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
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
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.
In Cisco IOS XE Denali 16.3 release, the set ip next-hop
unchanged/next-hop-unchanged allpaths IPv4/IPv6 feature extends the support for
BGP Next Hop Unchanged to specific prefixes, specific next-hop, and iBGP and
eBGP path prefixes while sending to eBGP neighbor. With the set ip next-hop
unchanged/next-hop-unchanged allpaths IPv4/IPv6 feature you can configure
unchanged next-hop for specific prefixes, for iBGP path prefixes while sending
to eBGP neighbor, and can also be configured to set peer-address or a specific
address as next-hop.
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