8.6 BGP Attributes
8.6.3
Next Hop behavior on multiaccess media
Recall that a network link is considered multiaccess if more than two hosts can potentially connect to it. Routers on a multiaccess link share the same IP subnet and can physically access all other connected routers in one hop. Ethernet, 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 multiaccess link as the speaker. In other words, if RTC is advertising a route learned from RTB, and if RTC and RTB share a common multiaccess media, then when RTC advertises the route, it should indicate RTB as being the source of the route. If not, routers on the same medium would have to make an unnecessary hop via RTC to get to a router that is sitting in the same segment.

In the figure, RTA, RTB, and RTC share a common multiaccess medium. 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 routing 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 medium with RTC.

When the media is broadcast, such as Ethernet and FDDI, physical connectivity is a given and the Next Hop behavior is no problem. However, when the media is nonbroadcast, such as Frame Relay and ATM, special care should be taken as described in the following section.