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Examples: Configuring BGP Flap Damping

 

Understanding Damping Parameters

BGP route flapping describes the situation in which BGP systems send an excessive number of update messages to advertise network reachability information. BGP flap damping is a method of reducing the number of update messages sent between BGP peers, thereby reducing the load on these peers, without adversely affecting the route convergence time for stable routes.

Flap damping reduces the number of update messages by marking routes as ineligible for selection as the active or preferable route. Marking routes in this way leads to some delay, or suppression, in the propagation of route information, but the result is increased network stability. You typically apply flap damping to external BGP (EBGP) routes (routes in different ASs). You can also apply flap damping within a confederation, between confederation member ASs. Because routing consistency within an AS is important, do not apply flap damping to internal BGP (IBGP) routes. (If you do, it is ignored.)

There is an exception that rule. Starting in Junos OS Release 12.2, you can apply flap damping at the address family level. In a Junos OS Release 12.2 or later installation, when you apply flap damping at the address family level, it works for both IBGP and EBGP.

By default, route flap damping is not enabled. Damping is applied to external peers and to peers at confederation boundaries.

When you enable damping, default parameters are applied, as summarized in Table 1.

Table 1: Damping Parameters

Damping Parameter

Description

Default Value

Possible Values

half-life minutes

Decay half-life—Number of minutes after which an arbitrary value is halved if a route stays stable.

15 (minutes)

1 through 45

max-suppress minutes

Maximum hold-down time for a route, in minutes.

60 (minutes)

1 through 720

reuse

Reuse threshold—Arbitrary value below which a suppressed route can be used again.

750

1 through 20,000

suppress

Cutoff (suppression) threshold—Arbitrary value above which a route can no longer be used or included in advertisements.

3000

1 through 20,000

To change the default BGP flap damping values, you define actions by creating a named set of damping parameters and including it in a routing policy with the damping action. For the damping routing policy to work, you also must enable BGP route flap damping.

Example: Configuring BGP Route Flap Damping Parameters

This example shows how to configure damping parameters.

Requirements

Before you begin, configure router interfaces and configure routing protocols.

Overview

This example has three routing devices. Device R2 has external BGP (EBGP) connections with Device R1 and Device R3.

Device R1 and Device R3 have some static routes configured for testing purposes, and these static routes are advertised through BGP to Device R2.

Device R2 damps routes received from Device R1 and Device R3 according to these criteria:

  • Damp all prefixes with a mask length equal to or greater than 17 more aggressively than routes with a mask length between 9 and 16.

  • Damp routes with a mask length between 0 and 8, inclusive, less than routes with a mask length greater than 8.

  • Do not damp the 10.128.0.0/9 prefix at all.

The routing policy is evaluated when routes are being exported from the routing table into the forwarding table. Only the active routes are exported from the routing table.

Figure 1 shows the sample network.

Figure 1: BGP Flap Damping Topology
BGP Flap Damping Topology

CLI Quick Configuration shows the configuration for all of the devices in Figure 1.

The section Step-by-Step Procedure describes the steps on Device R2.

Configuration

CLI Quick Configuration

To quickly configure this example, copy the following commands, paste them into a text file, remove any line breaks, change any details necessary to match your network configuration, and then copy and paste the commands into the CLI at the [edit] hierarchy level.

Device R1

Device R2

Device R3

Step-by-Step Procedure

The following example requires you to navigate various levels in the configuration hierarchy. For information about navigating the CLI, see Using the CLI Editor in Configuration Mode in the CLI User Guide.

To configure damping parameters:

  1. Configure the interfaces.

  2. Configure the BGP neighbors.
  3. Create and configure the damping parameter groups.

  4. Configure the damping policy.

  5. Enable damping for BGP.

  6. Apply the policy as an import policy for the BGP neighbor.

    Note

    You can refer to the same routing policy one or more times in the same or different import statements.

  7. Configure an export policy.
  8. Apply the export policy.
  9. Configure the autonomous system (AS) number.

Results

From configuration mode, confirm your configuration by issuing the show interfaces, show protocols, show policy-options, and show routing-options commands. If the output does not display the intended configuration, repeat the instructions in this example to correct the configuration.

If you are done configuring the device, enter commit from configuration mode.

Verification

Confirm that the configuration is working properly.

Causing Some Routes to Flap

Purpose

To verify your route flap damping policy, some routes must flap. Having a live Internet feed almost guarantees that a certain number of route flaps will be present. If you have control over a remote system that is advertising the routes, you can modify the advertising router's policy to effect the advertisement and withdrawal of all routes or of a given prefix. In a test environment, you can cause routes to flap by clearing the BGP neighbors or by restarting the routing process on the BGP neighbors, as shown here.

Action

From operational mode on Device R1 and Device R3, enter the restart routing command.

Caution

Use this command cautiously in a production network.

user@R1> restart routing




user@R3> restart routing

Meaning

On Device R2, all of the routes from the neighbors are withdrawn and re-advertised.

Checking the Route Flaps

Purpose

View the number of neighbor flaps.

Action

From operational mode, enter the show bgp summary command.

user@R2> show bgp summary

Meaning

This output was captured after the routing process was restarted on Device R2’s neighbors four times.

Verifying Route Flap Damping

Purpose

Verify that routes are being hidden due to damping.

Action

From operational mode, enter the show route damping suppressed command.

user@R2> show route damping suppressed

Meaning

The output shows some routing instability. Eleven routes are hidden due to damping.

Displaying the Details of a Damped Route

Purpose

Display the details of damped routes.

