NEXT_HOP Behavior Over Nonbroadcast Multiaccess Media
Media such as Frame Relay and ATM are referred to as nonbroadcast multiaccess media. Themany-to-many direct interaction between routers is not guaranteed unless virtual circuits areconfigured from each router to all other routers. This is referred to as a full-mesh topology,and it is not always implemented for a number of reasons. In practice, Frame Relay or ATMvirtual circuits are provided by the access carrier at a certain dollar amount per circuit, andadditional circuits translate into extra money. In addition to this prohibitive cost, manyorganizations use a hub and spoke approach, in which multiple remote sites have virtualcircuits built to one or more concentration routers at a central site (the hub site) whereinformation resides. Figure 6-18 illustrates an example of next-hop behavior in anonbroadcast multiaccess environment.
Figure 6-18. Nonbroadcast Multiaccess NEXT_HOP Example
The only difference between the environments illustrated in Figure 6-17 and Figure 6-18 isthat the medium in Figure 6-18 is a Frame Relay cloud that is NBMA. RTC is the hub router,and RTA and RTB are the spokes. Notice how the virtual circuits are laid out between RTCand RTA, and between RTC and RTB, but not between RTA and RTB. This is called apartial-mesh topology.RTA gets a BGP routing update about 11.11.11.0/24 from RTC, which in turn learns aboutthe prefix from the originator RTB. RTA will try to use RTB (10.10.10.3) as the next hop (thesame behavior as on MA media). Packet forwarding will fail because no virtual circuit existsbetween RTA and RTB.Cisco IOS software supports a special-case parameter that remedies this situation. The nexthop-self parameter (configured as part of the BGP neighbor command) forces the router (inthis case, RTC) to advertise 11.11.11.0/24 with itself as the next hop (10.10.10.2). RTAwould then direct its traffic to RTC to reach destination 11.11.11.0/24.
Figure 6-18. Nonbroadcast Multiaccess NEXT_HOP Example
The only difference between the environments illustrated in Figure 6-17 and Figure 6-18 isthat the medium in Figure 6-18 is a Frame Relay cloud that is NBMA. RTC is the hub router,and RTA and RTB are the spokes. Notice how the virtual circuits are laid out between RTCand RTA, and between RTC and RTB, but not between RTA and RTB. This is called apartial-mesh topology.RTA gets a BGP routing update about 11.11.11.0/24 from RTC, which in turn learns aboutthe prefix from the originator RTB. RTA will try to use RTB (10.10.10.3) as the next hop (thesame behavior as on MA media). Packet forwarding will fail because no virtual circuit existsbetween RTA and RTB.Cisco IOS software supports a special-case parameter that remedies this situation. The nexthop-self parameter (configured as part of the BGP neighbor command) forces the router (inthis case, RTC) to advertise 11.11.11.0/24 with itself as the next hop (10.10.10.2). RTAwould then direct its traffic to RTC to reach destination 11.11.11.0/24.
