Cisco IOS Software Releases 12.0 T

Table of Contents

BGP Soft Reset Enhancement

This feature module describes the Border Gateway Protocol (BGP) Soft Reset Enhancement feature. It includes information on the benefits of the new feature, supported platforms, related documents, and so forth.

This document includes the following sections:

Feature Overview

Routing policies for a peer include all the configurations such as route-map, distribute-list, prefix-list, and filter-list that may impact inbound or outbound routing table updates. Whenever there is a change in the routing policy, the BGP session must be cleared, or reset, for the new policy to take effect. There are two types of reset, hard reset and soft reset.

Clearing a BGP session using a hard reset invalidates the cache and results in a negative impact on the operation of networks as the information in the cache becomes unavailable.

Soft reset is recommended because it allows routing tables to be reconfigured and activated without clearing the BGP session. Soft reset is done on a per-neighbor basis. There are two types of soft reset:

Previously, in order to perform a soft reset for inbound routing table updates, the neighbor soft-reconfiguration command directed the Cisco IOS software in the local BGP router to store all received (inbound) routing policy updates without modification. This method is memory-intensive and not recommended unless absolutely necessary. (Outbound updates have never required the extra memory and are not affected by this feature.)

With this software release, the BGP Soft Reset Enhancement feature provides automatic support for dynamic soft reset of inbound BGP routing table updates that is not dependent upon stored routing table update information. The new method requires no preconfiguration (as with the neighbor soft-reconfiguration command) and requires much less memory than the previous soft reset method for inbound routing table updates.

Managing Routing Policy Changes

When the routing policy of a BGP neighbor changes, the session must be reset (cleared) for the changes to take effect. Because resetting a BGP session can be disruptive to networks, a soft reset method is recommended for reconfiguring the routing table.

Previously, in order to reconfigure the inbound routing table, both the local BGP router and the BGP peer first needed to be configured to store incoming routing policy updates using the neighbor soft-reconfiguration command. Additional resources, particularly memory, were required to store the inbound routing table updates. The clear ip bgp command could then initiate the soft reset, which generated a new set of inbound routing table updates using the stored information.

This feature provides an additional method for soft reset that allows the dynamic exchange of route refresh requests and routing information between BGP routers, and the subsequent re-advertisement of the respective outbound routing table. Soft reset using the route refresh capability does not require preconfiguration and consumes no additional memory resources.

To use this new method, both BGP peers must support the soft route refresh capability, which is advertised in the OPEN message sent when a peer sends its routing table update. Any router running BGP with this software release automatically supports the route refresh capability. Routers running previous Cisco IOS software releases do not support the route refresh capability and must use the older soft reset method.

If the soft reset fails, you can still clear the BGP session, but it will have a negative impact upon network operations and should only be used as a last resort.

Benefits

Allows Dynamic Route Refresh Requests

This feature provides a way to initiate nondisruptive routing policy changes by allowing the dynamic exchange of route refresh requests between BGP routers, and the subsequent re-advertisement of the respective outbound routing tables.

Requires No Preconfiguration

Because support for the soft reset using the route refresh capability is included in this release of the Cisco IOS software, no further router configuration is required. You can initiate a soft inbound reset using only the clear ip bgp in command.

Requires No Additional Memory Resources

Unlike a soft reset using the stored inbound routing table updates provided by the neighbor soft-reconfiguration command, when both BGP peers support the route refresh capability inbound routing table updates are not stored in the local BGP router. The soft reset requests are exchanged dynamically, and no additional memory is required.

Flexibility

There are now two available methods for inbound soft reset; the older method using stored inbound routing table updates, and the method provided by this feature using dynamic exchange of update information.

Restrictions

Route Refresh Support for BGP Peers

BGP peers must support the route refresh capability to use dynamic inbound soft reset capability. If a peer does not support the route refresh capability, then the only soft reconfiguration option is to use the neighbor soft-reconfiguration command, which initiates the storage of inbound routing table updates and requires additional memory.

Dynamic and Stored Inbound Soft Reset Functions are Mutually Exclusive

The dynamic inbound soft reset and inbound soft reset using stored information functions are mutually exclusive and cannot be configured together. If the inbound soft reset using stored routing table updates is configured for a neighbor, the dynamic inbound soft update method cannot be used.

Related Documents

Supported Platforms

Supported Standards, MIBs, and RFCs

Standards

MIB

No new or modified MIBs are supported by this feature.

RFCs

No new or modified RFCs are supported by this feature.

Prerequisites

Both BGP peers must support the soft reset route refresh capability, first included in this software release. If one router does not, then the only soft reconfiguration option is to use the neighbor soft-reconfiguration command, which initiates storage of inbound routing table updates and requires additional memory, followed by the clear ip bgp in command.

Configuration Tasks

See the following "Configuring BGP Soft Reset" section for configuration tasks for the BGP Soft Reset Enhancement feature. This task is required.

Configuring BGP Soft Reset

Whenever there is a change in the routing policy, the BGP session must be reset (cleared) for the new policy to take effect and the routing table to be reconfigured. Using a hard reset to clear a BGP session causes cache invalidation and results in a negative impact on network operation.

