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BGP Next Hop Unchanged

Table Of Contents

BGP Next Hop Unchanged

Finding Feature Information

Contents

Information About BGP Next Hop Unchanged

BGP Next Hop Unchanged

How to Configure BGP Next Hop Unchanged

Configuring the BGP Next Hop Unchanged for an eBGP Peer

Configuration Example for BGP Next Hop Unchanged

Example: BGP Next Hop Unchanged for an eBGP Peer

Additional References

Related Documents

Technical Assistance

Feature Information for BGP Next Hop Unchanged


BGP Next Hop Unchanged


Last Updated: May 19, 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 for BGP Next Hop Unchanged" section.

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

Contents

Information About BGP Next Hop Unchanged

How to Configure BGP Next Hop Unchanged

Configuration Example for BGP Next Hop Unchanged

Additional References

Feature Information for BGP Next Hop Unchanged

Information About BGP Next Hop Unchanged

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

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

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

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, Cisco IOS Release 15.1M&T

MPLS configuration tasks

MPLS Configuration Guide, Cisco IOS Release 15.1M&T

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

Configuring Internal BGP Features


Technical Assistance

Description
Link

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


Feature Information for BGP Next Hop Unchanged

Table 1 lists the release history for this feature.

Use Cisco Feature Navigator to find information about platform support and software image support. Cisco Feature Navigator enables you to determine which software images support a specific software release, feature set, or platform. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.


Note Table 1 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.


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.