BGP Next Hop Unchanged: Solving Common Routing Challenges

BGP Next Hop Unchanged: Solving Common Routing Challenges

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BGP is used to exchange routing information between multiple autonomous systems (AS) on the Internet. An autonomous system is a group of networks that share a common administrative area. BGP is used to route traffic across multiple independent systems and is the protocol used by Internet Service Providers (ISPs) to communicate routing information with one another.

Related : BGP: Path Selection Criteria - Path Vector Protocol

⭐ BGP Next hop Functionality 👇

When a router sends out a route in an external BGP (eBGP) session, by default, it modifies the BGP route's next hop attribute (to its own IP). If an eBGP multihop peer is setup with the BGP Next Hop Unchanged functionality enabled, BGP will deliver routes to them without changing the next hop, so next hop attribute is unchanged

Network design and migration are made more flexible by the BGP Next Hop Unchanged capability. It is limited to use amongst eBGP peers that have multihop configurations. It may be applied between two autonomous systems in a range of situations. 

⭐ Where we use "BGP Next hop unchanged" ?

One possibility is a network of linked autonomous devices sharing the same IGP, or at the very least, a different path for the routers to reach each other's next hops (thus the next hop can stay the same).

Multihop eBGP between RR
Fig 1.1- Multihop eBGP between RR

One may only configure the BGP Next Hop Unchanged feature between multihop eBGP peers; it cannot be configured for a neighbor that is directly linked.

This functionality is commonly used to configure MPLS inter-AS with multihop MP-eBGP for VPNv4 between RRs.

As a general rule, each BGP next-hop change resets the Label stack, and a new label is replaced. To construct an End-to-End LSP between two PE devices, the remote PE's BGP next-hop must not change when traversing AS borders. 

However, if the BGP next-hop on RRs is reset, a new label will be established. To avoid this, require the RRs not to alter the next-hop, which implies that the RR advertise the VPNv4 prefix without changing the next-hop. 

So we need to configure the specific command "next-hop-unchanged" as shown below

BGP Next Hop unchanged

Hope it clarifies the way we are using some of the 
BGP features to set up the things. This feature is only used in the case we have RR's in an MPLS scenario of Inter-AS option C.

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