Table of Contents
- About the Authors
- Icons Used in This Book
- Command Syntax Conventions
- Part I. Ethernet Switching
- Chapter 1. Configuring Advanced Switching on the Cisco Catalyst 3550 Ethernet Switch
- Part II. Controlling Network Propagation and Network Access
- Chapter 2. Configuring Route Maps and Policy-Based Routing
- Part III. Multicast Routing
- Chapter 3. Configuring Multicast Routing
- Part IV. Performance Management and Quality of Service
- Chapter 4. Router Performance Management
- Chapter 5. Integrated and Differentiated Services
- Chapter 6. QoS Rate Limiting and Queuing Traffic
- Part V. BGP Theory and Configuration
- Chapter 7. BGP-4 Theory
- Chapter 8. Introduction to BGP-4 Configuration
Chapter 9. Advanced BGP Configuration
- BGP Neighbor Authentication
- Simplifying Large BGP Networks
- Practical Example: BGP Confederations
- Route Aggregation
- Filtering BGP Routes
- Using BGP Attributes to Create Routing Policies
- Using Multiple Paths
- Practical Example: Multihoming a BGP Network
- Administrative Distance and Its Effects on BGP
- BGP Route Dampening
- Tuning BGP Performance
- Practice Scenarios
- Lab 15: Multihoming a BGP Network
- Further Reading
- Part VI. CCIE Practice Labs
- Chapter 10. CCIE Preparation and Practice Labs
- Part VII. Appendixes
- Appendix A. Cisco IOS Software Limitations and Restrictions
- Appendix B. RFCs
- Appendix C. Bibliography
- Appendix D. IP Prefix Lists
BGP Route Dampening
BGP route dampening controls the effects of route flapping between E-BGP peers. Route dampening is generally used to help service providers prevent one customer's router or circuit problems from affecting the stability of the provider's network by withdrawing problem BGP routes. There are two ways to enable route dampening: The first is to globally enable route dampening for all BGP peers using the bgp dampening command; the second is to use a route map to specify certain routes that are to be dampened and the parameters that are to be applied to the dampened networks. The following syntax shows the bgp dampening command and its optional parameters.
bgp dampening [[route-map route-map-name] | [half-life] | reuse-limit start- suppress suppress-duration]]
Using the bgp dampening command, route dampening can be configured three ways:
- Global route dampening using default parameters
- Global route dampening using custom parameters
- Specific route dampening using custom parameters
Table 9-12 shows the optional bgp dampening command parameters and their descriptions.
Table 9-12. BGP Route Dampening Parameters
The amount of time to wait before decrementing the dampening penalty, ranging from 1 to 45 minutes. The default half-life is 15 minutes.
The value between 1 and 20,000 that is compared to the penalty value to determine route reusability. If the penalty is greater than the suppress limit, the route will be suppressed; if not, it will be reused. The default suppress limit is 750.
This value between 1 and 20,000 specifies the penalty that will be used if a route is suppressed. The default route suppression penalty is 2000 for each route flap.
This value specifies the maximum duration that a route will be suppressed. The range for the suppress duration is from 1 to 255 minutes. The default suppress duration is 4 times the half-life, or in other words 60 minutes.
Specifies that a route map will be used to specify the route dampening parameters. Route maps are used to specify the routes that dampening policies should apply to. The same route dampening parameters apply when a route map is used.
After route dampening has been activated, a route flap penalty of 1000 points is assessed to the affected route. The router maintains a history for each route that has flapped, and that history stores the dampening information on a route-by-route basis. The half-life value is used to decrease the time suppression penalty by one-half after a route flaps. Therefore, if a route ceases to flap, it will not be dampened and the history will eventually be cleared. If the route flaps again, another penalty is imposed, and after the suppress-limit has been reached, the route is dampened. When a route has been dampened, it will not be advertised to other BGP peers until the suppress-duration has expired.
Look at the network shown in Figure 9-19. In this figure, the Service_Provider router in AS 18,901 is configured with a route dampening policy that dampens routes using the default dampening parameters with the exception of the half-life. In this case, the half-life is changed to 5 minutes, as shown in Example 9-93.
Example 9-93. BGP Configuration for the Service_Provider Router
Service_Provider# show run | begin bgp router bgp 18901 no synchronization bgp log-neighbor-changes bgp bestpath dampening 5 bgp dampening 5 network 184.108.40.206 mask 255.255.255.252 network 220.127.116.11 mask 255.255.255.252 neighbor 18.104.22.168 remote-as 60001 neighbor 22.214.171.124 remote-as 20148 no auto-summary
Figure 9-19 Service Provider to Customer Network
By default, the bgp best path dampening command is automatically entered after the bgp dampening command has been issued in later 12.2 releases of Cisco IOS Software. This command is also used to enable and disable route dampening. There are several ways to verify and track the BGP route dampening configuration, the most detailed of which is the show ip bgp dampened parameters command. Example 9-94 uses the show ip bgp dampening parameters command to show the BGP route dampening parameters for the Service_Provider router.
Example 9-94. show ip bgp dampening parameters Command
Service_Provider# show ip bgp dampening parameters dampening 5 750 2000 20 Half-life time : 5 mins Decay Time : 775 secs Max suppress penalty: 12000 Max suppress time: 20 mins Suppress penalty : 2000 Reuse penalty : 750
This command displays all the parameters for the local BGP route dampening policy, and in this case, it shows that the Service_Provider router has been configured with a 5-minute half-life. The alteration of the half-life parameter changed the maximum suppress time, so suppressed routes will not be as harshly penalized. Example 9-95 shows the default BGP route dampening parameters.
Example 9-95. Default BGP Route Dampening Parameters
Service_Provider# show ip bgp dampening parameters dampening 15 750 2000 60 (DEFAULT) Half-life time : 15 mins Decay Time : 2320 secs Max suppress penalty: 12000 Max suppress time: 60 mins Suppress penalty : 2000 Reuse penalty : 750
The show ip bgp dampening flap-statistics command displays detailed information for all dampened routes; in this case, the Service_Provider router has dampened the route to the 126.96.36.199/23 network because it flapped four times. Example 9-96 uses the show ip bgp dampening flap-statistics command to show that the route has been dampened for 3 minutes and 34 seconds and will be eligible for reuse in 10 minutes and 20 seconds.
Example 9-96. show ip bgp dampening flap-statistics Command
Service_Provider# show ip bgp dampening flap-statistics BGP RIB version is 13, local router ID is 188.8.131.52 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, Origin codes: i - IGP, e - EGP, ? - incomplete Network From Flaps Duration Reuse Path *d 184.108.40.206/23 220.127.116.11 4 00:03:34 00:10:20 60001
The clear ip bgp dampening command enables you to clear the dampened route and the flap statistics associated with the route.
Many steps can be taken to prevent BGP route dampening from occurring using the techniques covered earlier in this chapter; for instance, the Customer_B router could have been configured using some of the following BGP features:
- Multiple links and loopback interfaces to prevent network outages
- Aggregating routes into smaller, more stable prefixes so that one network failure will not affect all routes advertised to the upstream neighbor
- Creating static routes to Null0 to nail down unstable IGP routes
It will always be in your service provider's best interest to protect the stability of their network using BGP route dampening. Their policies might adversely affect a poorly configured network, so you should always try to configure your network with the most stable, redundant BGP configuration possible.