Home | Skip to Content | Skip to Search | Skip to Navigation | Skip to Footer |
Worldwide [change] |Register |About Cisco
Guest

Hierarchical Navigation

Cisco IOS Software Releases 12.3 T

BGP Support for Next-Hop Address Tracking

Downloads

Table Of Contents

BGP Support for Next-Hop Address Tracking

Contents

Prerequisites for BGP Support for Next-Hop Address Tracking

Restrictions for BGP Support for Next-Hop Address Tracking

Information About BGP Support for Next-Hop Address Tracking

Default BGP Scanner Behavior

BGP Support for Next-Hop Address Tracking

How to Configure BGP Next-Hop Address Tracking

Disabling BGP Next-Hop Address Tracking

Adjusting the Delay Interval for BGP Next-Hop Address Tracking

Configuration Examples for BGP Next-Hop Address Tracking

Enabling and Disabling BGP Next-Hop Address Tracking: Example

Adjusting the Delay Interval for BGP Next-Hop Address Tracking: Example

Where to Go Next

Additional References

Related Documents

Standards

MIBs

RFCs

Technical Assistance

Command Reference

bgp nexthop trigger enable

bgp nexthop trigger delay


BGP Support for Next-Hop Address Tracking

The BGP Support for Next-Hop Address Tracking feature introduces an event driven notification system to monitor the status of routes that are installed in the Routing Information Base (RIB) and to report next-hop changes that affect internal BGP (iBGP) or external BGP (eBGP) prefixes directly to the Border Gateway Protocol (BGP) process. This feature improves the overall BGP convergence time by allowing BGP to respond rapidly to next-hop changes for routes installed in the RIB.

Feature History for the BGP Support for Next-Hop Address Tracking feature

Release
Modification

12.0(29)S

This feature was introduced.

12.0(31)S

The default delay interval value was changed from 1 to 5 seconds.

12.3(14)T

This feature was integrated into Cisco IOS Release 12.3(14)T.


Finding Support Information for Platforms and Cisco IOS Software Images

Use Cisco Feature Navigator to find information about platform support and Cisco IOS software image support. Access Cisco Feature Navigator at http://www.cisco.com/go/fn. You must have an account on Cisco.com. If you do not have an account or have forgotten your username or password, click Cancel at the login dialog box and follow the instructions that appear.

Contents

•Prerequisites for BGP Support for Next-Hop Address Tracking

•Restrictions for BGP Support for Next-Hop Address Tracking

•Information About BGP Support for Next-Hop Address Tracking

•How to Configure BGP Next-Hop Address Tracking

•Configuration Examples for BGP Next-Hop Address Tracking

•Additional References

•Command Reference

Prerequisites for BGP Support for Next-Hop Address Tracking

This document assumes that BGP is enabled and peering has been established.

Restrictions for BGP Support for Next-Hop Address Tracking

This feature is not supported under the IPv6 address family.

Information About BGP Support for Next-Hop Address Tracking

•Default BGP Scanner Behavior

•BGP Support for Next-Hop Address Tracking

Default BGP Scanner Behavior

BGP monitors the next hop of installed routes to verify next-hop reachability and to select, install, and validate the BGP best path. By default, the BGP scanner is used to poll the RIB for this information every 60 seconds. During the 60 second time period between scan cycles, Interior Gateway Protocol (IGP) instability or other network failures can cause black holes and routing loops to temporarily form.

BGP Support for Next-Hop Address Tracking

The BGP Support for Next-Hop Address Tracking feature is enabled by default when a supporting Cisco IOS software image is installed. BGP next-hop address tracking is event driven. BGP prefixes are automatically tracked as peering sessions are established. Next-hop changes are rapidly reported to the BGP routing process as they are updated in the RIB. This optimization improves overall BGP convergence by reducing the response time to next-hop changes for routes installed in the RIB. When a bestpath calculation is run in between BGP scanner cycles, only next-hop changes are tracked and processed.

