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
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Step 1 |
enable
Router> enable |
Enables privileged EXEC mode. •Enter your password if prompted. |
Step 2 |
configure terminal
Router# configure terminal |
Enters global configuration mode. |
Step 3 |
router bgp as-number
Router(config)# router bgp 65535 |
Enters router configuration mode, and creates a BGP routing process. |
Step 4 |
address-family {ipv4 [unicast]| vpnv4 [unicast]}
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
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
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
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
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
Router(config-router-af)# end |
Exits address family configuration mode, and enters privileged EXEC mode. |
Step 10 |
show ip bgp
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.
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
Additional References
Related Documents
Technical Assistance
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The Cisco Support and Documentation website provides online resources to download documentation, software, and tools. Use these resources to install and configure the software and to troubleshoot and resolve technical issues with Cisco products and technologies. Access to most tools on the Cisco Support and Documentation website requires a Cisco.com user ID and password. |
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
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Feature Configuration Information
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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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