ranges from 1 to 20000. By default, it is 750. |
| suppress | Specifies the threshold value for the route to be suppressed. The route is not used when the penalty reaches the threshold. | The value is an integer ranges from 1 to 20000. By default, it is 2000. It must be greater than the value of reuse. |
| ceiling | Specifies the ceiling of the threshold. | The value is an integer ranges from 1001 to 20000. By default, it is 16000. It must be greater than the value of suppress. |
| route-policy route-policy-name | Specifies the name of the Route-Policy. | The name must be unique and is a string of 1 to 40 characters. It is case-sensitive. |
Views
BGP view, BGP-IPv4 unicast address family view, BGP-IPv6 unicast address family view, BGP-VPN instance IPv4 address family view, BGP-VPN instance IPv6 address family view, BGP-IPv4 multicast address family view, BGP-VPNv4 address family view
Usage Guidelines
Usage Scenario
If the suppression policy for route dampening is configured, routes preferentially match suppression parameters defined by route-policy in the command.
If no optional parameter is configured in the command, the default value of each parameter is used. The four parameters of the command are mutually dependent. If you configure one of the parameters, the other parameters also need to be configured in the command.
BGP route flap dampening measures the stability of a route by using a penalty value. The greater the penalty value, the less stable the route. Each time route flapping occurs, that is, when the router receives a Withdraw packet or an Update packet for a route, BGP adds a certain penalty value (1000) for the route.
When the penalty value of the route exceeds the suppression threshold, the route is suppressed. The router does not add the route to the IP routing table or advertise any Update packet to other BGP peers. BGP strips off the best flag of the route. If the route is marked with the d flag, the last packet received by the router is an Update packet; if the route is marked with the h flag, the last packet received by the router is a Withdraw packet. After the penalty value reaches a certain limit, it does not increase any more. The limit is called the penalty ceiling.
After the route is suppressed for a certain period, the penalty value is reduced by half. If the penalty value of a route marked with the d flag decreases to the reuse threshold, the d flag is removed, and the route becomes available and is selected preferentially. After that, the route can be added to the IP routing table and used to send Update packets to other BGP peers. If the penalty value of a route marked with the h flag decreases to 0, the route is deleted from the BGP routing table.
Configuration Impact
If the dampening command is run more than once, the latest configuration overrides the previous one.
After the dampening command is run, the system suppresses unstable routes. This means that the system does not add unstable routes to the BGP routing table or advertise them to other BGP peers.
Precautions
Note the following items when configuring BGP route flap dampening:
The value of suppress must be greater than that of reuse and smaller than that of ceiling.
If MaxSuppressTime obtained by using the formula of MaxSuppressTime = half-life-reach x 60 x (ln(ceiling/reuse)/ln(2)) is smaller than 1, suppression cannot be performed. You need to ensure that the value of MaxSuppressTime is equal to or greater than 1. This means that the value of ceiling/reuse must be great enough.
The dampening command is valid only for EBGP routes.
The dampening ibgp command is valid only for BGP VPNv4 routes.
Creating a route-policy before it is referenced is recommended. By default, nonexistent route-policies cannot be referenced using the command. If the route-policy nonexistent-config-check disable command is run in the system view and a nonexistent route-policy is referenced using the current command, the configured dampening parameters apply to all routes; if no dampening parameters are configured, the default dampening parameters apply to the routes.