Soft reset is recommended because it allows routing table policies to be reconfigured and activated without clearing the BGP session. Soft reset is done on a per-neighbor basis. Soft resets can be inbound or outbound:

There are two ways to perform an inbound soft reset: dynamically, and using stored routing update information.

There is only one way to perform an outbound soft reset.

Configuring BGP Dynamic Inbound Soft Reset

If both the local BGP router and the neighbor router support the route refresh capability, you can perform a dynamic soft inbound reset. This type of reset has the following advantages over a soft inbound reset using stored routing update information:

To determine whether a router supports the route refresh capability, use the show ip bgp neighbors command.

If all the BGP routers support the route refresh capability, you can use the dynamic soft reset method for resetting the inbound routing table.

Configuring BGP Outbound Soft Reset

Outbound soft resets do not require any preconfiguration. Using the keyword soft specifies that a soft reset be performed.

Configuring BGP Soft Reset Using Stored Routing Policy Information

If all of the BGP routers in the connection do not support the route refresh capability, use the soft reset method that generates a new set of inbound routing table updates from information previously stored. To initiate storage of inbound routing table updates, you must first preconfigure the router using the neighbor soft-reconfiguration command.

Keep in mind that the memory requirements for storing the inbound update information can become quite large.

Once you have initiated storage of inbound routing table updates for a specific neighbor or peer group, you can perform a soft inbound reset for that neighbor or peer group.

Verifying BGP Soft Reset

Enter the show ip bgp neighbors command to display information about the BGP and TCP connections to neighbors and verify the status and configuration of the BGP soft reset feature. The following sample output shows that a soft reset has been configured for neighbor 10.4.9.8:

Configuration Examples

This section provides the following configuration examples:

Dynamic Inbound Soft Reset

The following examples shows the clear ip bgp 131.108.1.1 soft in command used to initiate a dynamic soft reconfiguration in the BGP peer 131.108.1.1. This command requires that the peer supports the route refresh capability.

Inbound Soft Reset Using Stored Information

The following example enables inbound soft reconfiguration for the neighbor 131.108.1.1. All the updates received from this neighbor will be stored unmodified, regardless of the inbound policy. When inbound soft reconfiguration is performed later, the stored information will be used to generate a new set of inbound updates.

The following example clears the session with the neighbor 131.108.1.1.

Command Reference

This section documents new or modified commands. All other commands used with this feature are documented in the Cisco IOS Release 12.0 command reference publications.

In Cisco IOS Release 12.0(1)T or later, you can search and filter the output for show and more commands. This functionality is useful when you need to sort through large amounts of output, or if you want to exclude output that you do not need to see.

To use this functionality, enter a show or more command followed by the "pipe" character (|), one of the keywords begin, include, or exclude, and an expression that you want to search or filter on:

command | {begin | include | exclude} regular-expression

Following is an example of the show atm vc command in which you want the command output to begin with the first line where the expression "PeakRate" appears:

show atm vc | begin PeakRate

For more information on the search and filter functionality, refer to the Cisco IOS Release 12.0(1)T feature module titled CLI String Search.

clear ip bgp

To reset a Border Gateway Protocol (BGP) connection or session, use the clear ip bgp EXEC command.

clear ip bgp {* | address | peer-group-name} [soft [in | out]]

Syntax Description

Defaults

No default behavior or values.

Command Modes

EXEC

Command History

Usage Guidelines

You can reset inbound routing table updates dynamically or by generating new updates using stored update information. Using stored update information requires additional memory for storing the updates.

To reset inbound routing table updates dynamically, all BGP routers must support the route refresh capability. To determine whether a BGP router supports this capability, use the show ip bgp neighbors command. If a router supports the route refresh capability, the following message is displayed:

Received route refresh capability from peer.

If all BGP routers support the route refresh capability, use the clear ip bgp {* | address | peer-group name} in command. You need not use the soft keyword, because soft reset is automatically assumed when the route refresh capability is supported.

To generate new inbound updates from stored update information (rather than dynamically), without resetting the BGP session, you must first preconfigure the local BGP router using the neighbor soft-reconfiguration command. This preconfiguration causes the software to store all received updates without modification regardless of whether an update is accepted by the inbound policy. Storing updates is memory intensive and should be avoided if possible.

Outbound BGP soft configuration has no memory overhead and does not require any preconfiguration. You can trigger an outbound reconfiguration on the other side of the BGP session to make the new inbound policy take effect.

Use this command whenever any of the following changes occur:

Examples

The following example clears all entries from the host name-and-address cache (hard reset):

The following example clears the inbound session with the neighbor 131.108.1.1 without resetting the session:

The following example clears the outbound session with the peer-group corp without resetting the session:

Related Commands

show ip bgp neighbors

To display information about the TCP and BGP connections to neighbors, use the show ip bgp neighbors EXEC command.

show ip bgp neighbors [neighbor-address] [received-routes | routes | advertised-routes | {paths regexp} | dampened-routes]

Syntax Description

Command Modes

EXEC

Command History

Examples

The following is sample output from the show ip bgp neighbors command in privileged EXEC mode:

Table 1 describes the significant fields shown in the display.