Content

| 9.6 |  | BGP Attributes |
| 9.6.4 |  | Next Hop behavior on NBMA networks |

On an NBMA network, the many-to-many direct interaction between routers is not guaranteed unless virtual circuits are configured from each router to all other routers. The primary reason that most organizations implement a hub-and-spoke topology is because of cost considerations. In a hub-and-spoke topology, multiple remote sites have virtual circuits connected to one or more routers at a central site. Figure illustrates an example of next hop behavior in a nonbroadcast multiaccess environment.

The only difference between the environments illustrated is that the medium in Figure is a Frame Relay cloud that is NBMA. RTC is the hub router and RTA and RTB are the spokes. Notice that the virtual circuits are laid out between RTC and RTA, and between RTC and RTB, but not between RTA and RTB. This is 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. This is the same behavior as on multiaccess media. Routing will fail because no virtual circuit exists between RTA and RTB.

Cisco IOS supports a special case parameter that remedies this situation. The

next-hop-self keyword 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 then directs its traffic to RTC to reach destination 11.11.11.0/24. The syntax for this option is as follows:

> Router(config-router)#neighbor ip-address next-hop-self

For RTC, issue the following command:

> RTC(config-router)#neighbor 10.10.10.1 next-hop-self

| Lab Activity e-Lab Activity: Next Hop Behavior on NBMA Networks This lab is to fix the next hop behavior on NBMA networks.

| Web Links Configuring BGP http://www.cisco.com/univercd/cc/td/doc/product/ software/ios122/122cgcr/fipr_c/ipcprt2/1cfbgp.htm