How to Configure BGP Next-Hop Address Tracking

The tasks in this section show how configure BGP next-hop address tracking. BGP next-hop address tracking significantly improves the response time of BGP to next-hop changes in the RIB. However, unstable Interior Gateway Protocol (IGP) peers can introduce instability to BGP neighbor sessions. We recommend that you aggressively dampen unstable IGP peering sessions to reduce the possible impact to BGP. For more details about route dampening, see the

•Disabling BGP Next-Hop Address Tracking

•Adjusting the Delay Interval for BGP Next-Hop Address Tracking

•Configuration Examples for BGP Next-Hop Address Tracking

Disabling BGP Next-Hop Address Tracking

Perform this task to disable BGP next-hop address tracking. BGP next-hop address tracking is enabled by default under the IPv4 and VPNv4 address families. Disabling next hop address tracking may be useful if you the network has unstable IGP peers and route dampening is not resolving the stability issues. To reenable BGP next-hop address tracking, use the bgp nexthop command with the trigger and enable keywords.

SUMMARY STEPS

1. enable

2. configure terminal

3. router bgp as-number

4. address-family ipv4 [mdt | multicast | tunnel | unicast [vrf vrf-name] | vrf vrf-name] | vpnv4 [unicast]

5. no bgp nexthop trigger enable

6. end

DETAILED STEPS

 
Command or Action
Purpose

Step 1 

enable

Example:

Router> enable

Enables privileged EXEC mode.

•Enter your password if prompted.

Step 2 

configure terminal

Example:

Router# configure terminal

Enters global configuration mode.

Step 3 

router bgp as-number

Example:

Router(config)# router bgp 64512

Enters router configuration mod to create or configure a BGP routing process.

Step 4 

address-family ipv4 [mdt | multicast | tunnel | unicast [vrf vrf-name] | vrf vrf-name] | vpnv4 [unicast]

Example:

Router(config-router-af)# address-family ipv4 unicast

Enter address family configuration mode to configure BGP peers to accept address family-specific configurations.

•The example creates an IPv4 unicast address family session.

Step 5 

no bgp nexthop trigger enable

Example:

Router(config-router-af)# no bgp nexthop trigger enable

Disables BGP next-hop address tracking.

•Next-hop address tracking is enabled by default for IPv4 and VPNv4 address family sessions.

•The example disables next-hop address tracking.

Step 6 

end

Example:

Router(config-router-af)# end

Exits address-family configuration mode, and enters Privileged EXEC mode.

Adjusting the Delay Interval for BGP Next-Hop Address Tracking

Perform this task to adjust the delay interval between routing table walks for BGP next-hop address tracking.

Delay Interval Tuning to Match the Interior Gateway Protocol

You can increase the performance of this feature by tuning the delay interval between full routing table walks to match the tuning parameters for the Interior Gateway protocol (IGP). The default delay interval is 5 seconds. This value is optimal for a fast-tuned IGP. In the case of an IGP that converges more slowly, you can change the delay interval to 20 seconds or more, depending on the IGP convergence time.

Aggressive IGP Route Dampening

BGP next-hop address tracking significantly improves the response time of BGP to next-hop changes in the RIB. However, unstable Interior Gateway Protocol (IGP) peers can introduce instability to BGP neighbor sessions. We recommend that you aggressively dampen unstable IGP peering sessions to reduce the possible impact to BGP.

SUMMARY STEPS

1. enable

2. configure terminal

3. router bgp as-number

4. address-family ipv4 [mdt | multicast | tunnel | unicast [vrf vrf-name] | vrf vrf-name] | vpnv4 [unicast]

5. bgp nexthop trigger delay delay-timer

6. end

DETAILED STEPS

 
Command or Action
Purpose

Step 1 

enable

Example:

Router> enable

Enables privileged EXEC mode.

