8.7 The Routing Process
8.7.9 Next_Hop behavior over NBMA
Media such as Frame Relay and ATM are nonbroadcast MA (NBMA) networks. The many-to-many direct interaction between routers is not guaranteed unless virtual circuits are configured from each router to all other routers. This is called a fully meshed topology, and it is not always implemented for numerous reasons. In practice, Frame Relay or ATM virtual circuits are provided by the access carrier at a certain dollar amount per circuit, and additional circuits translate into extra money. In addition to this cost disincentive, most organizations use a hub-and-spoke approach, where multiple remote sites have virtual circuits built to one or more concentration routers at a central site (the hub site). Figure illustrates an example of next-hop behavior in a nonbroadcast MA environment.
Lab Activity
  In this lab, you will learn how to use the neighbor next-hop-self command to prevent routing failure between 2 routers that do not have a direct connection.

The only difference between the environments illustrated in Figure and Figure is that the media in Figure is a Frame Relay cloud that is NBMA. RTC is the hub router; RTA and RTB are the spokes. Notice how the virtual circuits are laid out between RTC and RTA, and between RTC and RTB, but not between RTA and RTB. This is called a partially meshed topology.

RTA gets a BGP routing update about 11.11.11.0/24 from RTC and would try to use RTB (10.10.10.3) as the next hop (the same behavior as on MA media). Routing will fail because no virtual circuit exists between RTA and RTB.

Cisco IOS® software supports a special case parameter that remedies this situation. The next-hop-self parameter (when configured as part of the BGP neighbor connection) forces the router (in this case, RTC) to advertise

11.11.11.0/24 with itself as the next hop (10.10.10.2). RTA would then direct its traffic to RTC to reach destination 11.11.11.0/24.

Note: Click on topology to view command outputs.