IP Routing: BGP Configuration Guide, Cisco IOS XE Release 3SE (Catalyst 3850 Switches)
BGP 4 Soft Configuration
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BGP 4 Soft Configuration

Last Updated: November 21, 2012

BGP4 soft configuration allows BGP4 policies to be configured and activated without clearing the BGP session, hence without invalidating the forwarding cache.

Finding Feature Information

Your software release may not support all the features documented in this module. For the latest caveats and feature information, see Bug Search Tool and the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the feature information table at the end of this module.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.

Information About BGP 4 Soft Configuration

BGP Session Reset

Whenever the routing policy changes due to a configuration change, BGP peering sessions must be reset by using the clear ip bgp command. Cisco software supports the following three mechanisms to reset BGP peering sessions:

  • Hard reset--A hard reset tears down the specified peering sessions including the TCP connection and deletes routes coming from the specified peer.
  • Soft reset--A soft reset uses stored prefix information to reconfigure and activate BGP routing tables without tearing down existing peering sessions. Soft reconfiguration uses stored update information, at the cost of additional memory for storing the updates, to allow you to apply new BGP policy without disrupting the network. Soft reconfiguration can be configured for inbound or outbound sessions.
  • Dynamic inbound soft reset--The route refresh capability, as defined in RFC 2918, allows the local device to reset inbound routing tables dynamically by exchanging route refresh requests to supporting peers. The route refresh capability does not store update information locally for nondisruptive policy changes. It instead relies on dynamic exchange with supporting peers. Route refresh must first be advertised through BGP capability negotiation between peers. All BGP devices must support the route refresh capability. To determine if a BGP device supports this capability, use the show ip bgp neighbors command. The following message is displayed in the output when the device supports the route refresh capability: 
         Received route refresh capability from peer.

The bgp soft-reconfig-backup command was introduced to configure BGP to perform inbound soft reconfiguration for peers that do not support the route refresh capability. The configuration of this command allows you to configure BGP to store updates (soft reconfiguration) only as necessary. Peers that support the route refresh capability are unaffected by the configuration of this command.

How to Configure BGP 4 Soft Configuration

Configuring Inbound Soft Reconfiguration When Route Refresh Capability Is Missing

Perform this task to configure inbound soft reconfiguration using the bgp soft-reconfig-backup command for BGP peers that do not support the route refresh capability. BGP peers that support the route refresh capability are unaffected by the configuration of this command. Note that the memory requirements for storing the inbound update information can become quite large.

SUMMARY STEPS

1.    enable

2.    configure terminal

3.    router bgp autonomous-system-number

4.    bgp log-neighbor-changes

5.    bgp soft-reconfig-backup

6.    neighbor {ip-address | peer-group-name} remote-as autonomous-system-number

7.    neighbor {ip-address | peer-group-name} soft-reconfiguration [inbound]

8.    neighbor {ip-address | peer-group-name} route-map map-name {in | out}

9.    Repeat Steps 6 through 8 for every peer that is to be configured with inbound soft reconfiguration.

10.    exit

11.    route-map map-name [permit | deny] [sequence-number]

12.    set local-preference number-value

13.    end

14.    show ip bgp neighbors [neighbor-address]

15.    show ip bgp [network] [network-mask]


DETAILED STEPS
 Command or ActionPurpose
Step 1
enable


Example:

Device> enable

 

Enables privileged EXEC mode.

  • Enter your password if prompted.
 
Step 2
configure terminal


Example:

Device# configure terminal

 

Enters global configuration mode.

 
Step 3
router bgp autonomous-system-number


Example:

Device(config)# router bgp 45000

 

Enters router configuration mode for the specified routing process.

 
Step 4
bgp log-neighbor-changes


Example:

Device(config-router)# bgp log-neighbor-changes

 

Enables logging of BGP neighbor resets.

 
Step 5
bgp soft-reconfig-backup


Example:

Device(config-router)# bgp soft-reconfig-backup

 

Configures a BGP speaker to perform inbound soft reconfiguration for peers that do not support the route refresh capability.

  • This command is used to configure BGP to perform inbound soft reconfiguration for peers that do not support the route refresh capability. The configuration of this command allows you to configure BGP to store updates (soft reconfiguration) only as necessary. Peers that support the route refresh capability are unaffected by the configuration of this command.
 
Step 6
neighbor {ip-address | peer-group-name} remote-as autonomous-system-number


Example:

Device(config-router)# neighbor 192.168.1.2 remote-as 40000

 

Adds the IP address of the neighbor in the specified autonomous system to the IPv4 multiprotocol BGP neighbor table of the local device.

 
Step 7
neighbor {ip-address | peer-group-name} soft-reconfiguration [inbound]


Example:

Device(config-router)# neighbor 192.168.1.2 soft-reconfiguration inbound

 

Configures the Cisco software to start storing updates.

  • All the updates received from this neighbor will be stored unmodified, regardless of the inbound policy. When inbound soft reconfiguration is done later, the stored information will be used to generate a new set of inbound updates.
 
