Media such as Frame Relay and ATM are
nonbroadcast MA (NBMA) networks. The many-to-many direct interaction
between routers is not guaranteed unless virtual circuits are
configured from each router to all other routers. This is called a
fully meshed topology, and it is not always implemented for numerous
reasons. In practice, Frame Relay or ATM virtual circuits are
provided by the access carrier at a certain dollar amount per
circuit, and additional circuits translate into extra money. In addition to this cost disincentive, most
organizations use a hub-and-spoke approach, where multiple remote
sites have virtual circuits built to one or more concentration
routers at a central site (the hub site). Figure
illustrates an example of next-hop behavior in a nonbroadcast MA
environment.
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Lab
Activity |
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In
this lab, you will learn how to use the neighbor
next-hop-self command to
prevent routing failure between 2 routers
that do not have a direct connection. |
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The only difference between the
environments illustrated in Figure
and Figure
is that the media in Figure
is a Frame Relay cloud that is NBMA. RTC is the hub router; RTA and
RTB are the spokes. Notice how the virtual circuits are laid out
between RTC and RTA, and between RTC and RTB, but not between RTA
and RTB. This is called 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 (the same behavior as on MA media). Routing will fail
because no virtual circuit exists between RTA and RTB.
Cisco IOS® software
supports a special case parameter that remedies this situation. The
next-hop-self parameter (when configured as part of the BGP neighbor
connection) 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 would then direct its traffic to RTC to reach
destination 11.11.11.0/24.
Note: Click on topology to view
command outputs.
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