8.7 The Routing Process
8.7.8
NEXT_HOP behavior on multiaccess media
A network link is considered multiaccess (MA) if routers connected to that media have the capability to exchange data in a many-to-many relationship. Routers on a MA link share the same IP subnet, and can physically access all other connected routers in one hop. Ethernet, Fiber Distributed Data Interface (FDDI), Token Ring, Frame Relay, and ATM are examples of multiaccess media.

BGP speakers should always advertise the actual source of the route if the source is on the same MA link as the speaker. In other words, if RTA is advertising a route learned from RTB, and RTA and RTB share a common MA media, when RTA advertises the route, it should indicate RTB as being the source of the route. If not, routers on the same media would have to make an unnecessary hop via RTA to get to a router that is sitting in the same segment.

In the figure, RTA, RTB, and RTC share a common MA media. RTA and RTC are running EBGP, while RTC and RTB are running OSPF. RTC has learned network 11.11.11.0/24 from RTB via OSPF and is advertising it to RTA via EBGP. Because RTA and RTB are running different protocols, you might think that RTA would consider RTC (10.10.10.2) as its next hop to reach 11.11.11.0/24, but this is incorrect. The correct behavior is for RTA to consider RTB (10.10.10.3) as the next hop because RTB shares the same media with RTC.

In situations where the media is broadcast, such as Ethernet and FDDI, physical connectivity is a given and the NEXT_HOP behavior is no problem. But in situations where the media is nonbroadcast, such as Frame Relay and ATM, special care should be taken as described in the following section.