•Enter your password if prompted.

Step 2 

configure terminal

Example:

Router# configure terminal

Enters global configuration mode.

Step 3 

router bgp as-number

Example:

Router(config)# router bgp 64512

Enters router configuration mode to create or configure a BGP routing process.

Step 4 

address-family ipv4 [mdt | multicast | tunnel | unicast [vrf vrf-name] | vrf vrf-name] | vpnv4 [unicast]

Example:

Router(config-router-af)# address-family ipv4 unicast

Enter address family configuration mode to configure BGP peers to accept address family-specific configurations.

•The example creates an IPv4 unicast address family session.

Step 5 

bgp nexthop trigger delay delay-timer

Example:

Router(config-router-af)# bgp nexthop trigger delay 20

Configures the delay interval between routing table walks for next-hop address tracking.

•The time period determines how long BGP will wait before starting a full routing table walk after notification is received.

•The value for the delay-timer argument is a number from 1 to 100 seconds. The default value is 5 second.

•The example configures a delay interval of 20 seconds.

Step 6 

end

Example:

Router(config-router-af)# end

Exits address-family configuration mode, and enters privileged EXEC mode.

Configuration Examples for BGP Next-Hop Address Tracking

The following examples show how to configure and tune this feature:

•Enabling and Disabling BGP Next-Hop Address Tracking: Example

•Adjusting the Delay Interval for BGP Next-Hop Address Tracking: Example

Enabling and Disabling BGP Next-Hop Address Tracking: Example

In the following example, next-hop address tracking is disabled under the IPv4 address family session: 

router bgp 50000 

 address-family ipv4 unicast

  no bgp nexthop trigger enable

Adjusting the Delay Interval for BGP Next-Hop Address Tracking: Example

In the following example, the delay interval for next-hop tracking is configured to occur every 20 seconds under the IPv4 address family session: 

router bgp 50000 

 address-family ipv4 unicast 

  bgp nexthop trigger delay 20

Where to Go Next

The BGP Support for Fast Peering Session Deactivation feature can also be configured to improve the response time of BGP to adjacency changes, improving overall BGP convergence. For information about BGP fast session deactivation, see the BGP Support for Fast Peering Session Deactivation feature.

Additional References

The following sections provide references related to the BGP Support for Next-Hop Address Tracking feature.

Related Documents

Related Topic
Document Title

BGP commands

•Cisco IOS IP Command Reference, Volume 2 of 4: Routing Protocols, Release 12.3T

BGP configuration tasks

•Cisco IOS IP Configuration Guide, Release 12.3

BGP Support for Fast Peering Session Deactivation

•BGP Support for Fast Peering Session Deactivation

IP Event Dampening

•IP Event Dampening


Standards

Standards
Title

No new or modified standards are supported by this feature, and support for existing standards has not been modified by this feature.

—


MIBs

MIBs
MIBs Link

No new or modified MIBs are supported by this feature, and support for existing MIBs has not been modified by this feature.

To obtain lists of supported MIBs by platform and Cisco IOS release, and to download MIB modules, go to the Cisco MIB website on Cisco.com at the following URL:

http://www.cisco.com/public/sw-center/netmgmt/cmtk/mibs.shtml


RFCs

RFCs
Title

No new or modified RFCs are supported by this feature, and support for existing RFCs has not been modified by this feature.

—


Technical Assistance

Description
Link

Technical Assistance Center (TAC) home page, containing 30,000 pages of searchable technical content, including links to products, technologies, solutions, technical tips, and tools. Registered Cisco.com users can log in from this page to access even more content.