Action

From operational mode, enter the show route damping suppressed 172.16.192.0/20 detail command.

user@R2> show route damping suppressed 172.16.192.0/20 detail

Meaning

This output indicates that the displayed route has a mask length that is equal to or greater than /17, and confirms that it has been correctly mapped to the aggressive damping profile. You can also see the route’s current (and last) figure of merit value, and when the route is expected to become active if it remains stable.

Verifying That Default Damping Parameters Are in Effect

Purpose

Locating a damped route with a /16 mask confirms that the default parameters are in effect.

Action

From operational mode, enter the show route damping suppressed detail | match 0/16 command.

user@R2> show route damping suppressed detail | match 0/16




user@R2> show route damping suppressed 172.16.0.0/16 detail

Meaning

Routes with a /16 mask are not impacted by the custom damping rules. Therefore, the default damping rules are in effect.

To repeat, the custom rules are as follows:

  • Damp all prefixes with a mask length equal to or greater than 17 more aggressively than routes with a mask length between 9 and 16.

  • Damp routes with a mask length between 0 and 8, inclusive, less than routes with a mask length greater than 8.

  • Do not damp the 10.128.0.0/9 prefix at all.

Filtering the Damping Information

Purpose

Use OR groupings or cascaded piping to simplify the determination of what damping profile is being used for routes with a given mask length.

Action

From operational mode, enter the show route damping suppressed command.

user@R2> show route damping suppressed detail | match "0 announced | damp"

Meaning

When you are satisfied that your EBGP routes are correctly associated with a damping profile, you can issue the clear bgp damping operational mode command to restore an active status to your damped routes, which will return your connectivity to normal operation.

Example: Configuring BGP Route Flap Damping Based on the MBGP MVPN Address Family

This example shows how to configure an multiprotocol BGP multicast VPN (also called Next-Generation MVPN) with BGP route flap damping.

Requirements

This example uses Junos OS Release 12.2. BGP route flap damping support for MBGP MVPN, specifically, and on an address family basis, in general, is introduced in Junos OS Release 12.2.

Overview

BGP route flap damping helps to diminish route instability caused by routes being repeatedly withdrawn and readvertised when a link is intermittently failing.

This example uses the default damping parameters and demonstrates an MBGP MVPN scenario with three provider edge (PE) routing devices, three customer edge (CE) routing devices, and one provider (P) routing device.

Figure 2 shows the topology used in this example.

Figure 2: MBGP MVPN with BGP Route Flap Damping
MBGP MVPN with BGP Route Flap
Damping

On PE Device R4, BGP route flap damping is configured for address family inet-mvpn. A routing policy called dampPolicy uses the nlri-route-type match condition to damp only MVPN route types 3, 4, and 5. All other MVPN route types are not damped.

This example shows the full configuration on all devices in the CLI Quick Configuration section. The Configuring Device R4 section shows the step-by-step configuration for PE Device R4.

Configuration

CLI Quick Configuration

To quickly configure this example, copy the following commands, paste them into a text file, remove any line breaks, change any details necessary to match your network configuration, and then copy and paste the commands into the CLI at the [edit] hierarchy level.

Device R1

Device R2

Device R3

Device R4

Device R5

Device R6

Device R7

Configuring Device R4

Step-by-Step Procedure

The following example requires you to navigate various levels in the configuration hierarchy. For information about navigating the CLI, see Using the CLI Editor in Configuration Mode in the CLI User Guide.

To configure Device R4:

  1. Configure the interfaces.

  2. Configure MPLS and the signaling protocols on the interfaces.

  3. Configure BGP.

    The BGP configuration enables BGP route flap damping for the inet-mvpn address family. The BGP configuration also imports into the routing table the routing policy called dampPolicy. This policy is applied to neighbor PE Device R2.

  4. Configure an interior gateway protocol.

  5. Configure a damping policy that uses the nlri-route-type match condition to damp only MVPN route types 3, 4, and 5.

  6. Configure the damping policy to disable BGP route flap damping.

    The no-damp policy (damping no-damp disable) causes any damping state that is present in the routing table to be deleted. The then damping no-damp statement applies the no-damp policy as an action and has no from match conditions. Therefore, all routes that are not matched by term1 are matched by this term, with the result that all other MVPN route types are not damped.

  7. Configure the parent_vpn_routes to accept all other BGP routes that are not from the inet-mvpn address family.

    This policy is applied as an OSPF export policy in the routing instance.

  8. Configure the VPN routing and forwarding (VRF) instance.

  9. Configure the router ID and the autonomous system (AS) number.

  10. If you are done configuring the device, commit the configuration.

Results

From configuration mode, confirm your configuration by entering the show interfaces, show protocols, show policy-options, show routing-instances, and show routing-options commands. If the output does not display the intended configuration, repeat the instructions in this example to correct the configuration.

Verification

Confirm that the configuration is working properly.

Verifying That Route Flap Damping Is Disabled

Purpose

Verify the presence of the no-damp policy, which disables damping for MVPN route types other than 3, 4, and 5.

Action

From operational mode, enter the show policy damping command.

Meaning

The output shows that the default damping parameters are in effect and that the no-damp policy is also in effect for the specified route types.

Verifying Route Flap Damping

Purpose

Check whether BGP routes have been damped.

Action

From operational mode, enter the show bgp summary command.

Meaning

The Damp State field shows that zero routes in the bgp.mvpn.0 routing table have been damped. Further down, the last number in the State field shows that zero routes have been damped for BGP peer 172.16.1.2.

Release History Table
Release
Description
Starting in Junos OS Release 12.2, you can apply flap damping at the address family level.