Step 8
neighbor {ip-address | peer-group-name} route-map map-name {in | out}


Example:

Device(config-router)# neighbor 192.168.1.2 route-map LOCAL in

 

Applies a route map to incoming or outgoing routes.

  • In this example, the route map named LOCAL will be applied to incoming routes.
 
Step 9
Repeat Steps 6 through 8 for every peer that is to be configured with inbound soft reconfiguration.  

--

 
Step 10
exit


Example:

Device(config-router)# exit

 

Exits router configuration mode and enters global configuration mode.

 
Step 11
route-map map-name [permit | deny] [sequence-number]


Example:

Device(config)# route-map LOCAL permit 10

 

Configures a route map and enters route-map configuration mode.

  • In this example, a route map named LOCAL is created.
 
Step 12
set local-preference number-value


Example:

Device(config-route-map)# set local-preference 200

 

Specifies a preference value for the autonomous system path.

  • In this example, the local preference value is set to 200.
 
Step 13
end


Example:

Device(config-route-map)# end

 

Exits route-map configuration mode and enters privileged EXEC mode.

 
Step 14
show ip bgp neighbors [neighbor-address]


Example:

Device# show ip bgp neighbors 192.168.1.2

 

(Optional) Displays information about the TCP and BGP connections to neighbors.

Note    Only the syntax applicable to this task is used in this example. For more details, see the Cisco IOS IP Routing: BGP Command Reference.
 
Step 15
show ip bgp [network] [network-mask]


Example:

Device# show ip bgp

 

(Optional) Displays the entries in the BGP routing table.

Note    Only the syntax applicable to this task is used in this example. For more details, see the Cisco IOS IP Routing: BGP Command Reference.
 

Examples

The following partial output from the show ip bgp neighbors command shows information about the TCP and BGP connections to the BGP neighbor 192.168.2.1. This peer supports route refresh. 

BGP neighbor is 192.168.1.2,  remote AS 40000, external link
 Neighbor capabilities:
   Route refresh: advertised and received(new)

The following partial output from the show ip bgp neighbors command shows information about the TCP and BGP connections to the BGP neighbor 192.168.3.2. This peer does not support route refresh so the soft-reconfig inbound paths for BGP peer 192.168.3.2 will be stored because there is no other way to update any inbound policy updates. 

BGP neighbor is 192.168.3.2,  remote AS 50000, external link
 Neighbor capabilities:
   Route refresh: advertised

The following sample output from the show ip bgp command shows the entry for the network 172.17.1.0. Both BGP peers are advertising 172.17.1.0/24, but only the received-only path is stored for 192.168.3.2. 

BGP routing table entry for 172.17.1.0/24, version 11
Paths: (3 available, best #3, table Default-IP-Routing-Table, RIB-failure(4))
Flag: 0x820
 Advertised to update-groups:
    1         
 50000
   192.168.3.2 from 192.168.3.2 (172.17.1.0)
     Origin incomplete, metric 0, localpref 200, valid, external
 50000, (received-only)
   192.168.3.2 from 192.168.3.2 (172.17.1.0)
     Origin incomplete, metric 0, localpref 100, valid, external
 40000
   192.168.1.2 from 192.168.1.2 (172.16.1.0)
     Origin incomplete, metric 0, localpref 200, valid, external, best

Configuration Examples for BGP 4 Soft Configuration

Examples: BGP Soft Reset

The following examples show two ways to reset the connection for BGP peer 192.168.1.1.

Example: Dynamic Inbound Soft Reset

The following example shows the command used to initiate a dynamic soft reconfiguration in the BGP peer 192.168.1.1. This command requires that the peer support the route refresh capability. 

clear ip bgp 192.168.1.1 soft in

Example: Inbound Soft Reset Using Stored Information

The following example shows how to enable inbound soft reconfiguration for the neighbor 192.168.1.1. 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. 

router bgp 100
 neighbor 192.168.1.1 remote-as 200
 neighbor 192.168.1.1 soft-reconfiguration inbound

The following example clears the session with the neighbor 192.168.1.1: 

clear ip bgp 192.168.1.1 soft in

Additional References

Related Documents

Related Topic Document Title

Cisco IOS commands

Cisco IOS Master Command List, All Releases

BGP commands

Cisco IOS IP Routing: BGP Command Reference

Standards and RFCs

Standard/RFC Title

RFC 2918

Route Refresh Capability for BGP-4

Technical Assistance

Description Link

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http://www.cisco.com/cisco/web/support/index.html

Feature Information for BGP 4 Soft Configuration

The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.

Table 1Feature Information for BGP 4 Soft Configuration
Feature Name Releases Feature Information

BGP 4 Soft Configuration

Cisco IOS XE Release 2.1

BGP 4 Soft Configuration allows BGP4 policies to be configured and activated without clearing the BGP session, hence without invalidating the forwarding cache.

This feature was introduced on the Cisco ASR 1000 Series Routers.

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Any Internet Protocol (IP) addresses and phone numbers used in this document are not intended to be actual addresses and phone numbers. Any examples, command display output, network topology diagrams, and other figures included in the document are shown for illustrative purposes only. Any use of actual IP addresses or phone numbers in illustrative content is unintentional and coincidental.

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