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It is important to control defaults in BGP
because if they are originated randomly they could cause everybody serious
problems; for example, if a BGP speaker that intends to originate default
to a specific peer ends up flooding the default to all of its neighbors
could pull in all the traffic from these surrounding ASs. Cisco provides a
way to target the default toward a specific neighbor.
Note: Click on topology to view command
outputs.
In the Figure, RTA is originating a
default route 0.0.0.0 0.0.0.0 toward RTC only; other IBGP neighbors, such
as RTF, will not get the default. RTA's configuration follows. The default-originate
option of the neighbor
command will cause 0/0 to be sent toward RTC. This is shown in the BGP and
IP routing tables of RTC. The routing table of RTC indicates that RTC has
dynamically learned the 0/0 default from RTA and has set its gateway of
last resort to 172.16.20.2, which is RTA.
Defaults can also be originated over all
BGP peers using the network 0.0.0.0 router command as long as the
router has its own default. The following configuration can be used,
assuming that RTA has a default route itself (default could be created via
a static route).
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