BGP Next Hop Unchanged

Last Updated: September 08, 2011

In an external BGP (eBGP) session, by default, the router changes the next hop attribute of a BGP route (to its own address) when the router sends out a route. The BGP Next Hop Unchanged feature allows BGP to send an update to an eBGP multihop peer with the next hop attribute unchanged.

Finding Feature Information

Your software release may not support all the features documented in this module. For the latest feature information and caveats, see 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 document.

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 Next Hop Unchanged

BGP Next Hop Unchanged

In an external BGP (eBGP) session, by default, the router changes the next hop attribute of a BGP route (to its own address) when the router sends out a route. If the BGP Next Hop Unchanged feature is configured, BGP will send routes to an eBGP multihop peer without modifying the next hop attribute. The next hop attribute is unchanged.


Note
There is an exception to the default behavior of the router changing the next hop attribute of a BGP route when the router sends out a route. When the next hop is in the same subnet as the peering address of the eBGP peer, the next hop is not modified. This is referred to as third party next-hop.

The BGP Next Hop Unchanged feature provides flexibility when designing and migrating networks. It can be used only between eBGP peers configured as multihop. It can be used in a variety of scenarios between two autonomous systems. One scenario is when multiple autonomous systems are connected that share the same IGP, or at least the routers have another way to reach each other’s next hops (which is why the next hop can remain unchanged).

A common use of this feature is to configure Multiprotocol Label Switching (MPLS) inter-AS with multihop MP-eBGP for VPNv4 between RRs.

Another common use of this feature is a VPNv4 inter-AS Option C configuration, as defined in RFC4364, Section 10. In this configuration, VPNv4 routes are passed among autonomous systems between RR of different autonomous systems. The RRs are several hops apart, and have neighbor next-hop unchanged configured. PEs of different autonomous systems establish an LSP between them (via a common IGP or by advertising the next-hops--that lead to the PEs--via labeled routes among the ASBRs--routes from different autonomous systems separated by one hop). PEs are able to reach the next hops of the PEs in another AS via the LSPs, and can therefore install the VPNv4 routes in the VRF RIB.

Restriction

The BGP Next Hop Unchanged feature can be configured only between multihop eBGP peers. The following error message will be displayed if you try to configure this feature for a directly connected neighbor:

%BGP: Can propagate the nexthop only to multi-hop EBGP neighbor

How to Configure BGP Next Hop Unchanged

Configuring the BGP Next Hop Unchanged for an eBGP Peer

SUMMARY STEPS

1.    enable

2.    configure terminal

3.    router bgp as-number

4.    address-family {ipv4 [unicast]| vpnv4 [unicast]}

5.    neighbor ip-address remote-as as-number

6.    neighbor ip-address activate

7.    neighbor ip-address ebgp-multihop ttl

8.    neighbor ip-address next-hop-unchanged

9.    end

10.    show ip bgp


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 65535

 

Enters router configuration mode, and creates a BGP routing process.

 
Step 4
address-family {ipv4 [unicast]| vpnv4 [unicast]}


Example:

Router(config-router-af)# address-family vpnv4

 

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

 
Step 5
neighbor ip-address remote-as as-number


Example:

Router(config-router-af)# neighbor 10.0.0.100 remote-as 65600

 

Adds an entry to the BGP neighbor table.

 
Step 6
neighbor ip-address activate


Example:

Router(config-router-af)# neighbor 10.0.0.100 activate

 

Enables the exchange of information with the peer.

 
Step 7
neighbor ip-address ebgp-multihop ttl


Example:

Router(config-router-af)# neighbor 10.0.0.100 ebgp-multihop 255

 

Configures the local router to accept and initiate connections to external peers that reside on networks that are not directly connected.

 
Step 8
neighbor ip-address next-hop-unchanged


Example:

Router(config-router-af)# neighbor 10.0.0.100 next-hop-unchanged

 

Configures the router to send BGP updates to the specified eBGP peer without modifying the next hop attribute.

 
Step 9
end


Example:

Router(config-router-af)# end

 

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

 
Step 10
show ip bgp


Example:

Router# show ip bgp

 

(Optional) Displays entries in the BGP routing table.

  • The output will indicate if the neighbor next-hop-unchanged command has been configured for the selected address.
 

Configuration Example for BGP Next Hop Unchanged

Example BGP Next Hop Unchanged for an eBGP Peer

The following example configures a multihop eBGP peer at 10.0.0.100 in a remote AS. When the local router sends updates to that peer, it will send them without modifying the next hop attribute.

router bgp 65535
 address-family ipv4
 neighbor 10.0.0.100 remote-as 65600
 neighbor 10.0.0.100 activate 
 neighbor 10.0.0.100 ebgp-multihop 255
 neighbor 10.0.0.100 next-hop-unchanged
 end

Additional References

Related Documents

Related Topic

Document Title

Cisco IOS commands

Cisco IOS Master Commands List, All Releases

BGP commands

Cisco IOS IP Routing: BGP Command Reference

BGP configuration tasks

IP Routing: BGP Configuration Guide

MPLS configuration tasks

MPLS Configuration Guide

BGP Outbound Route Map on Route Reflector to Set IP Next Hop for iBGP Peer

"Configuring Internal BGP Features"

Technical Assistance

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

Feature Information for BGP Next Hop Unchanged

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 1 Feature Information for BGP Next Hop Unchanged

Feature Name

Releases

Feature Configuration Information

BGP Next Hop Unchanged

12.0(22)S 12.0(16)ST 12.2 12.2(14)S 15.0(1)S

The BGP Next Hop Unchanged feature allows BGP to send an update to an eBGP multihop peer with the next hop attribute unchanged.

The following command was added by this feature: neighbor next-hop-unchanged.

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