TAC Home Page:

http://www.cisco.com/public/support/tac/home.shtml

BGP Support Page:

http://www.cisco.com/cgi-bin/Support/browse/psp_view.pl?p=Internetworking:BGP


Command Reference

This section documents the following new commands:

•bgp nexthop trigger enable

•bgp nexthop trigger delay

bgp nexthop trigger enable

To enable Border Gateway Protocol (BGP) next-hop address tracking, use the bgp nexthop trigger enable command in address family or router configuration mode. To disable BGP next-hop address tracking, use the no form of this command.

bgp nexthop trigger enable

no bgp nexthop trigger enable

Syntax Description

This command has no keywords or arguments.

Defaults

BGP next-hop address tracking is enabled by default under IPv4 and VPNv4 address-family sessions.

Command Modes

Address family configuration
Router Configuration

Command History

Release
Modification

12.0(29)S

This command was introduced.

12.3(14)T

This command was integrated into Cisco IOS Release 12.3(14)T.


Usage Guidelines

The bgp nexthop trigger enable command is used to enable the BGP Support for Next-Hop Address Tracking feature. BGP next-hop address tracking is event driven. BGP prefixes are automatically tracked as peering sessions are established. Next-hop changes are rapidly reported to BGP as they are updated in the RIB. This optimization improves overall BGP convergence by reducing the response time to next-hop changes for routes installed in the RIB. When a bestpath calculation is run in between BGP scanner cycles, only the changes are processed and tracked.

Note This feature improves BGP response time significantly. However, unstable IGP peers can introduce instability to the BGP session. We recommend that you aggressively dampen unstable IGP peering sessions to mitigate the possible impact to BGP.

Note This feature is not supported under the IPv6 address family.

Examples

In the following example, next-hop address tracking is disabled under the IPv4 address family session: 

Router(config)# router bgp 50000 

Router(config-router)# address-family ipv4 unicast

Router(config-router-af)# no bgp nexthop trigger enable

Router(config-router-af)# end

Related Commands

Command
Description

bgp nexthop trigger delay

Configures the delay interval between routing table walks for BGP next-hop address tracking.

neighbor fall-over

Configures BGP to use fast session deactivation for the specified peering session.


bgp nexthop trigger delay

To adjust the delay interval between routing table walks for Border Gateway Protocol (BGP) next-hop address tracking, use the bgp nexthop trigger delay command in address family or router configuration mode. To set the delay interval between routing table walks to the default value, use the no form of this command.

bgp nexthop trigger delay seconds

no bgp nexthop trigger delay

Syntax Description

seconds

The number of seconds for the delay. The range of values that can be entered is from 1 to 100 seconds.


Defaults

The routing table is walked every 5 seconds.

Command Modes

Address family configuration
Router Configuration

Command History

Release
Modification

12.0(29)S

This command was introduced.

12.0(31)S

The default delay interval was changed from 1 to 5 seconds.

12.3(14)T

This command was integrated into Cisco IOS Release 12.3(14)T.


Usage Guidelines

The bgp nexthop trigger delay command is used to adjust the delay interval between routing table walks for BGP next-hop address tracking. You can increase the performance of this feature by tuning the delay interval between full routing table walks to match the tuning parameters for the Interior Gateway protocol (IGP). The default delay interval is 5 seconds. This value is optimal for a fast-tuned IGP. In the case of an IGP that converges more slowly, you can change the delay interval to 20 seconds or more, depending on the IGP convergence time.

Note This feature improves BGP response time significantly. However, unstable IGP peers can introduce instability to the BGP session. We recommend that you aggressively dampen unstable IGP peering sessions to mitigate the possible impact to BGP.

Examples

In the following example, the delay interval is set to occur every 20 seconds under an IPv4 address family session: 

Router(config)# router bgp 50000 

Router(config-router)# address-family ipv4 unicast 

Router(config-router-af)# bgp nexthop trigger delay 20 

Router(config-router-af)# end

Related Commands

Command
Description

bgp nexthop trigger enable

Enables or disables BGP next-hop address tracking

neighbor fall-over

Configures BGP to use fast session deactivation for the specified peering session.


Copyright © 2005 Cisco Systems, Inc. All rights reserved.