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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.
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